SOLID SOLUTIONS. 



IF we define solutions as homogeneous mixtures of sub- 

 stances in variable proportions, we are at once obliged 

 to admit the existence of solid solutions, for there are many 

 mixed solids which fulfil the requirements of this definition. 

 Common potash alum, for example, can crystallise together 

 with ammonia alum, and form mixed crystals which are 

 perfectly homogeneous and of the same composition through 

 out, although the proportions of the two constituents may 

 be varied at will by proper selection of the aqueous solution 

 from which the crystals separate. 



We are inclined, however, to look in solutions for some- 

 thing more than mere homogeneity and uniformity of com- 

 position, and perhaps one of the most obvious characters of 

 a liquid solution is this, that should it at first be of different 

 composition in different parts of its mass, there is always 

 present the tendency of the dissolved body to attain a uni- 

 form distribution throughout the solvent. The process of 

 equalisation of the composition, or diffusion, occurs in all 

 solutions which are more concentrated in one part than in 

 another, the dissolved substance moving from the place of 

 greater to the place of less concentration. Diffusion in 

 solution goes forward very slowly if the liquid is protected 

 from mechanical disturbance and sudden change of tem- 

 perature, months being requisite for the attainment of uni- 

 form concentration if a comparatively short column of pure 

 solvent is placed above a denser layer of strong solution 

 contained in the bottom of a cylinder. If diffusion takes 

 place in solids we might expect it to proceed even more 

 slowly. 



A class of substances which form in some sort a connect- 

 ing link between liquids and solids, and are specially suited 

 to the study of diffusion phenomena, is to be found in jellies. 

 Graham, to whom we owe our first exact knowledge of 

 diffusion in liquids, prepared a stiff jelly containing common 

 salt in solution in one part, and compared the rate at which 



