48 Dr. Woods on the Heat of Cliemical Combination. 



one being the more apparent effect, the changes are ascribed to 

 attraction ; but when one body is gaseous, expansion being 

 the more perceptible action, the changes are said to be owing to 

 repulsion. These changes, however, manifestly result from 

 the same cause in both cases — the law that relative volumes must 

 be attained, and opposite movements simultaneously exist. 



(21.) I showed in (14.) that the nearer the particles of a body 

 are to each other, the greater expansion or contraction do they 

 compensate in other bodies by a reverse movement ; so that if 

 we could compress iron, suppose at 32° F., ever so little, it would 

 occasion a great increase in the bulk of a gas. We also saw, 

 that the further out the particles were removed, the greater the 

 distance they afterwards moved for the same amount of contrac- 

 tion in another body ; or, in other words, that the volume gained 

 by any body in compensating a contraction depended on the 

 volume it already possesses. Let us see : — does this rule hold 

 good in all the expansions of bodies ? 



(22.) All liquids in becoming gases expand inversely as their 

 atomic volume, or some multiple of it. — This law I thought I dis- 

 covered, but found on consideration it resulted as a matter of 

 course from the equality of combining volumes of gases ; for if 

 the volumes of the combining proportion of two gases are equal, 

 their expansion from their liquid state must be inversely as their 

 liquid volume. The smaller the one was in a liquid state, the 

 greater its expansion to make it equal to the other in the gaseous. 

 But the smaller the atom is, the greater the space for equal sizes ; 

 therefore the greater the space between the particles, the greater 

 the expansion when a liquid becomes a gas. We saw the same 

 law in the expansion of solids (14.). 



(23.) When a solid becomes a liquid, it is not so easy to tell 

 the amount of expansion on account of crystallization, &c. influ- 

 encing the result. But what is called the latent heat, that is, 

 the volume of expansion necessary for the change of state, reveals 

 the distance the particles separate; and Person has shown 

 (Comptes Rendus, vol. xxiii. p. 162) that this depends on the di- 

 stance of the fusing-point from a certain number of degrees be- 

 low zero ; or in other words, the higher the fusing-point, the 

 greater the expansion : the amount of expansion when a solid 

 becomes a liquid depends therefore on the degree of expansion 

 existing already in it. Person has also shown {Comptes Rendus, 

 vol. xxiii. pp. 327, 524), that the researches of Favre and Silber- 

 man prove that the latent heat of vapours is as their expansion. 

 These two paragraphs, (22.) and (23.), show that the expansion into 

 liquids or gases follows the same law as the expansion of a solid. 



(24.) Another law connected with the expansion of liquids and 

 solids into gases, and one which has hitherto been unnoticed, is 



