Modern history

Sunday, September 3



BY SUNDAY MORNING, A STRANGE QUIET HAD OVERTAKEN the streets of Soho. The usual chaos of the streetsellers had disappeared; most of the neighborhood’s residents had either evacuated or were suffering behind their doors. Seventy of them had perished over the preceding twenty-four hours, hundreds more were at the very edge of death. Out in front of 40 Broad, the pump attracted only a handful of stragglers. The most common sight on the streets were the priests and doctors making their frantic rounds.

Word of the outbreak had traveled through the wider city and beyond. The chemist’s son who had enjoyed his pudding days before on Wardour Street died on that Sunday at his home in Willesden. The entire city held its breath as it took in refugees from the embattled neighborhood, waiting to see if the outbreak in Golden Square would be re-created on a larger scale in the coming days. Seventy deaths in a single parish was not an uncommon number to hear in an age of cholera epidemics. But it normally took months for the disease to chalk up so many victims. The Broad Street strain of cholera—whatever it was, wherever it had come from—had managed that terrible feat in a single day.

While the disease had remained largely confined to an area of roughly five square blocks, the rest of Soho was on high alert. Many packed their bags and visited friends or family who lived in the country or other parts of the city; some locked the doors and shuttered the windows. The vast majority steered clear of the Golden Square neighborhood at all cost.

But one Soho regular had been following the case closely from his residence at Sackville Street on the southwestern edge of the neighborhood. Sometime near dusk he set out from his home, marching through the empty streets, directly into the heart of the outbreak. When he reached 40 Broad, he stopped and examined the pump for a few minutes in the fading light. He drew a bottle of water from the well, stared at it for a few seconds, then turned and made his way back to Sackville Street.

JOHN SNOW WAS IN HIS FORTY-SECOND YEAR, AND SINCE his early thirties he had by any measure enjoyed a remarkable streak of professional achievement. Unlike most members of the medical establishment or the sanitary reform movement, Snow had been born into a family of modest means, the eldest son of a Yorkshire laborer. A quiet, serious child with intellectual ambitions beyond his humble origins, Snow had apprenticed at the age of fourteen to a surgeon in Newcastle-on-Tyne. At the age of seventeen he read John Frank Newton’s influential 1811 manifesto The Return to Nature: A Defence of the Vegetable Regimen and promptly converted to vegetarianism. Shortly thereafter, he became a strict teetotaler. He would largely avoid meat and alcohol for the rest of his adult life.

As an apprentice in Newcastle, Snow saw the ravages of cholera firsthand when the disease struck in late 1831. He treated the survivors of a particularly brutal outbreak in a local mine, the Killingworth Colliery. The young Snow observed that the sanitary conditions in the mine were dreadful, with workers granted no separate quarters to relieve themselves, thus forcing them to eat and defecate in the same dark, stifling caverns. The idea that the cholera outbreak was rooted in the social conditions of these impoverished workers—and not in any innate susceptibility to the disease—lodged in the back of Snow’s mind as the cholera ran its course. It was only a partially realized thought, nowhere near a genuine theory. But it stayed with him, nonetheless.

A young Englishman interested in the medical life during the first half of the nineteenth century had three primary career paths open to him. He could apprentice with an apothecary and then eventually land a license from the Society of Apothecaries, which would grant him the right to concoct medicines prescribed by physicians. After some training, he would be free to embark on his own practice, treating patients with the woeful remedies of the day, probably dabbling in minor surgery or dentistry on the side. The more ambitious individual would go on to study at a medical school, and later join the Royal College of Surgeons of England, becoming a bona fide general practitioner and surgeon, performing a host of different tasks: everything from treating minor colds to excising bunions to amputating limbs. Beyond that lay the university degree Doctor of Medicine, whose recipients were conventionally called physicians, as opposed to the lower orders of surgeons and apothecaries. A university degree opened doors to the private hospitals, where one could rub shoulders with the wealthy benefactors who endowed them.

