MOLECULES AND ATOMS 293 



for example, water contains eight parts by weight of oxygen to one 

 part by weight of hydrogen, and their densities are 16 and 1, conse- 

 quently their volumes (or the above-mentioned factors) 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, know- 

 ing that nitric oxide contains fourteen parts of nitrogen and sixteen 

 parts of oxygen, and knowing that the specific gravities of both these 

 gases are fourteen and sixteen, we find that the volumes in which nitro- 

 gen and oxygen combine for the formation of nitric oxide are in the 

 proportion of 1:1. Consequently, nitric oxide is a combination of equal 

 volumes of nitrogen and oxygen. 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 there- 

 fore 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. N0 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 2 , 

 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 sub- 



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

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

 volume V in metrical units (cubic centimetres or cubic metres, &c.), if 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, form the most important province of chemical learning and the most important 

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

 relations are determined by the densities of gases and vapours, therefore the methods 

 of determining the densities of vapours (and also of gases) form important means 

 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 

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

 hydrogen, see Chap. II. Note 28) 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 thermo- 



