234 EECENT PROGRESS IN RELATION TO THE THEORY OF HEAT. 



Tlie experiment of M. Tyndall, of wliicli I have attempted to give an idea, 

 seemed to be of a nature to decide tlie question in favor of the English physicist. 

 But it is in reality a very complex question, as is shown by the latest observa- 

 tions of M. Wild, of Berne, and those of M. Magnus. The state of the wall of 

 the tube exerts a highly important influence on the effects obtained. It is true 

 that humid air communicates less heat to the battery than dry, if a tube polished 

 on the inner side be employed. But if the tube be blackened or lined with 

 velvet an inverse effect is observed ; humid air then conveys more heat to the 

 battery than dry. The complication of the phenomenon is connected with the 

 condensation of the vapor of water on the walls of the tube. It appears, there- 

 fore, extremely difficult to measure the absorbing power of aqueous vapor by the 

 methods which have heretofore been practiced; all that can be concluded from 

 numerous observations up to the present time is that this power is not so con- 

 siderable as M. Tyndall thinks. If we consider, however, the terrestrial atmos- 

 phei-e in relation to tlie vesicular water it contains, absorption will appear due 

 principally to that water, and the climatological conclusion of M. Tyndall be 

 free from objection. 



In what light should we regard the mechanism of absorption ? If we admit 

 in bodies the existence of ponderable particles and an ether, we can suppose that 

 the movement of the ether is transformed into a different movement, effected by 

 the particles of the body when absorption occurs. We are led to believe that 

 this transformation is more facile in compound bodies that in simple, from seeing 

 that the absorbent power of the former is in general gTeater, and we can imagine 

 compound bodies to be aggregations of particles whose form is opposed to the 

 vibrations of the ether. M. Tyndall says that simple gases are to compound 

 gases what a smooth cylinder, revolving in water, is to a wheel with paddles. 

 The verification of such a law is of very great importance, and we thus see how 

 the research respecting the absorbent powers of gases and vapors may disclose 

 remarkable correlations between the diff'erent properties of matter. 



In the same way that we just represented to ourselves the absorption of heat 

 by gases, we can also represent its emission ; it will be the transmission of the 

 movement of the particles of gas to the ether, and two gases will present the 

 same relation between their absorbent and their emissive powers. The emission 

 of heat by gases is well established by the experiments of MM. Tyndall and 

 Magnus ; there remains no uncertainty but with regard to the numbers which 

 measure it. On this head new researches are indispensable. 



There exist other phenomena calculated to reveal to us the relations of heat 

 and light. It has been long known that the refractive properties of bodies vary 

 with the temperature, and the study of this variation must greatly contribute to 

 our knowledge of the constitution of bodies. M. Fizeau has been occupied with 

 this study for many years, and he has been led to the origination of a new expe- 

 rimental method, the principle of which I proceed to explain. 



The heat absorbed by a body is employed to })roduce many effects, among 

 which is the change of its volume. When the question relates to abody sufficiently 

 voluminous, the methods practiced leave nothing to desire. It is not so, however, 

 in regard to bodies which can be obtained only in small fragments, such as 

 crystals. The process of M. Fizeau is essentially as follows : The solid frao-- 

 ment, having the form of a lamina with two parallel faces, is placed on a hori- 

 zontal metallic plane, supported by three long adjusting screws. The upper 

 points of these screws support a plane of glass, beneath which is the solid lamina 

 designed to be studied. By working these screws the lower face of th(^ plane 

 of g-lass is brought parallel to the upper lace of the solid, at a distance of about 

 two hundredths of a millimetre. By causing rays of simple light to fall perpen- 

 dicularly on the lamina, rings, alternately brilliant and obscure, will be seen 

 reflected on the latter. If the thickness of the small stratum of air interposed 

 between the glass and the solid be gradually increased, ^the rings approach the 