Snow realized at an early age that his ambitions extended beyond that of a provincial apothecary. He had moved back to York in 1835 and involved himself in the growing temperance movement there. But at the age of twenty-three, he decided to follow the classic itinerary of the bildungsroman genre that dominated the nineteenth-century novel: a provincial young man with dreams of greatness sets out for the big city to make a name for himself. Snow’s journey to London was typical of the earnest young doctor-in-training: he eschewed both horse and carriage and walked a meandering two-hundred-mile route alone.

In London, Snow settled in Soho and enrolled in the Hunterian School of Medicine. Within two years he had received both his apothecary and surgeon’s license and established a general practice at 54 Frith Street in London, about a five-minute walk east from Golden Square. Setting up shop as a doctor in those days required an entrepreneurial spirit. The competition was intense among London’s new medical middle class—four other surgeons had offices within a few blocks of Snow, though the only physicians nearby resided across Soho in Golden Square. Despite the proximity of so many rivals, Snow quickly established a successful practice. Temperamentally he was not the cliché of the friendly, garrulous general practitioner; his bedside manner was taciturn and emotionally flat. But he was a superb doctor: observant, quick-witted, and possessed of an exceptional memory for past cases. Snow was as free from superstition and dogma as it was possible to be in those days, though he was inevitably limited in his effectiveness by the conceptual dead ends and distortions of early Victorian medicine. The idea of microscopic germs spreading disease would have been about as plausible as the existence of fairies to most practicing doctors of the day. And as Surgeon-in-Chief G. B. Childs’ letter-writing campaign to the Times suggested, laudanum was regularly prescribed for almost any ailment. The Victorian medical refrain was, essentially: Take a few hits of opium and call me in the morning.

Seemingly bereft of anything resembling a traditional social life, Snow spent his time away from patients working on side projects that grew out of his surgeon’s practice but which also suggested the ultimate range of his ambition. He began writing in to the local journals, opining on medical and public-health issues of the day. His first published paper, addressing the use of arsenic in the preservation of cadavers, appeared in The Lancet in 1839. He went on to publish nearly fifty articles in the following decade, on a staggering range of subjects: lead poisoning, resuscitating stillborn children, blood vessels, scarlet fever, and smallpox. He wrote in to The Lancet with so many critiques of sloppy science that the editor eventually scolded him gently in print, suggesting that “Mr. Snow might better employ himself in producing something, than in criticizing the production of others.”

Snow clearly had his mind set on producing something of his very own, and he saw advanced degrees as a crucial bridge to that end. In 1843, he earned his bachelor of medicine degree from the University of London. A year later he had passed the challenging M.D. exams, placing in the first division of students. He was now, officially, Dr. John Snow. By most standards, he was already a remarkable success story: a laborer’s son who now had a thriving medical practice and a vibrant career as a researcher and lecturer. At the recommendation of one of his former professors, he had been invited to join the Westminster Medical Society, where he quickly became a respected and active member. Any number of physicians would have settled into that comfortable realm, pursuing only the incremental advances of tending to increasingly well-to-do clientele and elevating their own social prestige in the process. But Snow was oblivious of the trappings of London’s polite society; what drove him, more than anything, were problems that needed solutions, filling in the blind spots in the medical establishment’s vision of the world.

Snow would continue to work as a practicing physician for the rest of his life, but his eventual fame would come from his pursuits outside the consulting room. Snow did not aim low in his investigations. He would play a defining role in the battle against the era’s most relentless killer. But before he could tackle cholera, John Snow set his sights on one of the most excruciating deficiencies of Victorian medicine: pain management.

WHERE SHEER PHYSICAL BRUTALITY WAS CONCERNED, THERE was little in Victorian society that rivaled the professional medical act of surgery. Lacking any form of anesthesia beyond opium or alcohol—both of which could only be applied in moderation, given their side effects—surgical procedures were functionally indistinguishable from the most grievous forms of torture. Surgeons prided themselves on their speed above all else, since extended procedures were unbearable for both doctor and patient. Procedures that would now take hours to complete were executed in three minutes or less, to minimize the agony. One surgeon boasted that he could “amputate a shoulder in the time it took to take a pinch of snuff.”

