10 Composition of the Atmosphere 



Among other early modifications of the nitric-oxide eudiometer are 

 those of Magellan. 1 These are also referred to in a postscript in a letter 

 to Priestley, 2 dated London, November 30, 1776, in which he writes: 



The other contrivances I want to show you are two new eudiometers to measure the 

 degree of the salubrity of the air in different places. One of them, which I reckon the 

 better, being the more simple and the neater of the two, is according to the original 

 idea of your experiments on this subject; but in neither of them do I make use of any 

 cock, both on account of its being difficult to be made, and likewise subject to be out of 

 order. Those of Mess, the Chevalier Landriani and the Abbe Fontana seem to be liable 

 to this inconvenience. The experiments already made in most parts of Italy by the 

 former of those gentlemen, with his own eudiometer, deserve the greatest praises; and 

 it is to be wished that philosophers would more generally apply themselves to this inter- 

 esting subject of inquiry. 



Dobson's 3 instrument was used for analyzing the air of "sea weed 

 pods" and also, for comparison, the air of Liverpool. 



Lavoisier 4 reports three experiments made with nitric oxide in which 

 he found 25.3, 25, and 25.2 per cent of oxygen, respectively, an agreement 

 which he says he had not dared to hope for. From these results, therefore, 

 he concludes that the atmosphere, as he had previously announced, con- 

 sisted of about 3 parts of mephitic air and 1 part of vital air. In 1777 

 Lavoisier made determinations of the quantity of vital air contained in 

 the atmosphere and found it to be about 27.5 parts in 100, but maintains 

 it is possible that this larger quantity of vital air depends upon the season. 



Undoubtedly this quest for air with the highest oxygen content led to 

 innumerable analyses of atmospheric air in the latter part of the eight- 

 eenth century which otherwise would not have been made. We find that 

 Cavendish is reported as having made over 500 analyses of air by the 

 nitric-oxide eudiometer before 1 790. 5 Similarly the method was employed 

 "daily for three years" by de Saussure 6 in a comparison of this method 

 with the phosphorus eudiometer which was later to play an important role 

 in air-analyses. 



While many observers early found difficulties in the nitric-oxide 

 method and soon discarded it for other methods, it seems to have been 

 reserved for the genius of Cavendish so to adjust the conditions of experi- 

 mentation with this agent as to secure approximately accurate infor- 

 mation regarding the proportion of oxygen and nitrogen in the air. His 

 results are marvelously accurate when judged by analyses made by the 

 most approved methods of modern times. Cavendish was more interested 



1 J. Hyacynth de Magellan, Description of a glass apparatus for making mineral 

 waters like those of Pyrmont Spa, Seltzer, Seydschnitz, etc., together with the descrip- 

 tion of some new eudiometers, etc., London, 1777. 



1 Priestley, loc. cit., 1777, 3, p. 379. 



See Dobson's letter to Priestley, loc. cit., 1779, 4, p. 469. 



4 Lavoisier, Memoires de l'Academie des Sciences, 1782, p. 486. See also Oeuvres 

 de Lavoisier, 1862, 2, p. 503. 



6 Wilson, Life and works of Cavendish, London, 1851, p. 227. 



5 de Saussure, Journal de Physique, 1798, 47, p. 470. See also Gilbert's Annalen 

 der Physik, 1799, 1, p. 505. 



