800 PRINCIPLES, OF CHEMISTRY 



water contains eight parts by weight of oxygen to one part by weight 

 of hydrogen^ and their densities are 16 and 1, consequently "their 

 volumes (or the above-mentioned factors) are 1 and , and therefore 

 it is seen without direct experiment that water contains two volumes 

 of hydrogen for every one volume of oxygen. So also, knowing flbat 

 nitric oxide contains fourteen parts of nitrogen and sixteen parts of 

 oxygen, and knowing that the specific gravities of these last two gases are 

 fourteen and sixteen, we find that the volumes in which nitrogen and 

 oxygen combine for the formation of nitric oxide are in the proportion 

 of 1 : 1. We will cite another example. In the last chapter we saw 

 that the density of NO 2 only becomes constant and equal to twenty- 

 three (referred to hydrogen) above 135, and as a matter of fact a method 

 of direct observation of the volumetric composition of this substance 

 would be very difficult at so high a temperature. But it may be 

 easily calculated. NO 2 , as is seen from its formula and analysis, contains 

 thirty-two parts by weight of oxygen to fourteen parts by weight of 

 nitrogen, forming forty-six parts by weight of NO..,, and knowing the 

 densities of these gases we find that one volume of nitrogen with two 

 volumes of oxygen gives two volumes of nitrogen peroxide. Therefore, 

 knowing the amounts by weight of the Substances participating in a 

 reaction or forming a given substance, and knowing the density of the 

 gas or vapour, 2 the volumetric relations of the substances acting in a 



the expression of chemical composition, then, the weight of an atom is taken as unity. 

 The density of gases, D, is also taken in teference to the density of hydrogen, and the 

 Volume V in metrical units (cubic centimetres)^ it be a matter of absolute magnitudes 

 of volumes, and if it be a matter of chemical transformations that is, of relative volumes 

 then the volume of an atom of hydrogen, or of one part by weight of hydrogen, is taken 

 as unity, and all volumes are expressed according to these units. 



2 As the volumetric relations of vapours and gases, next to the relations of substances 

 by weight, foi*m the most important province of chemistry, and a most important means 

 for the attainment of chemical conclusions, and inasmuch as these volumetric relations 

 are determined by the densities of gases and vapours, necessarily the methods of deter- 

 mining the densities of vapours (and also of gases) are important factors in chemical, 

 research. These methods are described in detail in works on physics and physical 

 and analytical chemistry, and therefore we here only touch on the general principles of 

 the subject. 



If we know the weight p- and volume v, occupied by the vapour of a given substance 

 at a temperature t and pressure h, then its density may be directly obtained by dividing 1 

 p by the weight of a volume v of hydrogen (if the density be expressed according to 

 hydrogen, see Chapter II., Note 23) at t and h. Hence, the methods of determining the 

 density- of vapours and gases are based on the determination of p, v, t, and h. The two 

 last data (the temperature t and pressure h) are given by the thermometer and barometer 

 and the heights of mercury or other liquid -confining the gas, and therefore do not 

 require further explanation. It need only be remarked that : (1) In the case of easily 

 volatile liquids there is no difficulty in procuring a bath with a constant temperature, 

 but that it is nevertheless best (especially considering the inaccuracy of thermometers) 

 to have a medium of absolutely constant temperature, and therefore to take either & 

 bath in which some substance is melting such as melting ice at or crystals .of 



