304 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
that the elementary molecules of the body considered do not 
undergo temporary changes in their individual constitution du- 
ring the variations of temperature which the mass undergoes. 
The numerous experiments made by M. Regnault upon specific 
heats, with unusual care and precision, render the contrary pro- 
position extremely probable; for by them he has demonstrated 
that the specific heat of bodies, that is to say the quantity of 
heat necessary to raise their temperature one degree, has two 
distinct functions; the effect of the one being to dilate the body, 
by forcing its constituent particles to separate further from one 
another; the other, absorbed or dissimulated, so to speak, and 
uniting with these particles in their new conditions of distance 
in order to become a physical element of their modified consti- 
tution*. This result is entirely conformable to the mechanical 
conceptions employed by Laplace, and above all by Poisson, to 
establish the mathematical peculiarities of the equilibrium and 
movement of heat in material bodies. Now the formula (1.) 
which we have just obtained for the optical action of homoge- 
neous liquids will, in many cases, render manifest the distinction 
of these two portions of the total specific heat, and may even, 
in some cases, lead to an appreciation of their relation. For the 
values of the density @ being determinable by experiment for 
the different degrees of temperature to which one and the same 
active liquid is successively subjected, without any permanent 
alteration, if the value of [«], calculated with these densities 3, 
is variable, we may be certain that some portion of the total 
specific heat has become attached to the constituent particles, 
and has modified, at least temporarily, their constitution, since 
it has changed one of their properties. If, on the contrary, the 
value of [«] is constant, we can only conclude thence this alter- 
native, namely, that the specific heat has been consumed entirely 
in dilating the liquid under consideration, or that, if a portion is 
attached to the particles, it has not changed the energy of their 
action upon polarized light. 
10. These tests are much more sure and more sensitive than 
we should be led to think, considering the little extent of the 
variations which the densities of liquids experience when they 
* Recherches sur la Chaleur Spécifique des Corps Simples et Composés, by 
M. V. Regnault, Annales de Chimie et de Physique, tome Ixxiii. 2nd series, 
p- 67; tome i. 3rd series, pp. 189, 200, 207; tome ix. 3rd series, pp. 334, 341 
et passim. 
