140 ON SUPERSATURATED SOLUTIONS. 



The sulphate is much the most sensitive, and specks of soot 

 have often been seen to cause crystallisation : it is advisable to 

 keep the sulphate covered in rooms, especially in cold weather. 

 I imagine that these nuclei settle very lightly on the surface and 

 therefore are not at once saturated, but are able to act by absorption. 

 The temperature also affects the results : at a low temperature an 

 absorbent may be active which is inactive at a higher one, because 

 then the tendency of the salt to separate out is less. 



Adhesio7i of Crystals. — The apparent sensitiveness of these 

 solutions can in some cases be explained by the remarkable power 

 of adhesion, exhibited by the salts to any surface on which they 

 have been allowed to crystallise : the minutest trace of a crystal is 

 sufficient to make a drop crystallise when brought in actual 

 contact. — I was much puzzled once by a glass plate which had 

 been used for the acetate, and then washed carefully in cold water. — 

 Drops of the solution remained liquid on all parts of the plate, but 

 if a drop was scratched with a sharp point it crystallised instantly. 

 At first I attributed the effect to the scratching, but I found, at 

 last, that on washing the plate in hot water scratching produced no 

 effect at all. — The drops remained liquid, because the crystals were 

 adhering in the depression of the rough glass, and were covered 

 with a layer of water. So a pin which has been heated red hot 

 several times gets a rough surface on which the acetate may be 

 allowed to crystallise. It often then becomes almost impossible to 

 clean the pin in cold water. It can be laid in a drop and produce 

 no immediate effect, but is instantly active on scratching the plate. 

 I left a pin for three and a half hours in water and it was then 

 active on scratching. When a pin thus washed is left in a drop of 

 the solution a curious effect is often produced. When a crystal is 

 introduced, the drop, as a rule, crystallises with a sort of flash, the 

 whole becoming solid at once ; but in the case of these washed 

 pins no immediate effect is seen, but on standing for an hour or 

 two, well marked, large, distinct crystals are formed, which grow 

 very slowly, being constantly surrounded by an impoverished 

 layer of the solution. 



