MTI;<H;KN AND .\n; 231 



T) lumlysia of air is effected by converfcing the oxygen into a non- 

 gaseous compound, so as to separate it from the air. The original 

 volume of the air is first measured, and then the volume of the remain- 

 ing nitrogen. The quantity of oxygen is calculated either from the 

 difference between these volumes or by the weight of the oxygen com- 

 pound formed. All the volumetric measurements have to be corrected 

 for pressure, temperature, and moisture (Chapters I. and II.). The 

 medium employed for converting the oxygen into a non-gaseous sub- 

 stance should enable its being taken up from the nitrogen to the very 

 end without evolving any gaseous substance. So, for instance, 19 a mix- 

 ture of pyrogallol, C 6 H 6 O 3 , and a solution of a caustic alkali absorbs 

 oxygen with great ease at the ordinary temperature (the solution turns 

 black), but it is unsuited for accurate analysis because it requires an 

 aqueous solution of an alkali, and it alters the composition of the air 

 by acting on it as a solvent. 20 However, for approximate determina- 

 tions this simple method gives entirely satisfactory results. 



The determinations in a eudiometer (Chapter III.) give much more 

 exact results, if all the necessary corrections for changes of pressure, 

 temperature, and moisture be taken into account. This determination 

 is essentially carried on as follows : A certain amount of air is intro- 

 duced into the eudiometer, and its volume is determined. Then about 



gen, and that the variation in the densities of both gases with the height is determined 

 by the pressure of each gas separately. Details of the calculations and considerations 

 here involved are contained in my work On Barometric Levellings, 1876, p. 48. 



On the basis of the law of partial pressure and of hypsometrical formulae, expressing 

 the laws of the variation of pressures at different altitudes, the conclusion may be deduced 

 that at the upper strata of the atmosphere the proportion of the nitrogen with respect 

 to the oxygen increases, but the increase will not exceed a fraction per cent., even at 

 altitudes of four and a half to six miles, the greatest height within the reach of men either 

 by climbing mountains or by means of balloons. This conclusion is confirmed by the 

 analyses of air collected by Welsh in England during his aeronautic ascents. The ques- 

 tion of the distribution of gases in the upper strata of the atmosphere is of particular 

 importance for understanding in what state the gaseous or vaporous masses occur which 

 are borne in space, and are one of the elementary forms of the heavenly bodies (accord- 

 ing to Laplace's and Kant's theory). I touch on this subject in speaking of the origin of 

 naphtha in my work On the Naphtha Industry, 1879. 



19 The complete absorption of the oxygen may be attained by introducing moist phos- 

 phorus into a definite volume of air ; this is recognised by the fact of the phosphorus 

 becoming non-luminous in the dark. The amount of oxygen may be determined by 

 measuring the volume of nitrogen remaining. This method, however, cannot give accu- 

 rate results, owing to a portion of the air being dissolved in the water ; to the combination 

 of some nitrogen with oxygen ; to the necessity of introducing and withdrawing the 

 phosphorus, which cannot be accomplished without introducing bubbles of air ; and to the 

 numerous corrections of the volume (for moisture, temperature, and pressure), &c. 



20 For rapid and approximate analyses (especially technical and hygienic), such a mix- 

 ture is very suitable for determining the amount of oxygen in mixtures of gases, from 

 which the substances absorbed by alkalis have first been removed. According to certain 

 observers, this mixture evolves a small quantity of carbonic oxide after absorbing oxygen. 



