813 



COSMOS. 



may probably be considered as older than the existence at 

 organic beings on the surface of the earth. The sources from 

 \vhich carbonic acid* may be yielded to the atmosphere, are 

 most numerous. In the first place we would mention the 

 respiration of animals, who receive the carbon which they 

 inhale from vegetable food, whilst vegetables receive it from 

 the atmosphere ; in the next place, carbon is supplied from the 

 interior of the earth in the vicinity of exhausted volcanoes and 

 thermal springs, from the decomposition of a small quantity of 

 carburetted hydrogen gas in the atmosphere, and from the elec- 

 tric discharges of clouds, which are of such frequent occurrence 

 within the tropics. Besides these substances which we have 

 considered as appertaining to the atmosphere, at all heights 

 that are accessible to us, there are others accidentally mixed 

 with them, especially near the ground, which sometimes in 

 the form of miasmatic and gaseous contagia, exercise a noxious 

 influence on animal organisation. Their chemical nature has 

 not yet been ascertained by direct analysis ; but from the con- 

 sideration of the processes of decay which are perpetually 

 going on in the animal and vegetable substances with which 

 the surface of our planet is covered, and judging from analo- 

 gies deduced from the domain of pathology, we are led to 

 infer the existence of such noxious local admixtures. Ammo- 

 niacal and other nitrogenous vapours, sulphuretted hydrogen 

 gas, and compounds analogous to the polybasic ternary and 

 quaternary combinations of the vegetable kingdom, may pro- 

 duce miasmata,f which under various forms may generate 

 ague and typhus fever (not by any means exclusively on wet 

 marshy ground, or on coasts covered by putrescent mollusca, 

 and low bushes of Rhizophora mangle and avicennia.) Fogs, 

 which have a peculiar smell at some seasons of the year, 

 remind us of these accidental admixtuies in the lower strata 



* In this enumeration, the exhalation of carbonic acid by plants 

 during the night, whilst they inhale oxygen, is not taken into account, 

 because the increase of carbonic acid from this eource is amply counter- 

 balanced by the respiratory process of plants during the day. See Boussin- 

 gault's Econ. rurale, t. i. pp. 53-68, and Liebig's Organisclie Chemie, 

 B. 16, 21. 



f Gay-Lussac, in Annalcs de Chimie, t. liii. p. 120 ; Payen, Mem, 

 tur la composition chimique des Vegetaux, pp. 36, 42 ; Liebig, Org, 

 Chemie, s. 229-345 ; Boussinfault, Econ. rurale, t. i. pp. 142-153. 



