Prof. Graham on the Diffusion of Liquids. 263 



nitrate of potash, gave on a former occasion, in similar cir- 

 cumstances, and at 64 0, 9, nearly the same temperature, a dif- 

 fusion product of 15*80 grs. The quantity of nitrate of pot- 

 ash (15*32 grs.) which diffused into the solution of nitrate of 

 ammonia approaches so closely to the number quoted, that 

 we may safely conclude that the diffusion of nitrate of potash 

 is not sensibly resisted by nitrate of ammonia, although these 

 two salts are closely isomorphous. They are still therefore 

 inelastic to each other, like two different gases. 



These experiments have been made upon dilute solutions, 

 and it is not at all impossible that the result may be greatly 

 modified in concentrated solutions of the same salts, or when 

 the solutions approach to saturation. But there is reason to ap- 

 prehend that the phenomena of liquid diffusion are exhibited 

 in the simplest form by dilute solutions, and that concentration 

 of the dissolved salt, like compression of a gas, is attended 

 often with a departure from the normal character. 



On approaching the degree of pressure which occasions the 

 liquefaction of a gas, an attraction appears to be brought into 

 play, which impairs the elasticity of the gas ; so on approach- 

 ing the point of saturation of a salt, an attraction of the salt 

 molecules for each other, tending to produce crystallization,, 

 comes into action, which will interfere with and diminish that 

 elasticity or dispersive tendency of the dissolved salt which 

 occasions its diffusion. 



We are perhaps justified in extending the analogy a step 

 further between the characters of a gas near its point of lique- 

 faction and the conditions which we may assign to solutions. 

 The theoretical density of a liquefiable gas may be completely 

 disguised under great pressure. Thus, under a reduction by 

 pressure of 20 volumes into 1, while the elasticity of air is 

 19*72 atmospheres, that of carbonic acid is only 1670 atmo- 

 spheres, and the deviation from their normal densities is in 

 the inverse proportion. Of salts in solution the densities may 

 be affected by similar causes, so that although different salts 

 in solution really admit of certain normal relations in density, 

 these relations may be concealed and not directly observable. 



The analogy of liquid diffusion to gaseous diffusion and 

 vaporization is borne out in every character of the former 

 which has been examined. Mixed salts appear to diffuse in- 

 dependently of each other, like mixed gases, and into a water 

 atmosphere already charged with another salt as into pure 

 water. Salts also are unequally diffusible, like the gases, and 

 separations, both mechanical and chemical (decompositions), 

 are produced by liquid as well as by gaesous diffusion. But 

 it still remains to be found whether the diffusibilities of differ- 

 ent salts are in any fixed proportion to each other, as simple 