In 1811, the British author—and longtime Soho resident—Fanny Burney underwent a mastectomy in Paris. She described the experience in a letter to her sister a year later. After drinking a wine cordial as her sole form of painkiller, she settled into the ominous closet that had been assembled by the team of seven doctors in her home, lined with compresses and bandages and gruesome surgical tools. She lay down on the makeshift bed, and the doctors covered her face with a light handkerchief. “When the dreadful steel was plunged into the breast, cutting through veins, arteries, flesh, nerves, I needed no injunction not to restrain my cries. I began a scream that lasted unintermittingly during the whole time of the incision, and I almost marvel that it rings not in my ears still! So excruciating was the agony.… I then felt the knife rackling against the breastbone, scraping it! This performed, while I remained in utterly speechless torture.” Before passing out in near shock after the procedure, she caught a glimpse of her primary doctor—“pale nearly as myself, his face streaked with blood and its expression depicting grief, apprehension, and almost horror.”

In October of 1846, at Massachusetts General Hospital in Boston, a dentist named William Morton gave the first public demonstration of the use of ether as an anesthetic. Word quickly spread across the Atlantic, and by mid-December, a London dentist, James Robinson, had begun using ether on his patients, usually with a small audience of amazed medical men in attendance. On December 28, he performed another successful extraction. In the room, watching, with his usual quiet and observant manner, was John Snow.

By the turn of the new year, the excitement over ether had spilled beyond the medical community and into the popular press. Punch was running mock editorials advocating its use on difficult wives. But the miracle anesthetic was unreliable in practice. Some applications would work flawlessly: the patient would nod off for the length of the surgery, and then awaken minutes later with no memory of the procedure, and a greatly minimized feeling of pain. But other patients would fail to go under, or return to consciousness abruptly in the middle of a particularly delicate operation. More than a few patients never woke up at all.

Snow quickly hypothesized that the unreliability of ether was likely a problem of dosage, and embarked on a series of interlinked experiments to determine the best mechanism for delivering the miracle gas. From his earlier studies, Snow knew the concentration of any gas varied dramatically with temperature, and yet the early adopters of etherization had failed to take room temperature into account in their procedures. A patient etherized in a chilly room would end up with a significantly lower dose than one etherized in a room warmed by a roaring fire. By mid-January, Snow had compiled a “Table for Calculating the Strength of Ether Vapour.” Increasing the temperature by twenty degrees Fahrenheit would nearly double the dosage. The Medical Times published Snow’s table at the end of January.

While compiling the data for his numerical breakdown of ether’s properties, Snow had begun collaborating with a surgeon’s instrument maker named Daniel Ferguson in making an inhaler that would allow maximum control of the dosage. Snow’s idea was to adapt the well-known Julius Jeffrey vaporizer for the purposes of delivering ether, forcing it through a metal spiral at the center of the device, thus maximizing the surface area of metal exposed to the gas as it traveled to the patient’s mouth. The unit would be placed in a vat of heated water that would transmit its warmth to the metal contraption, where it would raise the temperature of the ether. All the doctor needed to control was the temperature of the water; the device would do the rest. Once the doctor had a reliable fix on the ether’s temperature, he could determine the proper dose with little variation. Snow first presented his device to the Westminster Society on January 23, 1847.

Snow’s productivity during this period is truly astounding, when you think that the very concept of etherization simply hadn’t existed three months before. Not only had Snow detected one of the fundamental properties of the gas within two weeks of first seeing it applied, he had also engineered a state-of-the-art medical device to deliver it. And his research had only begun: in the following months, he explored the biology of etherization: everything from the initial intake of the gas in the lungs, to its circulation through the bloodstream, all the way to its psychological effects. When the medical community shifted its focus to the rival anesthetic chloroform later in 1847, Snow immersed himself in its properties as well. By the end of 1848 he had published a seminal monograph on the theory and practice of anesthesia: On the Inhalation of the Vapour of Ether in Surgical Operations.

