EECENT TROGEESS IX RELATION TO THE THEORY OF HEAT. 235 



centre by a centripetal movement ; the central dark ring- becomes a black point 

 and disappears ; the second dark ring has taken its place, disappears in its turn, 

 and so on in succession. Inversely, it" the tliickness of the stratum of air is 

 diminished, the movement of the rings is centrifugal ; a point appears at the centre, 

 grows larger, becomes a ring ; then a new ring is formed at the <?entre, and so 

 in succession. When a ring proceeds thus to occupy the place of another ring, 

 we know, according to the laws of light, that the thickness of the stratum of 

 air has varied by a half length of the wave; for yellow light this variation is 

 294 millionths of a millimetre. Observation of the rings, therefore, will enable 

 ns to know the slightest variations in the thickness of the stratum of air. 



The apparatus is placed in an air-bath, and is gradually heated. If the solid 

 lamina dilates it tends to diminish the thickness of the stratum of air; the three 

 screws, on the contrary, by dilating, tend to increase that thickness. The result- 

 ing ellect will be a diminution or augmentation of the thickness, and consequently 

 the centrifugal or centripetal movement of the rings will be observed, according 

 as the dilatation of the lamina shall be greater or less than that of the screws. 

 From the degree of displacement of the rings we deduce the dilatation of the 

 lamina. 



This method, the precision of which is extremely great if we take all the pre- 

 cautions indicated by M. Fizeau, enables us to resolve a great number of ques- 

 tions relating to the properties of crystals, and to establish new relations between 

 heat and light. Thus there exist in a crystal three rectangular directions, which 

 are called axes of elasticity, around which are grouped the most remarkable opti- 

 cal phenomena, and also the phenomena of couductibility and electricity discovered 

 by I)e Senarmont. These axes play the same part in the phenomena of dilatation 

 by heat, and the ingenious researches of M. Fizeau have now completed our 

 knowledge of the admirable structure of crystallized solids. Among the numerous 

 unexpected results at which he has arrived, I may cite the contraction of the 

 ioduret of crystallized or amorphous silver at every temperature which has be(in 

 employed, and the existence of a maximum of density for the beiwl, the protoxide 

 of copper, and the diamond. 



I pass now to the second part of my subject, the relations which exist between 

 heat and movement. During the heating or the cooling of a body, there are in 

 general three sorts of eftects to be considered, the variation of tem[)erature, tho 

 external mechanical labor which results from the change of the volume of bodies 

 and from pressures exerted on their surface, and the int(Mnal ineclianical labor 

 which consists in the change of aggregation. Tliere are delinite relations between 

 these effects and the quantities of heat lost or gained by the body, and the dis- 

 covery of these relations is one of the most remarkable advances of modern 

 physics. It serves as the basis for the mechanical theory of heat. 



It is now well established by experiment that a given quantity of heat is 

 equivalent to a definite mechanical labor, as if heat were convertilde into labor, 

 and vice versa. This experimental law h^ads us to regard the effects of heat as 

 the result of the movement of the particles of bodies, and to frame hypotheses 

 which enable ns to conceive of this movement; l)ut such is not the object of the 

 mecliani(;al theory of heat. Without forming any hypothesis respecting tho 

 nature of heat, it only sets f(n-th a small number of principles, a sort of postu- 

 lata suggested by experiments, and it links together all the known facts by means 

 of general relations deduceil mathematically from those j)rinciples. It is a pliysical 

 theory in the rigorous acce[)tation of tlie word. 



Till now two buuhunental principles have served as a point oi' dcpariure ; but 

 according to the recent researches of M. Ilirn, the second principle would bo a 

 rational consequence of the first, so that thcnnodijnuinici woidd seem based iu 

 reality on the sole principle of the eqiiivulcnci/ of heat and. of labor. So impmlant 

 is such a proposition that I could not pass it by in silence. 



The thermodynamic theory has opened a new horizon to all those who study 



