NITROGEN AND All: '233 



a tube, which is previously weighed, and contains shavings of metallic 

 copper. A long layer of such copper heated to redness absorbs all 

 the oxygen from the air, and leaves pure nitrogen, whose weight 

 must be determined. This is done by collecting it in a weighed and 

 exhausted globe, and the amount by weight of oxygen is shown by the 

 increase in weight of the tube with the copper after the experiment. 



Air free from moisture and carbonic anhydride 22 contains 23*15 

 parts of oxygen and 76*85 of nitrogen by weight, 23 which, taking the 

 density of oxygen =16 and of nitrogen =14, gives the volumetric 

 composition of air as 2O84 volumes of oxygen and 79*16 of nitrogen. 24 



The possibility of the composition of air being altered by the mere 

 action of a solvent very clearly shows that the component parts of air 

 are in a state of mixture, in which any gases may occur ; they do not 

 in this case form a definite compound, although the composition of the 

 atmosphere does appear constant under ordinary conditions. The fact 

 that its composition varies under different conditions confirms the 

 truth of this conclusion, and therefore the constancy of the composition 

 of air must not be considered as in any way dependent on the nature 

 of the gases entering into its composition, but only as proceeding from 

 cosmic phenomena co-operating towards this constancy. It must be 

 admitted, therefore, that the processes evolving oxygen, and chiefly the 

 processes of the respiration of plants, are of equal force with those 

 processes which absorb oxygen over the entire surface of the earth. 25 



22 Air free from carbonic anhydride indicates after explosion the presence of a 

 small quantity of carbonic anhydride, as De Saussure remarked, and air free from moisture, 

 after being passed over red-hot copper oxide, seems invariably to contain a small 

 -quantity of water, as Boussingault has observed. These observations cause one to think 

 that air always contains a certain quantity of gaseous hydrocarbons, like marsh gas, 

 which, as we shall afterwards learn, is evolved from the earth, marshes, &c. Its amount, 

 however, does not exceed some hundredths of a per cent. 



23 The analyses of air are accompanied by errors, and there are variations of composi- 

 tion attaining hundredths per cent. ; therefore one must limit oneself to the first places 

 in decimals in expressing the average normal composition of air. 



24 The weight of a litre of hydrogen at and 760 mm. pressure is 0'08958 gram, 

 therefore 20'8 litres of oxygen weigh 29'87 grams, and 79'2 litres of nitrogen 99'28 grams, 

 which gives the weight of a litre of air as T2914, instead of T293. This difference corre- 

 sponds with the possible errors of both the analysis of air and of the other data entering 

 into the calculation. 



25 In Chapter III. note 4, an approximate calculation made for the determination of 

 the amount of oxygen in the entire atmosphere is evidently without solid foundation 

 that is, it may be supposed that the composition of air varies from time to time when the 

 relation between vegetation and the processes absorbing oxygen changes ; but such a 

 supposition may be met by an argument of the following kind : the atmosphere of the 

 arth has not, and should not have, a definite limit, and we have already seen (Chapter IV. 

 note 88) that there are observations confirming this, consequently ourVtmosphere should 

 vary in its component parts with the entire heavenly space. If the equilibrium now exist- 

 ing were destroyed, it would be rectified by means of the immense mass of rarefied air 