Snow managed to build his mastery of this embryonic field almost entirely through research conducted in his own home. He maintained a small menagerie in his Frith Street quarters—birds, frogs, mice, fish—where he spent countless hours watching the creatures’ response to various dosages of ether and chloroform. He also used his medical practice as a source of experimental data, but was not above using himself as a test subject. There is something wonderful—and more than a little ironic—in this image of Snow the teetotaler, arguably the finest medical mind of his generation, performing his research. He sits alone in his cluttered flat, frogs croaking around him, illuminated only by candlelight. After a few minutes tinkering with his latest experimental inhaler, he fastens the mouthpiece over his face and releases the gas. Within seconds, his head hits the desk. Then, minutes later, he wakes, consults his watch through blurred vision. He reaches for his pen, and starts recording the data.


SNOW’S MASTERY OF ETHER AND CHLOROFORM RAISED HIM to a new echelon in the London medical world. He became the most sought-after anesthesiologist in the city, assisting with hundreds of operations a year. By the 1850s, a growing number of doctors were recommending chloroform as a palliative for the discomfort of childbirth. As the birth of her eighth child approached in the spring of 1853, Queen Victoria decided to give chloroform a try, encouraged by the scientifically astute Prince Albert. Her choice of an anesthesiologist was an obvious one. Snow gave the episode a few more words than usual in his casebooks, though his tone did not betray the magnitude of the professional honor that had been bestowed upon him:

Thursday 7 April: Administered Chloroform to the Queen in her confinement. Slight pains had been experienced since Sunday. Dr. Locock was sent for about nine o’clock this morning, stronger pains having commenced, and he found the os uteri had commenced to dilate a very little. I received a note from Sir James Clark a little after ten asking me to go to the Palace. I remained in an apartment near that of the Queen, along with Sir J. Clark, Dr. Ferguson and (for the most part of the time) Dr. Locock till a little a [sic] twelve. At a twenty minutes past twelve by a clock in the Queen’s apartment I commenced to give a little chloroform with each pain, by pouring about 15 minims [0.9 ml] by measure on a folded handkerchief. The first stage of labour was nearly over when the chloroform was commenced. Her Majesty expressed great relief from the application, the pains being very trifling during the uterine contractions, and whilst between the periods of contraction there was complete ease. The effect of the chloroform was not at any time carried to the extent of quite removing consciousness. Dr. Locock thought that the chloroform prolonged the intervals between the pains, and retarded the labour somewhat. The infant was born at 13 minutes past one by the clock in the room (which was 3 minutes before the right time); consequently the chloroform was inhaled for 53 minutes. The placenta was expelled in a very few minutes, and the Queen appeared very cheerful and well, expressing herself much gratified with the effect of the chloroform.

Snow’s research into anesthesia had elevated him from a surgeon of humble origins to the very apogee of Victorian London. But, in a way, the most impressive thing about his research was not the levels of social class that he traversed but rather the intellectual strata, the different scales of experience that his mind crossed so effortlessly. Snow was a truly consilient thinker, in the sense of the term as it was originally formulated by the Cambridge philosopher William Whewell in the 1840s (and recently popularized by the Harvard biologist E. O. Wilson). “The Consilience of Inductions,” Whewell wrote, “takes place when an Induction, obtained from one class of facts, coincides with an Induction obtained from another different class. Thus Consilience is a test of the truth of the Theory in which it occurs.” Snow’s work was constantly building bridges between different disciplines, some of which barely existed as functional sciences in his day, using data on one scale of investigation to make predictions about behavior on other scales. In studying ether and chloroform, he had moved from the molecular properties of the gas itself, to its interactions with the cells of the lungs and the bloodstream, to the circulation of those properties through the body’s overall system, to the psychological effects produced by these biological changes. He even ventured beyond the natural world into the design of technology that would best reflect our understanding of the anesthetics. Snow was not interested in individual, isolated phenomena; he was interested in chains and networks, in the movement from scale to scale. His mind tripped happily from molecules to cells to brains to machines, and it was precisely that consilient study that helped Snow uncover so much about this nascent field in such a shockingly short amount of time.

And yet, there was a ceiling to his intellectual pursuit of ether and chloroform: his research stopped at the scale of the individual subject. The next step up the chain—the larger, connected world of cities and societies, of groups, not individuals—did not factor into his anesthesia investigations. He might have attended on the queen’s body, but the body politic remained outside Snow’s frame of reference.

Cholera would change all that.

