14 Composition of the Atmosphere 



He evidently wished to take into consideration not only the tempera- 

 ture of the air examined, but the possibility of differences in expansion, 

 and referred to one of his researches indicating such a difference. 



About the time that his book was published, von Humboldt started 

 on a scientific expedition to Spanish America, and in a letter written by 

 him to Delam^therie 1 from Cumana, South America, he shows his intense 

 interest in the composition of the air, and in the possible sources of con- 

 tamination or alteration of the oxygen content, by citing his experiment 

 with a sample of air which he had collected in a bottle from the crater of 

 a volcano. After having determined the purity of his nitric oxide by 

 means of ferrous sulphate, he found only 19 per cent of oxygen in this 

 sample, while at the sea-level the oxygen content was 27.8 per cent. 

 The idea of a geographical difference in composition of the air was also 

 evidently present, since he cites the fact that he was able to analyze the 

 air on board ship with as much ease as in his laboratory, and found that 

 the sea air at 10 30' "on a beautiful moonlight night" contained over 30 

 per cent of oxygen. So firm was von Humboldt's belief in the nitric-oxide 

 eudiometer that he was in constant polemical discussion with Berthollet 

 with regard to the relatively new phosphorus eudiometer. 



As facility in experimental technique was acquired by scientists in the 

 new field of pneumatic chemistry, errors in the nitric-oxide eudiometer 

 were early recognized and it is not surprising to find Seguin 2 stating that 

 this method has 20 different errors. Even von Humboldt, 3 its most ar- 

 dent supporter, pointed out that nitric acid of different strengths yielded 

 nitric oxide of different character which combined with different amounts 

 of oxygen from the air. A few years later 4 he acknowledged completely 

 the errors of the method, which he then discarded for the hydrogen eudi- 

 ometer of Volta. Subsequently, Berger 5 in Geneva laid especial em- 

 phasis upon the errors of the nitric-oxide method and opposed von Hum- 

 boldt's belief that the nitric-oxide solution in ferrous sulphate was a 

 suitable reagent for accurate oxygen determinations, concluding that the 

 phosphorus eudiometer was very much superior. 



Although the original nitric-oxide method of determining oxygen was 

 destined to be relegated to the ranks of impracticable chemical operations, 

 the experience with it naturally led to the employment by Davy 6 of a 

 solution of nitric oxide in ferrous sulphate. This means of testing the 

 purity of the nitric oxide had been advocated by von Humboldt, and 

 Davy first made use of the oxygen-absorbing power of such a solution to 

 analyze air. Davy strongly criticized the old Fontana nitric-oxide eudiom- 

 eter, but pointed out that 1 c.c. of a reasonably strong solution of ferrous 



1 von Humboldt, Gilbert's Annalen der Physik, 1800, 4, p. 443. 



2 Seguin, Annates de Chimie, 1791, 9, p. 293. 



3 von Humboldt, Annales de Chimie, 1799, 28, p. 123. 



4 von Humboldt and Gay-Lussac, Journal de Physique, 1805, 60, p. 129. 



5 Berger, Journal de Physique, 1802, 56, p. 253. 



6 Davy, Journal of the Royal Institution, 1802, I, p. 45. 



