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THIS MEMOIR IS the result of experiments upon heat which M. de Laplace and I made together during the last winter; the mildness of the season did not permit us to perform a greater number ... . We have devised the following apparatus [for making measurements of the heat which is developed by combustion, the respiration of animals, combinations of oil of vitriol with water, and the like], all of which have been impossible by the means hitherto known.
The plate represents a vertical section showing the interior of the device. Its volume is divided into three chambers: we shall distinguish them by the terms inner chamber, middle chamber, and outer chamber.
The inner chamber f is constructed of a meshwork of iron wire reinforced by strips of the same metal; the experimental object is placed in this chamber; the top of the chamber is fitted with a cover which is entirely open above; its bottom consists of an iron wire netting.
The middle chamber is intended to contain ice which entirely surrounds the inner chamber and which is melted by the heat of the experimental object; this ice is supported and retained by a grill under which is a sieve. In proportion as the ice is melted by the heat of the object in the innermost vessel, the water runs down through the grill and the sieve; it then runs down the cone and the tube and is collected in the vessel placed under the apparatus; the stopcock permits one to stop the outflow of the water at will. Finally, the outer chamber is designed to be filled with ice, the purpose of which is to prevent the entrance of heat from the external air or surrounding objects; the water produced by the melting of this ice runs down pipe s-t, which can be opened or closed by means of stopcock r. The entire apparatus is covered by a lid entirely open above and closed beneath; it is constructed of tin painted with oil to prevent rust.
In performing an experiment, one fills the middle chamber and the inner cover with crushed ice, as well as the outer chamber and the outside lid. One must be careful to crush the ice fine and to pack it down well into the apparatus. The inside ice (as we shall call the ice enclosed in the middle chamber and inner lid) is allowed to drain; when it has drained sufficiently, the device is opened, the desired object is placed inside, and the lid is replaced immediately. One waits until the object is completely cooled and the apparatus sufficiently drained; then one weighs the water which has collected in the vessel. Its weight exactly measures the heat emitted by the object; for clearly all its heat has been absorbed by the inner ice which has been protected from the effects of any other heat by the ice contained in the lid and in the outer container.
It is essential that there be no communication between the middle chamber and the outer chamber, which can be easily tested by filling the outer chamber with water. If there were communication between these two chambers, the ice melted by the atmosphere, the heat of which affects the wall of the outer chamber, might pass into the middle chamber, and then the water flowing out from the latter would no longer be a measure of the heat lost by the experimental object.
When the atmospheric temperature is above zero, heat enters the middle cavity only with difficulty because it is stopped by the ice in the lid and the outer chamber; but if the external temperature is below zero, the atmosphere may cool the inner ice; it is therefore essential to work at temperatures above zero; in cold weather the apparatus must be kept in a heated room; furthermore, the ice used must not be colder than zero degrees; if it is, it must be crushed and spread out in thin layers for a time in a place where the temperature is above zero.
The inner ice always retains a small amount of water which adheres to its surface, and one might think that this water would affect the experimental results; but it must be pointed out that at the beginning of each experiment the ice is already saturated with all the water it can thus retain, so that though a small part of the melted ice remains adhering to the inner mass of ice, a very nearly equal quantity of water, originally adhering to the surface of the ice, must run down into the collecting vessel, since the surface of the inner ice changes very little during the course of the experiment ... .
We have had two machines constructed as described; one of them is intended for experiments in which it is not necessary to change the air within the chamber; the other apparatus is designed for experiments in which the air must be renewed, such as those involving combustion and respiration; the latter apparatus differs from the former only in that the two covers are pierced by two holes through which pass two thin pipes which serve for the passage of air between the outside and the inside; by this means it is possible to blow atmospheric air upon combustible objects ... .
Experiments on Heat, Carried out by This Method
We took a small earthen vessel which had been dried; after having placed it on a balance and tared it very exactly, we placed glowing coals in it, blowing upon them to keep them at red heat; at the instant when their weight was one ounce [30.59 grams] we transferred them quickly to one of our machines; their combustion, in the interior of the apparatus, was maintained by means of a bellows; they were consumed in 32 minutes. At the beginning of the experiment the outside thermometer stood at 1.5° and it rose to 2.5° during the experiment; the apparatus when well-drained yielded 6 pounds, 2 ounces [2,998 grams] of melted ice; this was produced by the combustion of one ounce of carbon.
