Dr. Woods on the Heat of Chemical Combination. 49 



that the amount of expansion of compound bodies, when changing 



from one state to another, as from a liquid to a gas, is deter in hied 



by one of the elements only. For instance, the atomic volume of 



9 

 the vapour of water is = say 14 : here there is only one 



proportion of oxygen. The vapour of fether has for its atomic 



volume ^h = say 14, and in it there is only one proportion of 



oxygen also. Now alcohol has two proportions of oxygen, and 



.46 

 the volume of the combining atom is .Yen— say 28, or twice 14, 



showing twice the expansion that occurred in the other cases. In 

 methylic aether and acetone there is only one proportion of oxygen, 

 and their atomic volume is expressed by 14; but in pyroxylic spirit, 

 and all compounds having more than one proportion, notwithstand- 

 ing the diversity of their composition, the atomic volume is 28. In 

 the same way, the mercury in calomel is twice the mercury in 

 the bichloride, and the atomic volume of the vapour of the former 

 is twice that of the latter, the chlorine not influencing the result : 



the atomic volume of vapour of calomel is = say 28, that of 



bichloride of mercury = say 14. In nitrous oxide and nitric 



oxide the same circumstance is observed : indeed in all cases of 

 expansion of solids or liquids into gases this law holds good. 

 It is an interesting subject of inquiry, whether the same law 

 influences the expansion of solids and liquids when not changing 

 their state. It would anticipate the result of some investigations 

 I am making on the change of solids and liquids into gas to say 

 more at present than what is contained in this paragraph (24.). 

 However, enough has been said to show, by combining it with 

 (22.), (23), &c, that bodies when expanding or contracting gain 

 and lose definite volumes depending on their previous state ; that 

 the expansions and contractions of different bodies which always 

 accompany each other have a fixed relation, or in other words, 

 the same amount of expansion or contraction in one body from the 

 same state is always accompanied by a fixed amount of opposite 

 motion in another ; and that in compound bodies, in cases where 

 the expansions and contractions are best marked, as in changing 

 their states, these expansions and contractions depend for their 

 amount on only ONE of the elements. 



(25.) Now before applying these principles to the explanation 

 of the heat of chemical combination, I will notice that, even in 

 what is called attraction of gravitation, equal and opposite motion 

 exists — in this case between the masses, as in what is called 

 the attraction of cohesion between the particles. I allude to it 

 here to speak of a circumstance to which I will refer by and by. 



Phil. Mag. 8. 4. Vol. 8. No. 15. Jan. 1852. E 