WE DON’T KNOW EXACTLY WHAT SEQUENCE OF EVENTS turned John Snow’s interest toward cholera in the late 1840s. For this working physician and researcher, of course, the disease would have been a constant presence in his life. There may in fact have been a direct link to his practice as an anesthesiologist, since chloroform had been (wrongly) championed as a potential cure for cholera by some early adopters who were less rigorous in their empiricism than Snow. Certainly, the outbreak of 1848–1849, the most severe British outbreak in more than a decade, made cholera one of the most urgent medical riddles of its time. For a man like Snow, obsessed with both the practice of medicine and the intellectual challenge of science, cholera would have been the ultimate quarry.

There were practically as many theories about cholera as there were cases of the disease. But in 1848, the dispute was largely divided between two camps: the contagionists and the miasmatists. Either cholera was some kind of agent that passed from person to person, like the flu, or it somehow lingered in the “miasma” of unsanitary spaces. The contagion theory had attracted some followers when the disease first reached British soil in the early 1830s. “We can only suppose the existence of a poison which progresses independently of the wind, of the soil, of all conditions of the air, and of the barrier of the sea,” The Lancet editorialized in 1831. “In short, one that makes mankind the chief agent for its dissemination.” But most physicians and scientists believed that cholera was disease spread via poisoned atmosphere, not personal contact. One survey of published statements from U.S. physicians during the period found that less than five percent believed the disease was primarily contagious.

By the late 1840s the miasma theory had established a far more prestigious following: the sanitation commissioner, Edwin Chadwick; the city’s main demographer, William Farr; along with many other public officials and members of Parliament. Folklore and superstition were also on the side of the miasmatists: the foul inner-city air was widely believed to be the source of most disease. While no clear orthodoxy existed regarding the question of cholera’s transmission, the miasma theory had far more adherents than any other explanatory model. Remarkably, in all the discussion of cholera that had percolated through the popular and scientific press since the disease had arrived on British soil in 1832, almost no one suggested that the disease might be transmitted by means of contaminated water. Even the contagionists—who embraced the idea that the disease was transmitted from person to person—failed to see merit in the waterborne scenario.

Snow’s detective work into cholera began when he noticed a telling detail in the published accounts of the 1848 epidemic. Asiatic cholera had been absent from Britain for several years, but it had recently broken out on the Continent, including the city of Hamburg. In September of that year, the German steamer Elbe docked in London, having left port at Hamburg a few days earlier. A crewman named John Harnold checked into a lodging house in Horsleydown. On September 22, he came down with cholera and died within a matter of hours. A few days later, a man named Blenkinsopp took over the room; he was seized by the disease on September 30. Within a week, the cholera began to spread through the surrounding neighborhood, and eventually through the entire nation. By the time the epidemic wound down, two years later, 50,000 people were dead.

Snow recognized immediately that this sequence of events posed a severe challenge to the opponents of the contagion model. The coincidence was simply too much for the miasma theory to bear. Two cases of cholera in a single room in the space of a week might be compatible with the miasma model, if one believed that the room itself contained some kind of noxious agent that poisoned its inhabitants. But it was stretching matters beyond belief to suggest that the room should suddenly become prone to those poisonous vapors the very day it was occupied by a sailor traveling from a city besieged by the disease. As Snow would later write: “Who can doubt that the case of John Harnold, the seaman from Hamburgh, mentioned above, was the true cause of the malady in Blenkinsopp, who came, and lodged, and slept, in the only room in all London in which there had been a case of true Asiatic cholera for a number of years? And if cholera be communicated in some instances, is there not the strongest probability that it is so in the others—in short, that similar effects depend on similar causes?”

But Snow also recognized the weakness of the contagionist argument. The same doctor attended both Harnold and Blenkinsopp, spending multiple hours in the room with them during the rice-water phase of the disease. And yet he remained free of the disease. Clearly, the cholera was not communicated through sheer proximity. In fact, the most puzzling element of the disease was that it seemed capable of traveling across city blocks, skipping entire houses in the process. The subsequent cases in Horsleydown erupted a few doors down from Harnold’s original lodging house. You could be in the same room with a patient near death and emerge unscathed. But, somehow, you could avoid direct contact altogether with the infected person and yet still be seized with the cholera, simply because you lived in the same neighborhood. Snow grasped that solving the mystery of cholera would lie in reconciling these two seemingly contradictory facts.

