of certain solid Homogeneous Bodies. 449 
A still graver objection to this hypothesis is that, contrary to 
all our theoretical notions and all the results of experience, we 
should be obliged to suppose that this secondary elongation 
produces no corresponding transverse contraction, since other- 
wise the ratio between the two observed quantities, namely the 
total longitudinal elongation and the total transverse contraction, 
would always remain that indicated by the ancient theory. 
I have been obliged to enter into these details in consequence 
_ of the perseverance with which certain physicists have for twelve 
years opposed this theory to mine, and the manner in which 
they insist on treating as a demonstrated scientific truth that 
which M. Clausius himself only regarded as a hypothesis, and 
one indeed to which no great importance was to be attached. 
MM. Lamé and Maxwell admit that the ratio above defined, 
or, what comes to the same thing, the ratio between the cubic 
and linear compressibilities, may vary in different substances. 
Experience alone can determine whether this is the case, as I 
have not failed to remark, both in my original memoir, and in 
several of those I have since published. M. Verdet is therefore 
wrong in asserting, as he does in the extract of a memoir which 
we shall have to discuss hereafter, and of which M. Kirchhoff is 
the author, that I have “ endeavoured to show by numerous ex- 
periments that this ratio has in all bodies the same constant 
value +.” On the contrary, while affirming and maintaining 
the exactitude of this value for those bodies which were the 
subject of my researches, I excluded those not yet submitted to 
experiment. 
According to an interesting experiment made by M. Clapeyron 
4 1 
on vulcanized caoutchouc, the fraction —, instead of being equal 
to 1 according to the ancient theory, or 2 as my experiments 
require, attains in the case of this substance the enormous value 
of 2201; this fact, to which we shall return hereafter, seems to 
me to be explained by the resuJts of the present memoir. 
Contrary to the opinion of M. Clausius, M. de Saint-Venant 
attributes the disagreement between the results of my experi- 
ments and the ancient theory to.a want of isotropism in the bodies 
on which I experimented: the author thinks “that there are as 
many species of mechanical homogeneousness as there are of 
possible curvilinear systems of coordinates, or of systems of con- 
jJugate orthogonal surfaces”—in fact, that we may imagine as 
many species as we please of non-isotropic homogeneousness ; 
but what he has failed to show is that any such heterotropy 
really exists in the bodies I experimented on, and, which is 
absolutely incredible, that it exists to the same degree in 
them all. 
Phil. Mag. 8S. 4. Vol. 21. No, 142. June 1861. 2G 