The outside thermometer being at 1.5°, we placed a guinea pig into one of our machines; its internal body temperature was about 32° [40° C.], i.e., not very different from that of the human body. To prevent its suffering during the experiment, we placed the animal in a little basket lined with cotton, the temperature of which was zero; the animal remained for 5 hours and 36 minutes in the apparatus; during this period we gave it four or five changes of air by means of a bellows. After removing the animal, we left the basket in the apparatus and waited until it had cooled off; the well-drained machine yielded about 7 ounces [214 grams] of melted ice. In a second experiment, the outside thermometer was still at 1.5°; the same guinea pig remained in the apparatus for 10 hours and 36 minutes, the air being renewed only three times; the machine yielded 14 ounces, 5 gros [447.38 grams]. The animal did not appear to suffer at all during these experiments.
According to the first experiment, the amount of ice which the animal could melt in 10 hours would be 12 ounces, 4 gros [382.38 grams]; according to the second experiment this quantity, for the same interval, would be 13 ounces, 6 gros, 27 grains [422.05 grams]; the average of these two results is 13 ounces, 1 gros, 13.5 grains [402.21 grams].
Combustion and Respiration
Until recently, only vague and imperfect ideas were current regarding the phenomena of the heat liberated in combustion and respiration. Experience had shown that bodies could not burn, nor could animals respire, in the absence of atmospheric air; but nothing was known of the manner in which it influences these two important natural processes and the resulting changes which the air undergoes. The most widespread opinion attributed to the air only the functions of cooling the blood as it passed through the lungs, and of holding the fire against a combustible object by its pressure. The important discoveries which have been made during the last few years on the nature of aerial fluids have greatly extended our knowledge of this subject; it is established that a single kind of air, known as dephlogisticated air, pure air, or vital air, is concerned in combustion, respiration, and the calcination of metals; this type of air comprises only about a quarter of the atmospheric air, and it is either absorbed, or altered, or converted into fixed air by the addition of a principle which we shall name the base of fixed air, in order to avoid any discussion as to its nature; thus, the air does not act simply as a mechanical force, but as an agency of new combinations. M. Lavoisier, having observed these phenomena, suspected that the heat and light liberated in combustion are due, at least to a great extent, to changes which the pure air undergoes. The facts pertaining to combustion and respiration are explained in such a natural and simple manner on this hypothesis, that he did not hesitate to propose it, if not as a demonstrated truth, at least as a very reasonable conjecture, worthy of the attention of natural philosophers... .
We have confined ourselves here to a comparison of the quantities of heat which are liberated in combustion and in respiration with the corresponding alterations of the pure air, without going into the question whether this heat comes from the air, or from the combustible substances and respiring animals. With the object of studying these alterations, we have performed the following experiments:
Upon a large trough filled with mercury we set a bell-jar full of dephlogisticated air [oxygen]; this air was not perfectly pure; it contained 16 parts of pure air [oxygen] per 19 parts and it included about 1/57th of its volume of fixed air [carbon dioxide].
[The bell jar image above is from a similar experiment, and is helpful in understanding what follows. Note however, that its letter markings do not correspond to the description below—Ed.]
We introduced under the bell-jar a small earthen jar, filled with coal which had previously been freed of its inflammable air by strong heat; upon the coal we placed a little tinder upon which was a small fragment of phosphorus, weighing at most a tenth of a grain [5 milligrams]. The earthen jar and all its contents had been weighed very exactly; we then raised the mercury within the bell-jar up to a marked level (E) by suction applied to the interior, in order that the expansion of the air produced by the burning of the carbon would not lower the level of the mercury too much below that of the outside mercury, which would have permitted the escape of air from within the bell-jar. Next, by means of a red-hot iron, passed very quickly through the mercury, we ignited the phosphorus, which set fire to the tinder and thus to the coal. Combustion lasted for 20 or 25 minutes, and when the ember was extinguished, and the inside air had cooled to room temperature, we marked a second line at the level (E’) where the mercury had risen by diminution of the volume of the enclosed air. We then introduced some caustic alkali under the bell-jar; all the fixed air was absorbed, and having allowed sufficient time for this to occur, when the mercury had ceased to rise in the bell-jar, we marked a third line (E”) at the level of the surface of the caustic alkali; we took care to observe, at the three positions E, E’, and E”, the heights of the mercury in the bell-jar above its level in the trough. Atmospheric air introduced into the bell-jar by means of a glass tube had the effect of lowering the mercury level to that of the outside. We then removed the earthen vessel, which we dried and weighed very exactly; the loss of weight gave us the quantity of carbon consumed. The external temperature varied very little during the course of the experiment, and the barometric pressure was about 28 inches.
