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this gelatinous silica occurs. This position is very different from that of the thermogenous quartz of the islands of Iceland and Ischia.-Annales des Mines 1826.

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Experiments to compare the specific Heat of Air, under a constant volume, with its specific Heat under a constant pressure. By Mr HENRY MEIKLE. (Communicated by the Author.)

IT has been long known, that gaseous bodies emit heat when

compressed, and absorb it when dilated, a property, by the by, which is not easily reconcileable with the creed of those who suppose heat to be mere motion. Little, however, was ascertained, for a considerable time, regarding the amount of the change of temperature accompanying a given change of density. The earliest experiments to determine this question seem to have been those of Professor Leslie. Mr Dalton and M. Gay Lussae have also engaged in the same inquiry *. As the heat evolved or absorbed by a change of density, depends on the difference between the specific heat under a constant pressure, and that under a constant volume, if we could find the ratio of these quantities, we should be enabled to determine their relation to the heat evolved or absorbed, and from this the change of temperature, and conversely. From certain experiments of MM. Delaroche and Berard, the Marquis de Laplace instituted some calculations †, which happened to come nearer the point than could have been expected; for these experiments were not at all suited to the purpose; and it is the more remarkable, that they should

According to the experiments of this last author, tinder or amadou is inflamed by the sudden compression of air into one-fifth of its bulk. 'Some have even questioned the fact, and others conjecture, that combustion commences at lower temperatures, as the air is denser. But may we not suppose, with more probability, that the pressure on the tinder, being suddenly augmented in an almost nine-fold ratio, should elicit much heat from this compressible substance itself? So that, till something else be known on the subject, we need neither doubt the fact, nor believe that a fivefold compression of air would of itself generate an inflammatory temperature. The melting of fine wires, or thin metallic leaves, would afford a surer test of the temperature in compressed air, than the kindling of soft spongy bodies.

✦ Annales de Chimie et de Phys. iii. 238.

have been used as the basis of such calculations, considering that, at as early a period (1812), MM. Desormes and Clement, had, with a very different view, made some better-contrived experiments, from which an approach to the true quantity could have been made with more certainty. Their method was very simple, and required no thermometer to shew the variations of temperature, a contrivance which is said to have been first sug"gested by Lambert. No notice, however, seems to have been taken of these latter experiments,-probably because they were Rassociated with a most fanciful inquiry after the absolute zero, till MM. Gay Lussac and Welter undertook a similar and more extensive series of experiments, giving nearly the same results. Of both of these and the inconsistent conclusions deduced from them by MM. Laplace and Poisson, I have had occasion to speak in the first volume of this Journal, where I have shewn that, whatever be the ratio of the specific heat of air under a constant pressure, to its specific heat under a constant volume; if that ratio only be constant, the variations of the quantity of heat in a mass of air must be uniform, while those of its volume,. under a constant pressure, form a geometrical progression; and it is remarkable, that our first-rate authorities on the subject, who admit the constancy of this ratio, did not see that it was directly at variance with the commonly received theory of the airthermometer.

But, although the value of the ratio referred to have nothing hito do with the true law of temperature, yet its exact determinaLotion would be of great moment in various researches. Consibliderable deference is due to the experiments of the illustrious philosophers above mentioned. They were well calculated for shewing that the ratio of the specific heats is constant; because, supposing any inaccuracy to attach to them, it would be common to all the cases. But I had always some doubt whether their apparatus was the most eligible for determining the value of that ratio. The apparatus mostly employed principally of a glass balloon, to the neck of which wa brass cap and stop-cock. From the side of the cap, n horizontal pipe, communicating with a vertical gla minating in some light liquid to act, the part of gauge or measure of the variations of pressure.

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rizontal tube could be connected with a pump or condenser, for the purpose of rarifying or condensing the included air at plea

sure.

Things being thus prepared, a 'slight change was effected in the density of the included air; and, after waiting a little till the former temperature was regained, the stop-cock was opened, and great care taken just to have it shut again by the very nick of time that the liquid within the gauge-tube had acquired the level of the outside, it being supposed that this was a proof that, at that instant, the included air had exactly regained the atmospheric pressure. A small interval being again allowed to restore the former temperature, the column of liquid in the gauge now shewed the change of pressure due to the last variation of temperature.

