120. MR JAMES CLERK MAXWELL ON THE EQUILIBRIUM OF SOLIDS. 
4th, In Coutome’s balance of torsion, the angles of torsion are proportional to the twisting 
forces. 
It would appear from these experiments, that compressions are always proportional to pressures. 
Professor Stoxes has expressed this by making one of his coefficients depend on the cubical 
elasticity, while the other is deduced from the displacement of shifting produced by a given tangential 
force. 
M. Cavcny makes one coefficient depend on the linear compression produced by a force acting 
in one direction, and the other on the change of volume produced by the same force. 
Both of these methods lead to a correct result ; but the coefficients of SrokEs seem to have more 
of a real signification than those of Cavcny; I have therefore adopted those of Stokzs, using the 
symbols m and p4, and the fundamental equations (4.) and (5.), which define them. 
Note C. 
As the coefficient @, which determines the optical effect of pressure on a substance, varies from 
one substance to another, and is probably a function of the linear elasticity, a determination of its 
value in different substances might lead to some explanation of the action of media on light. 