In order to determine the volumes of air contained [at levels E, E’, and E”], we filled them with plain water, the respective weights of which gave the volumes of these spaces in cubic inches. But, since the enclosed air had been unequally compressed as a result of the different heights ofthe mercury in the bell-jar, we reduced the volumes, by computation from the observed heights of the mercury, to that which the air would have occupied if it had been compressed by a 28-inch column of mercury. Finally, we reduced all our experimental results to the values which would have been obtained had the external temperature been 10°, utilizing the fact that, at a temperature of 10°, air expands 1/215th for each degree of temperature increase; therefore, the volumes of air which we shall report must be taken as the values for a temperature of 10° and a pressure of 28 inches of mercury.
In the preceding experiment, the bell-jar had contained 202.35 inches [4,026.8 cm3] of dephlogisticated air; its volume, by the sole combustion of carbon, was reduced to 170.59 inches [3,394.7 cm3]. After absorption of the fixed air by the caustic alkali, the volume of the remaining air was only 73·93 inches [1471.2 cm3]; the weight of the carbon consumed, apart from its ash, was 17.2 grains 1 [0.912 grams]; the weights of the tinder and the phosphorus together might have been half a grain [26 milligrams]; moreover, we have found, through many experiments, that the weight of ash formed by the coal is approximately 10 grains per ounce [17.4%]; it might therefore be estimated very nearly that 18 grains [0.954 grams] of carbon were consumed in the experiment, taking into account its ash.
The dephlogisticated air which we used contained about 1/57th of its volume of fixed air which had not been absorbed by the water over which it had been stored for several months; this intimate adhesion of fixed air to the pure air has led us to believe that, even after the fixed air was absorbed by caustic alkali in our experiments, the remaining air still contained a little fixed air, which we may without appreciable error estimate at 1/57th of its total volume. According to this hypothesis, in order to obtain the volume of all the pure air consumed by the carbon, one must take the difference between the volume of the air before combustion and the volume of the air remaining after absorption with caustic alkali, and then subtract 1/57th. Making a similar correction for the volume of air absorbed by the alkali, one may obtain the volume of fixed air formed in combustion; it will thus be found that one ounce of carbon, in burning, consumes 4037.5 inches of pure air and forms 3021.1 inches of fixed air. If one designates the volume of pure air consumed as unity, its volume, after combustion, would be reduced by 0.74828.
In order to estimate the weight of these volumes of pure air and fixed air, the weight of a cubic inch of each of these airs must be known; now, it has been observed that pure air is a little heavier than atmospheric air, approximately by a ratio of 187 to 185. The weight of atmospheric air has been determined very exactly by M. de Luc. Utilizing these determinations, it is found that at a temperature of 10° and a barometric pressure of 28 inches, a cubic inch of dephlogisticated air weighs 0.47317 grains. M. Lavoisier has observed that at the same temperature and pressure, a cubic inch of fixed air weighs very nearly 0.7 of a grain. According to these results, an ounce of carbon, in burning, consumes 3.3167 ounces of pure air and forms 3.6715 ounces of fixed air. Thus, in ten parts by volume of fixed air, there are about nine parts of pure air and one part of a principle supplied by the carbon, which is the base of fixed air; but a determination of such delicacy requires a greater number of experiments.
We have previously seen that an ounce of carbon, in burning, melts 6 pounds, 2 ounces of ice, from which it may be readily concluded that, in the combustion of carbon, the alteration of an ounce of pure air is capable of melting 29.547 ounces of ice, and that the production of one ounce of fixed air is capable of melting 26.692 ounces.
It is with the greatest circumspection that we present these results on the quantities of heat liberated by the alteration of an ounce of pure air by the combustion of carbon. We have performed only one experiment on the heat liberated by this combustion, and although it was carried out under quite favorable conditions, nevertheless we shall not be quite confident of its exactness until we have repeated it a number of times. As we have said before, and we cannot stress this too much, it is not so much the result of our experiments, as the method we have employed that we present to the natural philosophers, inviting them, if the method appears to offer advantages, to confirm these experiments, which we intend to repeat ourselves with the greatest care... .