We do not know if Snow hit upon the solution to this riddle sometime in the months that followed the initial 1848 outbreak, or if perhaps the solution had long lingered in the back of his mind, a hunch that had first taken shape more than a decade before, as he tended to the dying miners in Killingworth as a young surgeon’s apprentice. We do know that in the weeks after the Horsleydown outbreak, as the cholera began its fatal march through the wider city and beyond, Snow embarked on a torrid stretch of inquiry: consulting with chemists who had studied the rice-water stools of cholera victims, mailing requests for information from the water and sewer authorities in Horsleydown, devouring accounts of the great epidemic of 1832. By the middle of 1849, he felt confident enough to go public with his theory. Cholera, Snow argued, was caused by some as-yet-unidentified agent that victims ingested, either through direct contact with the waste matter of other sufferers or, more likely, through drinking water that had been contaminated with that waste matter. Cholera was contagious, yes, but not in the way smallpox was contagious. Sanitary conditions were crucial to fighting the disease, but foul air had nothing to do with its transmission. Cholera wasn’t something you inhaled. It was something you swallowed.

Snow built his argument for the waterborne theory around two primary studies, both of which showcased talents that would prove to be crucial five years later, during the Broad Street outbreak. In late July of 1849, an outbreak of cholera killed about twelve people living in slum conditions on Thomas Street in Horsleydown. Snow made an exhaustive inspection of the site and found ample evidence to support his developing theory. All twelve lived in a row of connected cottages called the Surrey building, which shared a single well in the courtyard they faced. A drainage channel for dirty water ran alongside the front of the houses, connecting to an open sewer at the end of the courtyard. Several large cracks in the drain allowed water to flow directly into the well, and during summer storms, the entire courtyard would flood with fetid water. And so a single case of cholera would quickly spread through the entire Surrey building population.

The layout of the Thomas Street flats provided Snow with an ingenious control study for his inquiry. The Surrey building backed onto a set of houses that faced another courtyard known as Truscott’s Court. These abodes were every bit as squalid as the Surrey building, with the exact same demographic makeup of poor working families living within them. For all intents and purposes, they shared the same environment, save one crucial difference: they got their water from different sources. During the two-week period that saw the deaths of a dozen residents in the Surrey building, only one person perished in Truscott’s Court, despite the fact that both groups lived within yards of each other. If the miasma were responsible for the outbreak, why would one squalid, impoverished group suffer ten times the loss of the one living next door?

The Thomas Street outbreak showcased Snow’s on-the-ground investigative skills, his eye for the details of transmission patterns, sanitary habits, even architecture. But Snow also surveyed the outbreak from the bird’s-eye view of citywide statistics. During his research, Snow had amassed an archive of information about the various companies that supplied water to the city, and that study had revealed a striking fact: that Londoners living south of the Thames were far more likely to drink water that had originated in the river as it passed through Central London. Londoners living north of the river drank from a variety of sources: some companies piped in water from the Thames above Hammersmith, far from the urban core; some drew from the New River in Hertfordshire to the north; others from the River Lea. But the South London Water Works drew its supply from the very stretch of the river where most of the city’s sewers emptied. Anything that was multiplying in the city’s intestinal tracts would be more likely to find its way into the drinking water of South London. If Snow’s theory of cholera was on the mark, Londoners living below the Thames should have been significantly more prone to the disease than those living above.

Snow next surveyed the tables of cholera death that had been compiled by William Farr, London’s registrar-general. What he found there followed the pattern that the water-supply routes predicted: of the 7,466 deaths in the metropolitan area during the 1848–1849 epidemic, 4,001 were located south of the Thames. That meant that the per capita casualty rate was near eight per thousand—three times that of the central city. In the growing suburbs of West and North London, the death rate was just above one per thousand. For the miasmatists, who were inclined to blame those death rates on the foul air of the working-class neighborhoods south of the river, Snow could point to the neighborhoods of the East End, which were probably the most destitute and overcrowded of any in the city. And yet their death rate was exactly half that of the area south of the Thames.

