Life and Writings of Tlieodore de Saussure. 31 



parts in 10,000, according to 104 observations, made both by 

 day and night, at four feet above the ground. The maximum 

 was 5*75, the minimum 3*15. Rainy weather, in general, 

 diminishes the proportion of the atmospheric carbonic acid ; 

 continued frost, on the contrary, like every other cause of dry- 

 ness, tends sensibly to increase it. The air on the lake con- 

 tains less of it than the air on the ground, whatever be the 

 season or hour of the day. There is more of it during the 

 day in a town than in the country, and it increases more un- 

 der the influence of the night in the country than in a town. 

 There is more of it on the mountains than in the plains ; 

 during a strong wind than in a calm. There is more atmo- 

 spheric carbonic acid in the country in the night than in the 

 day, in the proportion of from 4*32 to 3'98, taken as the mean 

 of 54 comparative observations. The most considerable and 

 sudden changes take place between the end of the night and 

 the earliest hours of the day ; and the natural explanation 

 of this is found in the decomposition, by means of light, of the 

 carbonic acid by the green parts of vegetables. A more than 

 usually intense atmospheric electricity tends to diminish the 

 proportion of carbonic acid in the air. Such, in a condensed 

 view, are the results obtained by Theodore de Saussure ; and, 

 although they are of real interest, how many look upon them 

 as disproportionate to the immense labour they cost ! 



After thus determining exactly the proportions and varia- 

 tions of the atmospheric carbonic acid, Theodore de Saussure 

 proposed a new method of estimating, in a more certain way 

 than the previously known processes admitted of, the proportion 

 of oxygen existing in the air. Such was the object of his 

 memoir on the use of lead in eudiometry. Taking advan- 

 tage of the property possessed by moistened small shot, made 

 of lead, to absorb the oxygen of the air at the ordinary tem- 

 perature, he introduces a known quantity of air into a mattrass, 

 shuts the stop-cock, then shakes the vessel with the moist 

 small-shot in it for three hours, and, when the process 

 appears completed, from the greyish tint assumed by the 

 yellow oxide of lead first produced, he terminates the opera- 

 tion by weighing the vessel, and measuring, under water, the 

 proportion of gas which has been absorbed. The series of 



