-1859] WRITINGS OF JOSEPH HENRY. * 163 



we find that the atmosphere consists of 20'01 parts of oxygen 

 to 75'29 of nitrogen in volume, or 23'01 parts, by weight, of 

 oxygen and 76'39 of nitrogen. These numbers are not pre- 

 cisely those which would result from a chemical union, as 

 was at first supposed, namely, one volume of oxygen and 

 four of nitrogen. They are not also entirely invariable, but 

 are found to differ slightly at different places at the level of 

 the sea. Observation has not shown any appreciable varia- 

 tion from year to year, though it is not improbable that dur- 

 ing the geological periods changes have taken place in its 

 proportions as well as in its amount. The quantity of car- 

 bonic acid is found, by the mode we have described, to vary 

 from the yoVo ^^ two ^^ ^^e weight of the whole. 



Oxygen, as we have seen in the exposition of the atomic 

 theory, is a very energetic element widely diffused through 

 nature, and performs an important part in the transforma- 

 tions of inert matter into plants and animals, and back again 

 into carbonic and other inorganic compounds. The nitro- 

 gen also is an important element in vital economy, and is 

 associated with all the most instable organic compounds. 

 Its atoms appear to exert a great repulsive energy on each 

 other; and hence, when confined in a solid state by sur- 

 rounding atoms of other substances, the slightest jar will 

 overturn the instable equilibrium, and produce a violent ex- 

 plosion. 



Carbonic acid is a transparent substance that is produced 

 when charcoal is burnt in air or oxj^gen, and is composed 

 of one atom of the former to two of the latter, or three parts 

 of one to eight of the other by weight. It furnishes the car- 

 bon of the plant, and though it exists in small quantities in 

 the atmosphere, animal and vegetable life could not be con- 

 tinued on the surface of the globe without it. The quantity 

 of carbonic acid contained in the air varies between the 

 hours of night and day, the quantity being at its maximum 

 towards morning, and its minimum towards the middle of 

 the day. In this respect it follows a law analogous to that 

 of the heat and moisture of the atmosphere. A part of this 

 variation may be referred to the absorption of carbonic acid 



