M<>LK<TLES AND ATOMS 301 



When a compound is formed from two or more components, there 

 is or is not a contraction ; the volume of the acting substances is in 

 this case either equal to or greater than the volume of the resultant 

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

 positions, when from one substance there are produced several of 

 simpler 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 replacement 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 substitution, the action of oxygen on nitric oxide a 

 combination, and so on. 



The degree of contraction proceeding 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 takes place, the properties of the resultant compound are very 

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

 Hence ammonia bears no resemblance in its physical or chemical 

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

 place in a state of vapour, and consequently approach ment of parts 

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

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

 easily liquefied. For 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 de- 

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

 liquefy as nitrogen and oxygen. In order to obtain a still more com- 



and B. A substance AB is produced by their combination. It may, according to the 

 law of multiple proportion, combine not only with substances C, D, &c., but also with A 

 and with B. In this new combination the volume of AB, combining with the volume of 

 A, should be in simple multiple proportion with the volume of A ; hence the volume of 

 the compound AB is in simple proportion to the volume of its component parts. There- 

 fore, only one law of volumes need be accepted. We shall afterwards see that the third 

 law of volumes may also embrace in itself the two first laws. 



