808 PRINCIPLES OF CHEMISTRY 



compound The reverse is naturally observed in the case of decom- 

 positions, when from one substance there are produced several of 

 simple? nature. Therefore in the future we shall term combination 

 a reaction in which a contraction is observed that is, a diminution in 

 the volume of the component bodies in a state of vapour or gas ; and 

 we shall term decomposition a reaction in which an expansion is pro- 

 duced ; while those reactions in which the volumes in a gaseous or 

 vaporous state remain constant (the volumes being naturally com- 

 pared at the same temperature and pressure) we shall term reactions 

 of substitution or of double decomposition. Thus the transition of 

 oxygen into ozone is a reaction of combination, the formation of nitrous 

 oxide from oxygen and nitrogen will also be a combination, the 

 formation of nitric oxide from the same will be a reaction of sub- 

 stitution, the action of oxygen on nitric oxide a combination, and 

 so on. 



The degree of contraction produced in the formation of chemical 

 compounds not unfrequently leads to the possibility of distinguishing 

 the degree of change which takes place in the chemical character of 

 the components when combined. In those cases in which a contrac- 

 tion occurs, the- properties of the. resultant compound are very dif 

 ferent from the properties of the substances of which it is composed. 

 Thus ammonia bears no resemblance in its physical or chemical pro- 

 perties to the elements from which it is derived ; a contraction takes 

 place in a state of vapour, indicating a proximation.of the elements 

 the distance between the atoms is diminished, and from gaseous sub- 

 stances there is formed a liquid substance, or at any rate one which is 

 feasily liquefied. For 1 this reason nitrous oxide formed by the conden- 

 sation of two permanent gases is a substance which is somewhat easily 

 converted into a liquid again, nitric acid, which is formed from 

 elements which are permanent gases, is a liquid, whilst, on the contrary, 

 nitric oxide, which is formed without contraction and is decomposed 

 without expansion, remains a gas which is as difficult to liquefy as 

 nitrogen and oxygen. In order to obtain a still more complete idea of 

 the dependence of the properties of a compound on the properties of 

 "the component substances, it is further necessary to know the quantity 

 of heat which is developed in the formation of the compound. If this 

 quantity be large as, for example, in the formation of water then 

 the amount of energy in the resultant compound will be considerably 

 less than the energy of the elements ejntering into its composition ; 

 whilst, on the contrary, if the amount of heat evolved in the formation 

 of a compound be small, or if there even be an absorption of heat, as 

 in the formation of nitrous oxide, then the energy of the elements is 



