320 PRINCIPLES OF CHEMISTRY 



of a series of fundamental chemical and physical data, inasmuch as a 

 number of the properties of substances are dependent on their vapour 

 density, or molecular weight and composition. Therefore, the vapour 



density D = . For instance, the formula of ethyl ether is C 4 H 10 O, 



hence it corresponds with the molecular weight 74, and the vapour 

 density of 37, which is the fact. Therefore, the density of vapours and 

 gases ceased to be an empirical magnitude obtained by experiment 

 only, and it acquired a rational meaning. It is only then needful to 

 remember that 2 grams of hydrogen, or the molecular weight of this 

 primary gas in grams, occupies, at and 760 mm. pressure, a volume 

 of 22'3 litres (or 22300 cubic centimetres), in order to directly reduce 

 the weights of cubical measures of gases and vapours from their 

 formulae, because the molecular weights of all other vapours at and 

 760 mm. occupy the same volume, 22*3 litres. Thus, for example, in the 

 case of carbonic anhydride, CO 2 , the molecular weight M=44, hence 44 

 grams of carbonic anhydride at and 760 mm. occupy a volume of 

 22 '3 litres consequently, a litre weighs 1*97 grams. By combining the 

 laws of gases Gay-Lussac's, Mariotte's, and Avogadro-Gerhardt's we 

 obtain 23 a general formula for gases 



where s is the weight in grams of a cubic centimetre of a vapour or gas 

 .at a temperature t and pressure p (expressed in centimetres of mer- 

 cury) if the molecular weight of the gas M. Thus, for instance, at 

 100 and 760 millimetres pressure (i.e., at the atmospheric pressure) 

 the weight of a cubic centimetre of the vapour of ether (M = 74) is 

 equal to s=0-0024. 24 



As the molecules of many elements (hydrogen, oxygen, nitrogen, 



23 This formula (which is given in my work on 'The Tension of (! uses,' and in a 

 somewhat modified form in the ' Comptes Rendus,' Feb. 1876) is deduced in the following 

 manner. According to the law of Avogadro-Gerhardt, M = 2D for all pises, where M i- 

 the molecular weight and D the density referred to hydrogen. But they equal the weight 

 S of a cubic centimetre of a gas in grams at and 76 c.m. pressure, divided by 

 0-0000896, for this is the weight in grams of a cubic centimetre of hydrogen. But the 

 weight s of a cubic centimetre of a gas at a temperature t and under a pressure /; 

 .(in centimetres) is equal to spjlft (l + at). Therefore, s s.76 (l + at)p ; hence 

 D = 76./S (l + a)/0.0000896j9, whence M = 152s (l+ai)/0*000089dp, which gives the above 

 expression, because I/a = 278. m/v, where tn is the weight and v the volume of a 

 vapour, may be taken instead of s. 



24 The above formula may be applied in order to ascertain directly the molecular 

 weight for a given vapour density, as s = the weight of vapour m, divided by the volume 

 v, and consequently by experiment, M = 6,255 m (278 + t)/pv. Therefore, instead of the 

 formula (see Chap. II. Note 38), pv = ~R('273 + t), where R varies with the mass and 

 nature of a gas, we may apply the formula ^>u = 6,255(?/i/M) (278 + ), and taking a 

 .weight of a gas m equal to its molecular weight, pv 6,255 (278 + t) for all gases. 