Whether you looked at the evidence on the scale of an urban courtyard or on the scale of entire city neighborhoods, the same pattern repeated itself: the cholera seemed to segment itself around shared water supplies. If the miasma theory were right, why would it draw such arbitrary distinctions? Why would the cholera devastate one building but leave the one next door unscathed? Why would one slum suffer twice the losses as a slum with arguably worse sanitary conditions?

Snow introduced his theory of cholera in two forms during the second half of 1849: first as a self-published thirty-one-page monograph, On the Mode and Communication of Cholera, intended for his immediate peers in the medical community, and then as an article in the London Medical Gazette, targeted at a slightly wider audience. Shortly after the publication, a country doctor named William Budd published an essay that came to similar conclusions about cholera’s waterborne transmission, though Budd left open the possibility that some cases of cholera might be transmitted through the atmosphere, and he claimed, erroneously, to have identified the cholera agent in the form of a fungus growing in contaminated water supplies. Budd would later make an observation regarding the waterborne transmission of typhoid, for which he is now best known. But Snow’s cholera theory had beaten Budd’s to the presses by a month, and it did not include the false lead of fungal agents or of atmospheric transmission.

The reaction to Snow’s argument was positive but skeptical. “Dr. Snow deserves the thanks of the profession for endeavouring to solve the mystery of the communication of cholera,” a reviewer wrote in the London Medical Gazette. But Snow’s case studies had not convinced: “[They] furnish no proof whatever of the correctness of his views.” He had convincingly demonstrated that the South London neighborhoods were more at risk for cholera than the rest of the city, but it did not necessarily follow that the water in those neighborhoods was responsible for the disparity. Perhaps there was special toxicity to the air in those zones of the city that was absent in the slums to the north. Perhaps cholera was contagious, and thus the cluster of cases in South London simply reflected the chain of infection thus far; if the initial cases had unfolded differently, perhaps the East End would have been attacked more grievously, and South London left relatively unscathed. There was a correlation between water supply and cholera—that much Snow had convincingly proved. But he had not yet established a cause.

The Gazette did suggest one scenario that might settle the matter convincingly:

The experimentum crucis would be, that the water conveyed to a distant locality, where cholera had been hitherto unknown, produced the disease in all who used it, while those who did not use it, escaped.

That passing suggestion stayed with Snow for five long years. As his anesthesia practice expanded, and his prominence grew, he continued to follow the details of each cholera outbreak, looking for a scenario that might help prove his theory. He probed, and studied, and waited. When word arrived of a terrible outbreak in Golden Square, not ten blocks from his new offices on Sackville Street, he was ready. So many casualties in such a short stretch of time suggested a central contaminated water source used by large numbers of people. He needed to get samples of the water while the epidemic was still at full force. And so he made the journey across Soho, into the belly of the beast.

Snow’s expectation was that contaminated water would have a cloudiness to it that was visible to the naked eye. But his initial glance at the Broad Street water surprised him; it was almost entirely clear. He drew samples from the other pumps in the area: Warwick Street, Vigo Street, Brandle Lane, and Little Marlborough Street. All were murkier than the Broad Street water. The Little Marlborough Street sample was worst of all. As he drew the water there, a handful of local residents on the street remarked that the pump water was notoriously poor—so poor, in fact, that many of them had taken to walking the extra blocks to Broad Street for their drinking water.

As Snow hurried back to his home on Sackville Street, he turned over the clues in his mind. Perhaps the Broad Street pump was not the culprit after all, given the lack of particles in the water. Perhaps one of the other pumps was the culprit? Or perhaps some other force was at work here? He would have a long night ahead of him, analyzing the samples, taking notes. He knew an outbreak of this magnitude could supply the linchpin for his argument. It was just a matter of finding the right evidence, and figuring out how to present that evidence in a way that would persuade the skeptics. Snow may well have been the only soul in Soho that day who found in the outbreak a glimmer of hope.

