[mcintosh] 



SUPERSATURATED SOLUTIONS 



269 



way. Pieces of apparatus of the form shown in the diagram were 

 constructed. The tube A was filled with the supersaturated solution; 

 a piece of iron was sealed into a glass tube and placed in B, and the 

 apparatus "sterilized" by heating to 45° for several hours. After 

 standing for a week, a few grains of finely-powdered sodium sulphate 

 were placed in C, a phosphoric anyhdride tube was sealed on and the 

 system exhausted to one one-thousandth of a millimeter of mercury with 

 a Toepler pump. The apparatus was washed out several times with 

 dry air, and was finally exhausted and sealed. After standing for a 

 month, F was raised with a magnet, the inner tube was broken, and 

 the powder allowed to fall into the solution. In four experiments 

 there was instant crystallization ; in the fifth hesitation for an instant. 

 A note in Faraday's Researches on Electricity (392) explains, I 

 think, why the crystals are not destroyed. Faraday observed that 

 hydrated crystals, if perfect, i.e. with unruptured surfaces, could re- 

 main indefinitely in an atmosphere where the water vapour pressure 



^P ^^s 



No.^50^ 



