300 Dr. Woods on the Heat of Chemical Combination. 



of proportionate volume in matter, or heat is always the same, and 

 can neither be increased or diminished. Having referred to an 

 experiment which shows that the nearer the particles of a body- 

 are to each other, the greater the effect they have in compensating 

 an expansion or contraction in another, I endeavoured to show 

 that this law holds good, not in some, but in all states in which 

 a body may be placed. And I did so for this reason ; that as it 

 is generally imagined, the expansion of a body when its particles 

 are more widely separated is greater than when they are more 

 nearly together, because attraction has to a certain extent been 

 overcome, if I could show that in those conditions of matter 

 where attraction is not said to exist the same law was followed, 

 it would argue that attraction is not necessary for the explana- 

 tion. To prove this point, I showed that liquids change into 

 vapours with an amount of expansion which is inversely as their 

 atomic volume, or a multiple of it ; and argued that, as in a 

 certain bulk of atoms and space, the smaller the atom the larger 

 the space, so the larger the space, or the greater the expansion 

 already existing in the liquid state, the greater the subsequent 

 expansion into vapour; but when liquids are expanding into 

 vapour, attraction is not said to exist. 



In proving that the same occurs when solids change into 

 liquids, I said the amount of expansion could not be measured; 

 I mean the amount of expansion between the atoms, because 

 crystallization, &c. influenced the result ; but as Person has 

 shown that the latent heat of vapours is proportionate to their 

 amount of expansion from the liquid state, I proposed to use the 

 quantity of heat rendered latent when a solid becomes a liquid 

 to express the atomic expansion that at the same time occurs, 

 and thus find whether in this case also the expansion depended 

 on the atomic volume. I here stated, that Person says the 

 latent heat depended on the height of the fusing-point ; and 

 this is the error I wish to correct : not so much that it would 

 materially affect my theory, but that when I discovered my mis- 

 take, and reasoned on the formula that Person really gives, I 

 saw it proved clearly, that the expansion occurring when a solid 

 changes to a liquid is influenced as to extent precisely as that of 

 a liquid when it becomes a vapour ; and that the amount of this 

 expansion in both cases is inversely as the atomic volume taken in 

 connexion with the boiling- and fusing-point. 



Person shows (Comptes Rendus, vol. xxiii. pp. 327, 524), that 

 if the temperature of the fusing-point of any body be increased 

 by 160° C, and the result be multiplied by the difference of the 

 specific heat in the solid and liquid state, the product will be 

 equal to the latent heat. His formula is (160 + /)xS = L; 

 where t is the temperature of the fusing-point, 8 the difference 



