temporarily produced in Isotropic Bodies, 347 



through the compressed portion of the curved glass ; the two 

 systems of fringes, extraordinary and ordinary, which are ob- 

 served with a birefracting prism, are displaced with reference to 

 one another, and move as a vernier on its scale. The measure 

 of the displacement gives a second equation between p and q. 



But in order to deduce numerical results from these experi- 

 ments, it is necessary to assume a determinate ratio between the 

 changes of length which occur in the directions of the three 

 mechanical axes. In theory, this ratio, or what is the same thing, 

 the law of change of volume, may rest indeterminate, as asserted 

 by MM. Lame and Maxwell* ; but not so when it is the question 

 of an experiment, the results of which we would know and 

 apply. 



M. Neumann has found himself compelled to adopt the law 

 of Poisson, the accuracy of which had not indeed been contested 

 when his memoir was published ; consequently we have 



and we find 



jo-9 = 0-054; jo= -0-085; g= -0-139. 



Introducing the new law into the formulae of M. Neumann, 

 we see that all the numerical coefficients are simplified, and we 

 obtain at the end of the calculation the following values : — 



y=+8; a=;S= + |; ;>-^=0'0505; 



^=_0-3006; g= -0-3511, 



which difi^er notably from the old ones. If, as in the present 

 memoir, we confine ourselves to the determination of the quan- 



* I profit by this occasion to protest against the inexact manner in which 

 Mr. Maxwell has reported my experiments (Trans, of the Roy. Soc. of Edin- 

 burgh, vol. XX. part 1, page 87). " M. Wertheim," he says, "has given 

 the results of some experiments on caoutchouc, by means of which he finds 



A 



K=A: or 11-=. — m, and he concludes that for all substances we have K=A;." 

 «j 



Here Mr. Maxwell speaks only of experiments which I expressly stated to 

 be preliminary ones, and he passes in silence others much more exact and 

 more varied, which I have made by means of hollow cyUnders of difi'erent 

 substances. 



To demonstrate afterwards that the law of the change of volume may 

 vary with the nature of the body, Mr. Maxwell cites cork, which has a 

 cubical elasticity smaller, and a linear elasticity greater, than the corre- 

 sponding elasticities of a jelly. It will certainly not be admitted without 

 difficulty that these two substances are types of homogeneous bodies ; but 

 even if they were, Mr. Maxwell has cited no experiment in support of his 

 opinion. 



3A2 



