Royal Society, 141 



duced by liquid diffusion ; the constituents of a double salt of so 

 much stability as common alum being separated, and the sulphate of 

 potash diffusing in the largest proportion. In fact the diffusive force 

 is one of great energy, and quite as capable of breaking up com- 

 pounds as the unequal volatility of their constituents. Many empiri- 

 cal operations in the chemical arts, it was said, have their foundation 

 in such decompositions. 



Again, one salt, such as nitrate of potash, will diffuse into a solu- 

 tion of another salt, such as nitrate of ammonia, as rapidly as into 

 pure water; the salts appearing mutually diffusible, as gases are 

 known to be. 



Lastly, the diffusibilities of the salts into water, like those of the 

 gases into air, appear to be connected by simple numerical relations. 

 These relations are best observed when dilute solutions of the salts 

 are diffused from the solution-cell, such as 4, 2 or even 1 per cent, 

 of salt. The quantities diffused in the same time from 4 per cent, 

 solutions of the three salts, carbonate of potash, sulphate of potash 

 and sulphate of ammonia, were 10*25, 10*57 and 10*51 grains re- 

 spectively ; and a similar approach to equality was observed in the 

 ] , 2, and 6f per cent, solutions of the same salts. It also held at 

 different temperatures. The acetate of potash appeared to coincide 

 in diffusibility with the same group, and so did the ferrocyanide of 

 potassium. The nitrate of potash, chlorate of potash, nitrate of am- 

 monia, chloride of potassium and chloride of ammonium formed an- 

 other equi-diffusive group. The times in which an equal amount of 

 diffusion took place in these two groups appear to be as 1 for the 

 second to 1*4142 for the first, or as 1 to the square root of 2. Now 

 in gases, the squares of the times of equal difiusion are the densities 

 of the gases. The relation between the sulphate of potash and nitrate 

 of potash groups would therefore fall, to be referred to the diffusion 

 molecule or diffusion vapour of the first group having a density re- 

 presented by 2, while that of the second group is represented by 1. 



The corresponding salts of soda appeared to fall into a nitrate and 

 sulphate group also, which have the same relation to each other as 

 the potash salts. 



The relation of the salts of potash to those of soda, in times of 

 equal diffusibility, appeared to be as the square root of 2 to the square 

 root of 3 ; which gives the relation in density of their diffusion mo- 

 lecules, as 2 to 3. Hydrate of potash and sulphate of magnesia were 

 less fully examined, but the first presented sensibly double the dif- 

 fusibility of sulphate of potash, and four times the diffusibility of the 

 sulphate of magnesia. If these times are all squared, the following 

 remarkable ratios are obtained for the densities of the diffusion mo- 

 lecules of these different salts, each of which is the type of a class of 

 salts, hydrate of potash 1, nitrate of potash 2, sulphate of potash 4, 

 sulphate of magnesia 16, with nitrate of soda 3 and sulphate of soda 6. 



In conclusion, it was observed, that it is these diffusion molecules 

 of the salts which are concerned in solubility, and not the Daltonian 

 atoms or equivalents of chemical combination ; and the application 

 was indicated of the knowledge of the diffusibilities of different sub- 

 stances to a proper study of endosmose. 



