66 Intelligence and Miscellaneous Articles. 
ON THE VALUE OF POISSON ’S COEFFICIENT FOR CAOUTCHOUC. 
BY H. H. AMAGAT. 
It is known that scientific men are far from being in agreement 
as to the numerical value to be given to whatis known as Potsson’s 
Coefficient. According to the theories of Poisson, Navier, M. de Saint 
Venant, and the experiments of Cagniard de Latour, and of M. 
Cornu, this coefficient should be equal to 4; according to Wertheim 
it is equal to 1; from the researches of Cauchy, of Lamé, and of 
Kirchhoff, all that can be affirmed is that it is between zero 
and 3; finally, according to MM. Schneebeli and Okatow it varies 
not only from one body to another, but also with the same body 
according to its physical condition. The latter physicist had the 
idea of utilizing the great extensibility of caoutchouc to determine 
the coefficient in question by the well-known method; and MM. 
Naccari and Bellati have made analogous experiments by Regnault’s 
method. 
Tt has been observed with justice that experiments made with 
caoutchouc are but little conclusive; they present great irregula- 
rities, due more especially to permanent deformations ; and, on the 
other hand, the body has but little homogeneity. It may be added 
that the very magnitude of the deformations has put it outside the 
theoretical conditions, which suppose the deformations to be very 
small. 
I propose to show that by following a totally different path we ~ 
may arrive, in the case of caoutchouc, at conclusions which can only 
be invalidated by supposing errors of experiment quite out of pro- 
portion to those which can be really committed. 
In order to make these experiments, and others which are not 
concluded, I have had a piezometer made, in which, as in that of 
Regnault, the pressure may be transmitted in the interior or on 
the exterior at the same time or separately. The apparatus is, 
however, double; two spheres or two cylinders may be placed in 
it side by side, which are under precisely the same pressure and 
at the same temperature—conditions which are very favourable for 
rather delicate comparative experiments. This is not the case of 
the present experiments, which, as we shall see, do not claim great 
accuracy. 
I call « Poisson’s coefficient, K the coefiicient of cubical compressi- 
bility, a the coefficient of elongation or the inverse of the coefficient 
of elasticity, and A and p the two constants. 
We have the ratios :— 
IN 
es arate a 
(2) K= 38a(1—2c), 
1 3A—2 
(3) Velho 
For two different bodies, 
“ K_sG~2e) 
