SOLID SOLUTIONS. 123 



mena which can only be explained by the assumption of 

 solid solutions. When equivalent proportions of barium 

 sulphate and sodium carbonate are finely powdered, 

 intimately mixed, and subjected to a very high pressure, a 

 double decomposition takes place with formation of barium 

 carbonate and sodium sulphate. The decomposition, how- 

 ever, is not complete, only 20 per cent, of the original 

 substances being transformed. If, on the other hand, we 

 start with a mixture of barium carbonate and sodium 

 sulphate and compress it, we find that the reverse trans- 

 formation now occurs, barium sulphate and sodium carbonate 

 being formed, and that to the extent of So per cent, of the 

 original substances present. Here we are evidently dealing 

 with a state of equilibrium between the four substances 

 above mentioned, which can only exist together permanently 

 under pressure in certain definite proportions. If these 

 proportions are departed from, the system so transforms 

 itself that the requisite state for equilibrium is attained. 

 Now this of itself points to the substances existing here in 

 a state analogous to that of bodies in liquid solution, for we 

 know that in general definite proportions are necessary in 

 solutions for stable equilibrium to exist. In the case of 

 solids the general rule is that when they are in equilibrium 

 under given conditions in one proportion, they are in 

 equilibrium under the same conditions in every other pro- 

 portion. The behaviour, then, of these solids under pressure 

 is analogous to the behaviour of substances in solution, and 

 different from the ordinary behaviour of solids. The con- 

 tinuance of the pressure is not essential to the establishment 

 of such a definite solid equilibrium, for Spring has shown 

 that by relieving the pressure after 73 per cent, of a 

 system of barium carbonate and sodium sulphate had been 

 transformed, the process continued, though less rapidly, 

 and after a week had reached the proportion of 80 per cent, 

 necessary for equilibrium. Here diffusion must have played 

 a part, for no matter how finely divided the reacting sub- 

 stances originally were, their surface of contact (where alone 

 the mutual decomposition could take place if there were no 

 diffusion) must have been comparatively small. 



