MOLECULES AND ATOMS 807 



in it. Consequently the volumetric composition of water is expressed 

 in the following terms : Two volumes of hydrogen combine with 

 one volume of oxygen to form two volumes of aqueous vapour. For 

 substances which are gaseous at the ordinary temperature, this direct 

 method of observation is sometimes very ea'sily conducted j for 

 instance, with ammonia, nitric and nitrous oxides. Thus to determine 

 the composition by volume of nitrous oxide, the above-described 

 apparatus may be employed. Nitrous oxide is introduced into the 

 tube, and after measuring its volume electric sparks are passed 

 through the gas ; it is then found that two volumes of nitrous oxide 

 have given three volumes of gases namely, two volumes of nitrogen 

 and one volume of oxygen. Consequently the composition of nitrous 

 oxide is similar to that of water ; two volumes of nitrogen and one 

 volume of oxygen give two volumes of nitrous oxide. By decomposing 

 ammonia it is found to be composed in such a manner that two volumes 

 give one volume of nitrogen and three volumes of hydrogen ; also two 

 volumes of nitric oxide are formed by the union of one volume of oxygen 

 with one volume of nitrogen. The same relations may be proved by 

 calculation from the vapour densities, as was described above. 



Comparisons of various results made by the aid of direct observa- 

 tions or calculation, an example of which has just been cited, led Gay- 

 Lussac to the conclusion that the volume of a compound in a gaseous or 

 vaporous state is always in simple multiple proportion to the volume 

 of each of the component parts of which it is formed (and consequently 

 to the sum of the volumes of the elements of which it is formed). This 

 is the second law of Gay-Lussac ; it extends the simplicity cf the 

 volumetric relations to compounds, and is of the same nature as 

 that presented by the elements entering into mutual combination. 

 Hence not only the substances forming a given compound, but also 

 the substances formed, exhibit a simple relation of volume when 

 measured as vapour or gas. 5 



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

 may or may not be a contraction ; the volume of the reacting substances 

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



5 This second law of volumes may be considered as a consequence of the first law, 

 'The first law requires simple ratios between the volumes of the combining substances A 

 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 J3. 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 there is a 

 third law of volumes embracing also the two firs'jb laws. 



