16 Composition of the Atmosphere 



content of air. Lavoisier was infinitely more interested in chemical prob- 

 lems involving combustion in the air than he was in the exact composition 

 of air itself, and hence we find his records of air composition always inci- 

 dental to other important data. 



Thus, in studying the properties of phosphorus, 1 he reports that in a 

 large number of experiments in which an excess of phosphorus was burned 

 in a bell jar containing 109 pouces 2 he found a diminution in volume of 20 

 to 21 pouces, that is, about one-fifth. Lavoisier also noted an increase in 

 the weight of the phosphorus. Of particular interest is the experiment 

 reported in his memoir on the respiration of animals, 3 in which he heated 

 mercury with a confined volume of oxygen. On heating 50 pouces of 

 common air with 4 ounces of mercury, he found at the end of 12 days that 

 the air was diminished one-sixth part. In the same year, 1777, in his re- 

 port of the research on the combustion of a candle, 4 he states that the air 

 contains about one-fourth of its volume of "air pur et respirable." Fi- 

 nally, in 1785, in a report of a study of the alteration of air by respiration, 5 

 he maintained that air contains 25 per cent of oxygen. 



Lavoisier's experience with all known methods for absorbing oxygen 

 justified his critical discussion of the subject 6 in which he maintains that 

 the eudiometers established on the principle which depends upon the 

 great affinity between oxygen and phosphorus, or the alkaline sulphides, 

 or the mixture of iron and sulphur, are much preferable to those of Priest- 

 ley, Fontana, and Ingen-housz using nitric oxide. In the first place, 

 it is always easy to have phosphorus, sulphur, and iron perfectly identi- 

 cal, while nitric oxide always differs in composition. Second, the nitric 

 oxide does not always absorb all of the vital air. Third, this gas is sus- 

 ceptible of different degrees of oxygenation varying with the temperature 

 and pressure, the rapidity of mixture, and the diameter of the vessel. 

 Fourth, the nitric oxide is capable of mixing with nitrogen in all propor- 

 tions. An excess of sulphuret of potash, iron, sulphur, or phosphorus 

 may be used without affecting the results, but with nitric oxide an excess 

 produces an error. Lavoisier finally decided to use sulphuret of potash, 

 preparing the air sample over water and at the end of 15 to 20 days de- 

 termining the contraction in volume. The importance of a general study 

 of the oxygen content of the air on the earth is emphasized in the following 

 paragraph from Lavoisier: 



II est a desirer que quelque physicien ait le courage d'entreprendre, par cette m6thode, 

 une suite d'exp6riences sur Pair atmospherique recueilli dans difterents lieux, dans dif- 



1 Lavoisier, Opuscules Physiques et Chimiques, 1773; Oeuvres de Lavoisier, 1, p. 643. 



1 A cubic pouce equals 19.6 c.c. 



' Lavoisier, Memoires de l'Academie des Sciences, 1777, p. 185; Oeuvres de Lavoisier, 



2, p. 174. 

 4 Lavoisier, Memoires de l'Academie des Sciences, 1777, p. 195; Oeuvres de Lavoisier 



2, p. 184. 

 6 Lavoisier, Recueil d. Memoires de Lavoisier, 3, p. 13; Oeuvres de Lavoisier, 2, p. 676. 

 * Lavoisier, Recueil d. Memoires de Lavoisier, 3, p. 154; Oeuvres de Lavoisier, 2, p. 



715. 



