310 Royal Society. 



3. Even the sulphates are unaffected by the dust of the open 

 air and generally of ordinary rooms. 



4. Anhydrous salts or modified salts, sometimes new, are pro- 

 duced by the spontaneous evaporation of the solutions in drops. 



5. Drops can be rapidly touched on their surface with crystals 

 of the same salt without crystallizing. 



6. Greasy surfaces, whether films or lenses, have no effect. 



7. The shape of the vessel has sometimes a material influence 

 on the possibility of obtaining a supersaturated solution. 



8. Air and dirty surfaces are active on salts which apparently 

 cannot exist in air. 



9. Scratching a hard surface will cause a particular solution to 

 crystallize. 



The crystal theory, modified as it now must be, seems on the 

 whole the best explanation of the phenomena. The case of the 

 hyperacid sodium sulphate, however, remains to be explained. If 

 the crystal theory is true, the order of sensitiveness of the solu- 

 tions should be the order of comparative rarity of the salts ; and 

 this remains to be proved. 



As to the cause of supersaturation, a good many facts seem to 

 show that it is the anhydrous salt which enters into solution. 

 The lower hydrates seem to be first formed, as in the case of 

 sodium sulphate and the alums. In the case of the hyperacid 

 sodium sulphate with two parts acid to one of salt, repeatedly 

 boiled, it seems to be the anhydrous salt which is first deposited. 

 When the aqueous solutions of sodium sulphate and the alums 

 are made to crystallize, the modified salts become opaque white, 

 while the hyperacid salt remains unchanged, and can be obtained 

 unchanged by heating the opaque variety from the top so as to 

 dissolve this, but not the anhydrous. 



Against the theory that it is the anhydrous sodium sulphate in 

 solution at low temperatures must be set the following fact. 



Lowel, in his Tables of the solubility of the three forms of 

 sodium sulphate, which are found in all our text-books, gives 412 

 parts of salt to 100 of water as the maximum solubility of the 

 10-atom salt ; and this is the highest number for any of the three 

 kinds. Now I have dissolved 600 parts of 10-atom salt in 100 

 of water at 37° C. without throwing down a trace of anhydrous. 



I then warmed it : at 45° a doubtful trace of anhydrous ; at 51° 

 very few ; at 60° still very few ; at 67° about as much as would lie 

 on a little-finger nail ; at 75° eight or ten times as much, the 

 liquid nearly opaque; at 80° a large quantity; boiled, the salt 

 thickly covered the bottom of a large flask. 



Now here the solution at 00° practically retained the whole of 

 the 6 oz. of salt to 1 of water, while according to Lowel it should 

 have retained only 2| oz. 



Then between 70° and 80° a sudden change takes place, and a 

 large quantity is thrown down. This agrees so far with Lowel's 

 Table, as, according to him, at 84° the whole of the excess was 

 practically thrown down. This looks very much like dissociation 