Snow didn’t realize it at the time, but as he walked home that Sunday night, the basic pattern of that experimentum crucis suggested five years before in the London Medical Gazette was finally taking shape, miles away from Broad Street, in the greenery of Hampstead. Susannah Eley had fallen ill earlier in the week after drinking her regular supply of Broad Street water, dutifully shipped to her by her children in Soho. By Saturday she was dead, followed on Sunday by her niece, who had returned to her home in Islington after visiting with her aunt. As Snow reviewed the pump-water samples in his microscope, Susannah Eley’s servant, who had also consumed a glass of Broad Street water, remained locked in a life-or-death struggle with the disease.

Not one other case of cholera in Hampstead would be recorded for weeks.

IT’S ENTIRELY LIKELY THAT HENRY WHITEHEAD PASSED JOHN Snow on the streets of Soho that early evening. The young curate had toiled through another exhausting day, and was still making rounds well after the sun had set. Whitehead had begun the day with a feeling of hope; the fact that the streets seemed less chaotic made him wonder whether the outbreak was abating. A few of his initial visits offered reason for hope as well: the Waterstone girl had improved, and her father, having lost an otherwise perfectly healthy wife and daughter in less than two days’ time, had begun consoling himself with the thought that life indeed might be worth living if his one remaining daughter somehow survived. Whitehead shared his upbeat assessment with a few of his colleagues on the sidewalk, and found some agreement.

But the quiet proved misleading: the streets were more tranquil because so much suffering was going on behind the shutters. In the end, another fifty would die over the course of the day. And new cases continued to appear at an alarming clip. When Whitehead returned to the Waterstones’ at the end of the day, he found the sister continuing her improvement. But in the room next door, the girl’s father was in the throes of cholera’s initial attack. Life might well be worth living if his daughter survived, but the decision might not be up to him after all.

When Whitehead finally returned to his quarters at the end of that punishing day, he poured a glass of brandy and water, and found himself thinking of the Waterstones’ ground-floor quarters. He had encountered the gossip that had been circulating in the past day, folk wisdom that would eventually find its way into the papers in the coming weeks: the residents of upper floors were dying at a more dramatic rate than those living on ground or parlor floors. There was a socioeconomic edge to this contention, one that reverses the traditional upstairs/downstairs division of labor: in Soho at the time, the bottom floors were more likely to be occupied by owners, with the upper floors rented out to the working poor. An increased death rate in the upper floors would suggest a fatal vulnerability in the constitution or sanitary habits of the poor. The notion, in its crude and haphazard way, was a version of Snow’s tale of two buildings in Horsleydown: put two groups of people in close proximity, and if one group turns out to be significantly more vulnerable than the other, then some additional variable must be at work. For Snow, of course, the variable was water supply. But for the upstairs/downstairs rumor mill, the difference was class. A better sort of people lived on the ground floors—no wonder they were more likely to fight off the disease.

But as Whitehead reviewed his experiences over the preceding days, those easy assumptions began to wither in his mind. Yes, it did seem as though more people were dying on upper floors, but far more people lived on upper floors. And the Waterstones were clear evidence that the disease could assault ground-floor dwellers with impunity. Whitehead didn’t have numbers in front of him, but he had a hunch from his anecdotal experiences that the lower floors had in fact been deadlier per capita over the previous forty-eight hours. It was certainly a fact worth investigating—if the pestilence ever moved on from Golden Square long enough to investigate anything.

Fifteen blocks away, on Sackville Street, John Snow was contemplating statistics as well. He had already sketched out a plan to ask William Farr for an early look at the mortality numbers. Perhaps there would be something in the distribution of deaths that would point to a contaminated water supply. Like Whitehead, Snow recognized that his work among the suffering of Golden Square had only begun. Whatever numbers William Farr provided him would have to be supplemented with local investigation. The longer he waited, the more difficult that investigation would become, if only because so many of the witnesses were dying.

Snow and Whitehead shared one other common experience that night. They both spent those last ruminating hours in the company of water drawn from the Broad Street pump. Snow was analyzing it in his home laboratory, his vision dimmed by the low light of candles. The young curate, however, had used the water in a different way, more recreational than empirical: he had mixed the water with a thimble of brandy and swallowed it.



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