ORGANIC MANURES. 57 



artificial soda water and to mineral springs, (as the Sara- 

 toga,) their sparkling appearance and acid flavor. It 

 abounds in certain caves, sunken pits, and wells, which de- 

 stroy animal life, not from any intrinsic poisonous qualities, 

 but from its excluding oxygen, which is essential to respi- 

 ration. And it is from the same cause, that death ensues 

 to such as are confined in a close room where charcoal is 

 burnt. 



This acid is an active and important agent in the inces- 

 sant changes of nature. It is everywhere formed in vast 

 quantities, by subterranean fires and volcanoes. Though 

 heavier than atmospheric air, it mingles with it, and is car- 

 ried as high as examinations have yet been made, consti- 

 tuting in bulk, about one part in 1,000 of the atmosphere, 

 and something more than this in weight. Gay Lussac 

 ascended in a balloon 21,735 feet, and there filled a bottle 

 with air, which analysis showed to be identical in composi- 

 tion with that on the surface of the earth. Carbon is one 

 of the great principles of vegetation, and it is only as car- 

 bonic acid, that it is absorbed by the roots, leaves and stems 

 of vegetables, and by them is condensed and retained as 

 solid matter. 



Oxygen, hydrogen and nitrogen, when uncombined with 

 other substances, exist only as gases. The first makes up 

 nearly one half of all the substances of the globe ; and with 

 the exception of chlorine and iodine, it constitutes a large 

 part of every material in the ash of plants. It forms rather 

 over 21 per cent, by measure, and 23 by weight, of the whole 

 atmosphere ; and about eight parts out of nine, by weight, of 

 water, hydrogen making up the remainder. It is absorbed 

 and changed into new products by the respiration of animals, 

 and it is an essential agent in combustion. Oxides are com- 

 posed of it, in union with the metals and alkalies ; and most 

 of the acids, when it is combined with other substances, as 

 nitrogen, sulphur and phosphorus. Its presence, indeed, is 

 almost universal, and the agency which it exerts in vegeta- 

 ble nutrition, is among the most varied and intricate mani- 

 fested in vegetable life. 



Hydrogen is the lightest of all the gases. It is but r 4 

 the weight of the atmosphere, and n, the weight of oxy- 

 gen ; and from its great levity, it is used for filling balloons. 

 On applying a lighted taper, when brought into contact 

 with atmospheric air, it burns with a light flame, the com- 

 bustion forming water. 



