Last autumn (of 1812) I resumed this subject, and my attention was chiefly directed to the habitudes of these impurities with the chemical re-agents. This I found at tended with considerable difficulties, none of the least of which was to procure a sufficient quantity of these impu rities in a separate state. The series of experiments I proposed to myself on this subject have not yet been com pleted, but I may remark, that the result of some of those I have made promises to afford practical hints of consi derable importance to those brewers whose products are intended to afford sphituous liquors.
From this notice it will be observed, that I have scarcely yet entered on the wide field of inquiry, for the cultivation of which, the method alluded to appears to offer so powerful an instrument. Alcohol only has been subjected to experiment ; it was the only liquid which had resisted all attempts to reduce it to the solid state by the abstraction of caloric. If these experiments be cor rect, we may now pronounce it a general law, to which there is no exception, that all liquids with which we are acquainted may be reduced to the solid state, by a suit able abstraction of caloric. Whether all gases may be susceptible of reduction to the solid state, by the abstrac tion of caloric, remains to be ascertained ; although, as I have mentioned, analogy renders it in the highest degree probable." Before finishing the consideration of artificial cold, we must offer a few remarks on a fact, the explanation of which seems to have given a great deal of difficulty to chemists,—we mean the apparent radiation of cold. The discussion of this subject is important, in a scientific point of view, from its connection with the question con cerning the existence of a frigorific principle, as an anta gonist to caloric. We do not pretend that we shall set this question at rest ; but we are satisfied we shall be able to prove, that the existence of cold, as a separate and in dependent principle or condition of matter, receives no additional support from the fact of its apparent radiation.
The effect of surface in modifying the heating and cooling of bodies by radiation, has been long known ; though we are indebted to Mr Leslie for the first scien tific investigation of the subject. Some experiments on radiation had, indeed, been made by Lambert and Scheele, but they were extremely obvious, and have little preten sions to precision.
If a plate of polished metal be exposed to the heat or a fire, almost all the rays of caloric are reflected from its surface, and it requires but little increase of temperature. But if a pane of glass be placed in similar circumstances, the greater part of the rays are absorbed, and the pane soon becomes very warm. The same effect is produced, if the surface of the metallic plate be covered with a coat ing of lamp-black. From those experiments, and a va riety of others which will be detailed under HEAT, it appears that the absorbing power of bodies for caloric is considerably affected by the nature of their surfaces. It is somewhat curious, that when their temperature is su perior to that of the surrounding medium, or adjacent bodies, their tendency to radiate, or throw off caloric. corresponds with their disposition in the former case to absorb it ; so that the absorbing and radiating power, or in common language, the tendency to heating and cool ing in opposite circumstances, arc equally affected by surface, and in a degree that has some relation to the dis tance and difference of temperature of the radiating and absorbing bodies ; the radiation and absorption being car ried on between them, until an equilibrium of tempera ture is established. It is difficult to determine, whether
a mutual interchange of temperature takes place between the warmer and the colder bodies, each radiating in a degree corresponding to its quantity of caloric, or whe ther the radiation proceeds from the warmer body alone, and the colder body is merely a recipient of its emana tions ; only, if we say that one body is hot, and the other cold, we must affirm, in conformity with the ordi nary acceptation of the terms, that, in the latter suppo sition, there is a radiation of cold as well as of heat. Thus, let the temperature of the hotter body be 60° of Fahrenheit, and that of the colder 30°: if it be supposed, that a mutual communication of temperature by radia tion takes place between them, we would admit without hesitation that the colder body radiates cold ; or, if it be supposed that the temperature of the one is 30°, and that of the other and that the radiation proceeds solely from the body at the more elevated temperature, we might still say that the emanations were cold. But if the temperature of the hotter body were 120°, and that of the colder 60°, we would scarcely be disposed to admit that the latter radiated cold, even on the supposition of a reciprocal exchange of temperature. In those instances, however, it is obvious that we assume our own feelings as the standard of reference ; and though, in the discus sion of the radiation of cold, this circumstance has been sometimes brought into view, we suspect it has been too little regarded ; and that it has contributed, either di rectly or indirectly, to produce all the obscurity in which the radiation of cold is at present involved. If we reject entirely the decision of our feelings, as vague and falla cious, and have recourse to the less objectionable means of describing the degrees of temperature by the expan sion and contraction of bodies, we shall he compelled to acknowledge, that as we are still ignorant of the true zero, or the point of total privation of temperature, we cannot, in any case, pronounce with certainty whether the radiation be of a calorific or frigorific nature. Nay, as we judge ultimately of the reduction of temperature in every case by the contraction of bodies, it is evident that so long as this contraction takes place, we must in fer that caloric is abstracting from them ; and as this may continue to go on until the bodies are reduced to mere physical points, the true zero and the annihilation of matter must be coincident ; in other words, matter cannot exist without, having a certain temperature, or a certain degree of caloric is essentially connected with it from its very nature. If this view of the subject be just, the opinion of the radiation of cold must be rejected, as founded entirely on the vague use of the terms which we apply to the radiation of temperature in different cir cumstances, and which, by a slight change of these, may become a radiation of caloric.