THEORY OF SOLUTION AND CRYSTALLISATION. 257 



solvent, forming well-defined compounds j and cases are known in 

 which benzene, and probably other liquids, unite with crystallising 

 solids in a similar manner. ^ 



Examples of Alcoholates. 

 Mg (N03)2 + 6 C2H6 O \ Common 

 CaCl. + 4C2H6O Ethylic 



r7 r>\ . /-I TT /-k 1 Alcohol 



Zn CL -f 2 CgHgO ) 



Ca CI2 + 4 CH^O Methylic Alcohol 



Ca Clo + 4C5H12O Amy lie Alcohol 



12. Attention has lately been drawn to a very curious action of 

 filter paper upon certain saline solutions, (Bayley, J., Chem. Soc, 

 July, 1878.) When a drop of a solution of a metallic salt is 

 placed upon filter paper, the water spreads away into the paper, 

 leaving a more concentrated solution of the salt in the centre of the 

 spot. A great difference however is observed in the behaviour of 

 the salts of various metals. Silver, lead, and mercuric salts give a 

 wide water ring, as also do solutions of copper, nickel, and cobalt, 

 when dilute. But cadmium salts differ from all the rest in 

 spreading to the edge of the blot.* 



13. In this place also we may recall to mind one of the results 

 of Graham's researches on liquid diffusion ; namely, that the rate 

 of diffusion of a soluble substance increases with the temperature. 

 This fact admits of various explanations, and therefore too much 

 stress must not be laid upon it in connection with this question. 

 It does, however, seem to accord with what might be expected 

 from the disposition manifested by all salts to associate themselves 

 with a larger number of water molecules at lower than at higher 

 temperatures, and so to produce molecular aggregates which might 

 a priori be supposed to move less actively in proportion to their 

 increased complexity. 



* Something of the same kind was observed many years ago by Chevreul, 

 who found that when cotton is immersed in a solution of alum, it withdraws 

 a liquid which contains less alum than the original. 



