of the principal Phcenomena of Heat. $4.^ 



pulsive force of these two portions of heat will overbalance the 

 forces of attraction. Thus we have an explanation of the phse* 

 nomena of vaporization. 



An account of thfe causes of this last class of phaenomena 

 somewhat resembling the above has been given by Laplace 

 in the twelfth book of the Mecanique Celeste. Laplace how- 

 ever gives no explanation of the difference between latent and 

 sensible heat : he attributes the phaenomena of expansion ex- 

 clusively lo sensible heat, but does not explain why heat when 

 it becomes latent should lose its repulsive force : he supposes 

 the particles of heat to form atmospheres about the particles 

 of solid matter, which are at rest unless disturbed ; and in 

 order to explain why sensible heat tends continually to fly 

 offi he supposes the particles of matter to l)e in a state of con- 

 tinual agitation. 



7. In the same chapter Laplace has given an explanation 

 of the phaenomena of liquefaction, which appears to me to 

 require modification. " Every molecule of solid matter," he 

 says, " is subjected to the action of three forces : 1st, the attrac- 

 tion of the surrounding molecules ; 2ndly, the attraction of 

 the caloric of those molecules, plus their attraction for its ca- 

 loric ; Srdly, the repulsion of its caloric by the caloric of those 

 molecules. The first two forces tend to bring the molecules 

 nearer together, the third tends to separate them. The three 

 states of solidity^ fluidity^ and gaseous elasticity depend on the 

 respective efficacy of those forces. In the state of solidity, the 

 first force is the most powerful ; the influence of the figure of 

 the molecules is very considerable, and they are united in the 

 direction of their greatest attraction. The increase of caloric 

 lessens this influence by dilating the body; and when this 

 increase is such that the influence in question is very small or 

 none, the second force predominates, and the body takes the 

 liquid form." 



I am unable to comprehend why the attractive force be- 

 tween two solid particles should be suddenly reduced to 

 nothing when they are separated to a certain distance one 

 from the other. And this suggestion appears still less pro- 

 bable when it is considered that many crystallizable bodies 

 cease to contract, and even undergo a degree of expansion, 

 at the moment of congelation. The act of changing from 

 fluid to solid depends then on some other cause than the ap- 

 proximation of the particles. Laplace has, I think, truly as- 

 signed the immediate physical cause of solidity, when he says 

 that in solid bodies the particles are placed relatively to each 

 other in the position of greatest attraction ; but he has not 

 given an adequate reason for their assuming this position. To 



Third Series. Vol. 10. No. G2. May 1837. 2 Y 



