VAPORIZATION. 65 



tion, estal)lished the opposite conclusion. Mercury was found 

 to yield a small quantity of vapour during summer, at a tempe- 

 rature varying from 60 to 80, but in winter no trace of vapour 

 could be detected. Dr. Faraday has proved that several che- 

 mical agents, which may be volatilized by a heat between 300 

 and 400o, did not undergo the slightest evaporation when kept 

 in a confined space with water during four years. 



Bodies, therefore, cease all at once to emit vapour, at some 

 particular temperature. In the case of mercury, this tempera- 

 ture lies between 40 and 60 Fahrenheit. But a progressive 

 and endless diminution of vaporizing power is certainly more 

 natural than an abrupt cessation. What puts a stop to vaporiza- 

 tion ? it may be asked. Liquids, we know, have a certain at- 

 traction for their own particles, evinced in their disposition to 

 collect together in drops. The particles of solids are attracted 

 more powerfully, and cohere strongly together. Dr. Faraday is of 

 opinion, that when the vaporizing power becomes very weak, at 

 low temperatures, it may be overcome and negatived completely 

 by this cohesive attraction, and no escape of particles in the 

 vaporous form be permitted. 



This supposition is conformable with the views of corpuscular 

 philosophy entertained by Laplace. According to that profound 

 philosopher, the form of aggregation which a body affects de- 

 pends upon the mutual relation of three forces : 1 . The attrac- 

 tion of each particle for the other particles which surround it, 

 which induces them to approach as near as possible to each 

 other. 2. The attraction of each particle for the heat which 

 surrounds the other particles in its neighbourhood. 3. The 

 repulsion between the heat which surrounds each particle, and 

 that which surrounds the neighbouring particles, a force which 

 tends to disunite the particles of bodies. When the first of 

 these forces prevails, the body is solid ; if the quantity of heat 

 augments, the second force becomes dominant, the particles 

 then move among each other with facility^ and the body is 

 liquid. While this is the case, the particles are still retained by 

 the attraction for the neighbouring heat, within the limits 

 of the space which the body formerly occupied, except 

 at the surface, where the heat separates them, that is to 

 say, occasions evaporation, till the influence of some pressure 

 prevents the separation from being effected. When the heat 

 increases to such a degree that the reciprocal repulsive force 



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