62' DR. R. ANGUS SMITH ON 



But there are other cases when evaporation occurs and 

 a liquid or solid is said to assume the gaseous state : 

 we imagine it to assume that state by the atoms at the 

 surface separating to great distances, not by expanding 

 into a gas below the surface, as in boiling or in effer- 

 vescing, and rising up with a mechanical force ready to 

 take a globe of liquid along with them. At the same 

 time the question naturally arises, at what period of boiling 

 or effervescing does this vesicle form or cease to form, 

 assuming that it does so ? If watery solutions of many 

 substances be boiled, solid matter is taken up readily. Is 

 there a time when evaporation is so calm that no vesicles 

 can be formed, and no solid matter raised? This we 

 cannot doubt. Again, is the solid matter taken up by an 

 agency more allied to chemical than mechanical laws ? 

 In examining these two classes of laws, we generally find 

 that they run into each other on their frontiers in such a 

 manner as to make a definite line impossible ; and I think 

 it would be unwise to suppose that in this case there would 

 be an exception. I would then suppose a series of methods. 

 The first is the formation of vesicles of visible size, as de- 

 scribed : we have in them light bodies ready to decompose, 

 making their escape when the surface of water rises ; for 

 with water they have been hitherto united, and they may 

 somewhat retain their connexion even when that water 

 has somewhat enlarged its boundaries. It is exceedingly 

 probable that the evaporation of a large quantity of water 

 raises in this way a small portion of the bodies with which 

 it was united. Such substances cannot long maintain 

 their independent position in the atmosphere ; they must 

 be removed from it. The last stage would be, that 

 every minute particle should assume the gaseous condi- 

 tion when it had sufficiently separated itself from other 

 bodies, or had become sufficiently liberated from their 

 attraction — at the surface^ for instance, of the liquid. 



