PRINCIPLES OF CHEMISTRY 



CHAPTER VII 



MOLECULES AND ATOMS. THE LAWS OP GAY-LUSSAC AND 

 AVOGADRO-GEBHARDT 



HYDROGEN combines with oxygen in the proportion of two volumes to 

 one. The composition by volume of nitrous oxide is exactly similar 

 it is composed of two volumes of nitrogen and one volume of oxygen. 

 By decomposing ammonia by the action of an electric spark it is easy 

 to prove that it contains one volume of nitrogen to three volumes of 

 hydrogen. So, similarly, it is found, whenever a compound is decom- 

 posed and the volumes of the gases proceeding from it are measured, 

 that the volumes of the gases or vapours entering into combination 

 are in a very simple proportion to one another. With water, nitrous 

 oxide, &c., this may be proved by direct observation ; but in the majority 

 of cases, and especially with substances which, although volatile that 

 is, capable of passing into a gaseous (or vaporous) state are liquid at 

 the ordinary temperature, such a direct method of observation presents 

 many difficulties. But, then, if the densities of the vapours and gases 

 be known, the same simplicity in their ratio is shown by calculation. 

 The volume of a substance is proportional to its weight, and inversely 

 proportional to its density, and therefore by dividing the amount by 

 weight of each substance entering into the composition of a compound 

 by its density in the gaseous or vaporous state we shall obtain factors 

 whicli will be in the same proportion as the volumes of the substances 

 entering into the composition of the compound. 1 So, for example, 



1 If the weight be indicated by P, the density by D, and the volume by V, then 



where K is a coefficient depending on the system of the expressions P, D, and V If D 

 be the weight of a cubic measure of a substance referred to the weight of the same 

 measure of water if, as in the metrical system. (Chapter I., Note 9), the cubic measure of 

 one part by weight of water be taken as a unit of volume then K = 1. But, whatever it 

 be, it is cancelled in dealing with the comparison of volumes, because comparative and not 

 absolute measures of volumes are taken. In this chapter, as throughout the book, the 

 weight P is given in grams in dealing with absolute weights ; and if comparative, as in 



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