In order to determine the alterations which the respiration of animals brings about in pure air, we filled the bell-jar of the apparatus previously described with this gas, and we introduced into it various guinea-pigs of nearly the same weight as the one used in our experiment on animal heat. In one of these experiments, the bell-jar contained 248.01 inches of pure air before the guinea pig was put in; the animal was kept there for an hour and a quarter. In order to introduce it into the bell-jar, we passed it through the mercury; it was removed in the same manner. After the inside air had been allowed to cool to room temperature, its volume was slightly diminished to 240.25 inches; finally after the fixed air had been absorbed by caustic alkali, 200.56 inches of air remained. In this experiment, 46.62 inches of pure air had been altered, and 37.96 inches of fixed air produced, correcting for the small amount of fixed air contained by the dephlogisticated air in the bell-jar. If the volume of altered pure air be designated as unity, the reduction in volume due to respiration would be 0.814; in the combustion of carbon the volume of air was diminished by a ratio of 1 to 0.74828; this difference may be ascribed in part to errors of measurement, but it also results from a cause which we had not at first suspected and which those who wish to repeat these experiments might well be warned against.
In order to keep the bell-jar stable in the trough, we raised the level of the mercury inside slightly above the outside level; now, in introducing the animal and in removing it from the bell-jar, we observed that a small amount of air was carried in along the body of the animal, although it was partly immersed in the mercury; the mercury does not adhere closely enough to the hair and skin to prevent all communication between the outside air and the air under the bell-jar; thus the air appears to be less reduced by respiration than is actually the case.
The weight of the fixed air produced in the previous experiment is 26.572 grains; from which it follows that in an interval of ten hours the animal would have produced 212.576 grains of fixed air.
At the beginning of the experiment, the animal, breathing an air much purer than atmospheric air, might in a given time produce a larger quantity of fixed air; but at the end it breathes with difficulty, because the fixed air, accumulating by its weight at the bottom of the bell-jar where the animal is located, displaces the pure air which rises to the top of the bell-jar, and probably also because the fixed air is itself noxious to animals. It may therefore be assumed, with no appreciable error, that the amount of fixed air produced is the same as if the animal had been breathing atmospheric air, the quality of which is about the average between that of the air in the bottom of the bell-jar at the beginning and at the end of the experiment.
We then determined directly the amount of fixed air produced by a guinea-pig breathing air the same as the atmosphere. For this purpose, we placed one in a jar through which we had set up a current of atmospheric air; the air, compressed in a suitable apparatus, entered the vessel through a glass tube, and emerged through a second curved tube, the concave part of which was immersed in mercury, and the lower end of which terminated in a second flask filled with caustic alkali. The air was then led by a third tube into a second flask full of caustic alkali, and thence out into the atmosphere. The fixed air formed by the animal in the jar was in large part retained by the caustic alkali of the first flask; whatever escaped was absorbed by the alkali of the second flask; the increase in weight of the flasks gave us the weight of the fixed air there combined. During a three-hour interval, the weight of the first flask increased 63 grains; that of the second increased 8 grains; thus the total weight of the two flasks increased by 71 grains. Assuming that this quantity of fixed air is due solely to the respiration of the animal, it would, in ten hours, have formed 236.667 grains of fixed air, which differs by about one-ninth from the results obtained in the previous experiment. This difference could be attributed to the difference in size and strength of the two animals and to their momentary state during the experiment.
If the vapors [expired water] of respiration, carried by the air current, had been deposited in the flasks, the increase in weight of the caustic alkali would not have given the amount of fixed air produced by the animal; it was to avoid this inconvenience that we used a curved tube with its concave part immersed in mercury; the vapors of respiration condensed against the walls of this part of the tube and collected in its concavity, with the result that the air entering the first flask contained no appreciable amount of moisture, as shown by the fact that the part of the tube entering the flask remained transparent; it may therefore be assumed that though the weight of the flasks might have been augmented by these vapors, this increase would have been compensated for by the evaporation of water from the alkali. It might still be feared that a part of the fixed air combined came from the atmospheric air itself. To reassure ourselves in this regard we repeated the same experiment, but without placing a guinea-pig in the vessel; no increase then occurred in the weight of the flasks; that of the second flask diminished by 4 or 5 grains, no doubt owing to evaporation of water from its alkali.
A third experiment performed on a guinea-pig in dephlogisticated air gave 226 grains as the amount of fixed air produced in ten hours.
Taking an average of these experiments and several similar ones performed with· a number of guinea-pigs, both in dephlogisticated air and in atmospheric air, we have obtained an estimate of 224 grains as the amount of fixed air produced in ten hours by the guinea-pig on which we had experimented in our apparatus to determine its animal heat.
