UB 237 



dry and free from carbonic anhydride. Taking the amount of the 

 latter as 0'03 per 100 volumes, wo obtain a greater weight namely, 

 1-000156 times greater (consequently at and 760 mm., instead of 

 l-L'9300 grains, the weight of a litre will then be 1 '29319 grams). The 

 weight of moist air in which the tension 34 of the aqueous vapour (par- 

 tial pressure) =fim\\. t and consequently the volume of vapour (its 



density referred to air =0'62) =%- ~ f the whole pressure of the moist 

 air =760 mm.) is to the weight of an equal volume of dry air as 

 + 0-62 , or as ~~ f , is to 1 ; that is, the weight of air 



containing carbonic anhydride and moisture at and 760 mm. pressure 

 is not 1-2930 grams, but this weight multiplied by 1-000156 and by 



7fin ^ a P ressure (total) of air of H millimetres, a temperature 



t, and elasticity of vapour f, the weight of a litre of air will be 

 (i.e., if at and 760 mm. the weight of dry air = 1'293) equal to 



x H ~ 38/ - For instance ' if H = 73 mm " ' = 20 > 



and f =. 10 mm. (the moisture is then slightly below 60 p.c.), the 

 weight of a litre of air = 1-1512 gram. 35 



The presence of ammonia, a compound of nitrogen and hydrogen,. 

 in the air, is indicated by the fact that all acids exposed to the air ab- 

 sorb ammonia from it after a time. De Saussure observed that aluminium 

 sulphate is converted by air into a double sulphate of ammonium and 

 aluminium, or the so-called ammonia alum. Quantitative determina- 

 tions have shown that the amount of ammonia 3n contained in air 

 varies at different periods. However, it may be accepted that 100 cubic 



"* The tension of the aqueous vapour in the air is determined by hygrometers and 

 other similar methods. It may also be determined by analysis (see Chapter I. note 1). 



35 In determining the weight of small and heavy objects (crucibles, &c. in analysis, and 

 in determining the specific gravities of liquids, &c.) a correction may be introduced for 

 the loN.-i of trchjJit in the air of the room, by taking the weight of a litre of air displaced 

 as ra gram, and consequently 0'0012 gram for every cubic centimetre. But if gases or, 

 in general, large vessels are weighed, and the weighings have to be accurate, it is neces- 

 sary to take into account all the data for the determination of the density of the air 

 (t, H, and/), because sensitive balances can determine the possible variations of the 

 weight of air, as in the case of a litre the weight of air varies in centigrams, even at a 

 constant temperature, with variations of H and /. The following method was long ago 

 (1859) proposed and applied by me for this purpose. A large light and closed vessel is 

 taken, and its volume and weight in a vacuum are accurately determined, and verified from 

 time to time. On weighing it we obtain the weight in air of a given density, and by sub- 

 tracting this weight from its absolute weight and dividing by its volume we obtain the 

 density of the air. 



56 Schloesing studied the equilibrium of the ammonia of the atmosphere and of the 

 rivers, seas, &c., and he showed that its amount is interchangeable between them. The 



