132 ON SUPERSATURATED SOLUTIONS. 



(2.) The theonj of M. M. Violette and Gernez. — This is the 

 theory most generally adopted. There are several a priori 

 objections to this theory. We are asked to believe that the air 

 everywhere and always contains crystals of all those salts which 

 form supersaturated solutions. This is exceedingly improbable, 

 because some salts form supersaturated solutions which cannot 

 exist in air ; others again such as acetate of uranium are so rare 

 that it is inconceivable that they should be everywhere present. 

 Besides, if the salts which form supersaturated solutions are every- 

 where present we must conclude that all other salts are present 

 also. The presence or absence of a salt does not depend on its 

 power to form a supersaturated solution. We should have to infer 

 then that the air and dust is a most extraordinary storehouse of 

 crystals of all kinds, which have never indeed been seen, but whose 

 presence is shown by the fact that when the cotton wool is 

 removed, or when a supersaturated solution is touched with a rod 

 drawn through the fingers it very often crystallises. This theory 

 is supposed to be strongly supported by an experiment of M. 

 Gernez, in which he drew a large volume of country air through 

 water, and then on evaporating it on a glass slide obtained crystals 

 of sodium sulphate. I do not of course dispute his facts, but I 

 have strong evidence to show that sodium sulphate is not generally 

 present In a letter to " Nature " I described how drops of a very 

 strong solution of the sulphate were placed on leaves, moss, wood, 

 flowerpots, and many other substances in my garden near Bristol, 

 and they quietly evaporated as the modified 7 -atom salt — a con- 

 siderable quantity of earth taken from the flower beds was inactive 

 when dropped into the solution. 



But it is not my garden only which is free from the sulphate ; 

 drops of this solution have remained liquid on the window sills in 

 various rooms of the house. The following is a remarkable 

 instance of what can be done with sodium sulphate, vvhich is the 

 most sensitive of all these solutions, and makes a fairly complete 

 answer to M. Gernez. 