In this mode of operating, there is some ground for suspecting two sources of error, but which fortunately would be op posed to each other. In the first place, the air would take a sensible time to pass through a moderately sized stop-cock; and, during that interval, a considerable portion of the change of heat due to the change of density, would be lost on the sides of the vessel; especially considering how quickly heat might be communicated between air in its then agitated state, and a vitreous surface. On the other hand, the liquid in the gauge-tube might have acquired a force from its motion capable of carrying it to the common level of the cistern, before the spring of the air within had come into equilibrio with the atmosphere. If so, it is evident that, in the above arrangement, the stop-cock has been shut before that equilibrium was attained; and which shutting would, therefore, have been too soon, were it not that it happens nearly to be balanced by the other source of fallacy. To illustrate the second case, let one end of a glass-tube be stopped with the finger, and then let the other be immersed vertically in a jar of water. On removing the finger, the water, which had been depressed by the included air, will start considerably above the common level; so that, were the finger only partially removed, and suddenly re-applied to shut the tube again, at such a nick of time that the water within did not spring higher than the common level, it is clear that the force of the included airmust still have exceeded the atmospheric pressure; and that it

was this excess which prevented the liquor from rising to the same height as before.

With the view of making similar experiments, which should be in a great measure free from such objections, I had an apparatus fitted up on purpose. It consists of a large flask, made of strong tinned iron, and capable of containing 2300 cubic inches of air. The neck is of brass, about two inches wide; and into this was fitted by grinding a brass stopper, hollow and open inward. At equal distances from each other, four apertures were cut through the sides of the neck and of the stopper. Each is 1.2 inch long and 0.6 broad; so that these together can form a communication between the atmosphere and included air, equal to 2.88 square inches, or the opening of fourteen half inch stopcocks, and which communication can be both opened and shut by simply turning the stopper one-fourth round, an operation which requires but a very small moment of time.

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Near the neck, a tube branches out, and joins a vertical glass tube, which, terminating in some light liquid, forms the same sort of gauge as in the apparatus first alluded to; and, on the opposite side, is an aperture for attaching a pump or condenser to change the density of the included air. The air-vessel is inclosed-in another, both for the purpose of keeping the temperature steady, and also for applying a bath to maintain any temperature required. But, during tempestuous weather, or when the barometer is very unsteady, no experiments can be made with such apparatus.

As a preliminary step in the use of this instrument, it is necessary to ascertain at what rate we should turn the stopper, in order that the included air, when its pressure has been previously changed from that of the atmosphere by about 0.4 inch of mercury, may have a sufficient opportunity of regaining the external pressure. To determine this, the following method was employed: Having injected air till the increase of pressure, when the temperature had settled, was indicated by a depressed column of water of about six inches, I turned the stopper onefourth round, by which it was both opened and shut. During this operation, I noted how far the previously depressed water in the gauge tube started above the common level. The same operation was repeated, with the difference of only turning the

stopper one-eighth round, so as to leave its apertures completely open; and, on observing the gauge, it just sprung to the same height as before. Repeated trials satisfied me, that, with such small variations of density, it would require considerable haste to turn the stopper too quickly. In both of the cases just mentioned, the range through which the stopper turned was limited by a catch. But in the experiments to be afterwards noticed, I generally used a lighter fluid than water.

It is evident, that, instead of injecting air, as I usually did, to increase the pressure above that of the atmosphere, it would come to the same thing, if we first close the large vessel at a temperature a few degrees below that at which we wish to oper ate, and then raise it to the temperature which is to remain constant during the experiment. This consideration affords, perhaps, the simplest means of explaining the rationale, or use of this sort of experiments. For, let the pressure of the air when just shut in, be in equilibrio with the atmosphere, but suppose that the temperature of the apparatus is next raised, so as to in crease the pressure and depress the liquor in the gauge b inches, which we may call 6 degrees; then, if, whilst this augmented temperature of the apparatus remains constant, the stopper be turned one-fourth round, as above described, the equilibrium with the atmosphere will be for a moment restored, the communication with it again cut off, and the included air cool ed by the dilatation, but it will soon absorb heat, and recover the former temperature, as will be indicated by a second depres sion of c inches or degrees. This is obviously the change of temperature due to the excess of the quantity of heat, which would raise the temperature 6 degrees, under a constant pres sure, above what raises it b degrees under a constant volume.

From this it would follow, that the quantity of heat which raises the temperature b degrees under a constant volume, would only raise it b―c degrees under a constant pressure; or, that the specific heat in the first case is to that in the second as bc to b.

Strictly speaking, neither the volume during the first increase of temperature nor that during the second is constant, because the depression of the liquor in the gauge tube makes a little more room for the air. This, to be sure, could be obviated by

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