Inasmuch as these experiments were carried out at a temperature of 14°–15°, it is possible that the amount of fixed air produced by respiration is a little less than at a temperature of zero degrees, which is that of the interior of our apparatus; for greater precision it would therefore be necessary to determine the production of fixed air at the latter temperature; we shall take up this question in further experiments which we intend to carry out.
The foregoing experiments are contrary to those which MM. Scheele and Priestley have reported on the alterations of pure air by the respiration of animals. Respiration, according to these two excellent naturalists, produces very little fixed air and a large amount of vitiated air, which the latter has designated as phlogisticated, but upon investigating the effect of respiration of birds and guinea-pigs on pure air with the greatest possible care, by a great number of experiments, we have constantly observed that the transformation of this air into fixed air is the main alteration produced by the respiration of animals. By having guinea-pigs breathe a large amount of pure air and observing by means of caustic alkali the amount of fixed air produced by their respiration, and by subsequently causing the residual air to be breathed by birds and absorbing the newly-formed fixed air once more by caustic alkali, we have been able to convert into fixed air a large part of the pure air which we have been using; the remaining air had nearly the same quality which it would have had assuming that the transformation of pure air into fixed air is the only effect of respiration upon the air. It therefore seems certain to us that if respiration produces other alterations in pure air, they are inconsiderable, and we have no doubt that any naturalist performing the same experiments with a large mercury apparatus will be led to the same conclusion. It was previously seen that in the combustion of carbon, the formation of an ounce of fixed air can melt 26.692 ounces of ice; on the basis of this result, it is found that the formation of 224 grains of fixed air must melt 10.38 ounces. This amount of melted ice consequently represents the heat produced by the respiration of a guinea-pig during ten hours.
In the experiment on animal heat of a guinea-pig, this animal emerged from our apparatus with nearly the same heat with which it entered, for it is known that the internal heat of animals is always nearly constant. Without the constant renewal of its heat, all the heat which it had at first would have been gradually dissipated, and we should have found it cold upon taking it out of the apparatus like all the inanimate objects which we have used in our experiments. But the animal’s vital functions continually restore to it the heat which it gives off to its environment, and which in our experiment is diffused into the inner ice, of which it melted 13 ounces in ten hours. This amount of melted ice thus represents approximately the amount of heat renewed during this time interval by the vital functions of the guinea-pig. Perhaps an ounce or two should be subtracted, or maybe more, on account of the fact that the extremities of the body of the animal were chilled in the apparatus, although the interior of the body retained nearly the same temperature; furthermore, the moisture which its internal heat had evaporated melted a small amount of ice as it cooled, adding to the water draining out of the apparatus.
On subtracting about 2.5 ounces from this quantity of ice, one obtains the amount melted by the effect of the respiration of the animal upon the air. Now, if one considers the inevitable errors in these experiments and in the factors which were the starting point for our calculations, it will be seen that it is not possible to hope for a more perfect agreement between these results. Thus the heat which is liberated in the transformation of pure to fixed air by respiration may be regarded as the principal cause of the conservation of animal heat and if other causes are involved, they are of lesser significance.
Respiration is therefore a combustion, very slow to be sure, but perfectly similar to that of carbon. It occurs in the interior of the lungs, without the liberation of any perceptible light because the fire, as fast as it is freed, is absorbed by the humidity of these organs. The heat developed by this combustion is transferred to the blood which passes through the lungs, and thence is transmitted throughout the animal system. Thus the air which we breathe serves two purposes equally necessary for our preservation: it removes from the blood the base of fixed air, an excess of which would be most injurious; and the heat which this combination releases in the lungs replaces the constant loss of heat into the atmosphere and surrounding bodies to which we are subject.
Translated by M. L. Gabriel
Reading and Discussion Questions
1.What instruments and methods do Lavoisier and Laplace develop or employ for the purposes of this experiment? How does this setup and approach compare to what you might encounter in a present day chemistry lab?
2.Describe the series of experiments that Lavoisier and Laplace report in this reading. What are they trying to accomplish in each case?
3.Despite the fact that the observed amount of ice melted differs from the predicted value, Lavoisier and Laplace attribute this discrepancy to various sources of experimental error and interpret the result as evidence for their hypothesis. Does this seem plausible, or should they have taken the results to falsify their theory? What insights might this episode give us into the nature of scientific reasoning and methodology?