NITROGEN. 199 



the regular proportion of oxygen 5 so does that obtained in the deepest 

 mines ; that transported from Egypt and the African sands, and from 

 Mont Blanc and Chimborazo, had the same constitution.* 



(b.) This constancy, as has been generally supposed, is maintained 

 by the agency of the vegetable kingdom. See carbonic acid and veg- 

 etables. Living vegetables in the sun's light, give out oxygen gas 

 and decompose carbonic acid for food ; in the night, they absorb 

 oxygen and give out carbonic acid, but Priestley and Davy say, 

 that they give out more oxygen than they consume, and therefore 

 they purify the air. 



(c.) According to Prevost, 100 years would consume only T ^Vo tn 

 part of the weight of the oxygen in the atmosphere, making due allow- 

 ance for all the consuming processes that are going on, and therefore 

 if they had gone on even at the same rate from the creation of man, 

 the consumption would have been but the T ^ part, and doubtless it 

 has not been half of that, that is, o-j-^. Some have supposed, that 

 volcanic fires expel oxygen from various mineral bodies ; some, that 

 nitrogen is absorbed into the bodies of animals, and others, that hy- 

 drogen is obtained by plants from the decomposition of water ; all of 

 which processes would either throw oxygen into the air, or tend to give 

 it a preponderance, but none of these suggestions are proved to be true. 



7. AGENCY IN RESPIRATION. 



(.) Animal life universally, in all its forms, is sustained by the 

 oxygen of the air. 



(6.) The nitrogen appears to be merely a diluent,^ and not to act 

 except under certain peculiar' circumstances, but it is not improbable 

 that it answers some positive purpose in the animal economy, whose 

 nature is not yet understood. 



(c.) The principal effect in respiration, appears to be the abstrac- 

 tion of carbon from the blood. See carbonic acid and respiration. 



7. THERE ARE OTHER BODIES IN THE ATMOSPHERE. 



(a.) Perhaps the only ones that are constant, are carbonic acid, 

 about ToW or sVu an ^ it never exceeds yj^ and aqueous vapor; 

 T^ot by weight. Saussure found carbonic acid at the top of Mont 

 Blanc, and it exists at every height hitherto attained, but the aque- 

 ous vapor varies with the temperature; air at 60 may contain 10 

 grains of water to a cubic foot, and 4.5 at 43, and the quantity in- 

 creases in a high ratio as the temperature is raised. On high mountains, 



* Mr. Faraday's analysis of air from the Arctic regions, shows a decided and con- 

 stant difference between it and the air of London, of at least 1.374 per cent. See 

 Appendix to Parry's 3d voyage, Lond. Ed. p. 240. No explanation is given to ac- 

 count for the cause of this difference, but ! have little doubt that it is owing to the 

 deficiency of vegetation in high northern latitudes. (Communicated.) J. T. 



t \Ve cannot be positive on this point ; it is certainly possible that it has some more 

 important agency. 



t Mr. Dalton found it rather more than this in the air which an assembly of two 

 hundred people had breathed for more than two hours. 



