THE VOLATILE PART OF PLANTS. 39 
Nitrogen cannot maintain respiration, so that animals 
perish if confined in it. For this reason it was formerly 
called Azote (against life). Decay does not proceed in an 
atinosphere of this gas, and in general it is difficult to ef- 
fect its direct union with other bodies. At a high tem- 
perature, especially in presence of baryta, it unites with 
carbon, forming cyanogen—a compound existing in Prus- 
sian-blue. 
The atmosphere is the great store and source of nitrogen 
in nature. In the mineral kingdom, especially in soils, 
it occurs in small quantity as an ingredient of saltpeter 
and ammonia. It isa small but constant constituent of 
all plants, and in the animal it is a never-failing component 
of the working tissues, the muscles, tendons and nerves, 
and is hence an indispensable ingredient of food. 
Hydrogen.— Water, which is so abundant in nature, 
and so essential to organic existence, is a compound of 
two elements, viz.: oxygen, that has already been con- 
sidered, and hydrogen, which we now come to notice. 
Hydrogen, like oxygen, is a gas, destitute, when pure, 
of eithér odor, taste, or color. It does not occur naturally 
in the free state, except in small quantity in the emana- 
tions from boiling springs and volcanoes, Its preparation 
almost always consists in-abstracting oxygen from water 
by means of agents which have no special affinity for hy- 
drogen, and therefore leave it uncombined. 
Sodium, a metal familiar to the chemist, has such an at- 
traction for oxygen that it decomposes water with great 
rapidity. 
Exe. 11.—Hydrogen is therefore readily procured by inverting a bot- 
tle full of water in a bowl, and inserting into it a bit of sodium as large 
asapea. Thesodium must first be wiped free from the naphtha in” 
which it is kept, and then be wrapped tightly in several folds of paper. 
On bringing it, thus prepared, under the mouth of the bottle, it floats 
upward, and when the water penetrates the paper, an abundant escape 
of gas occurs. 
Metallic iron and zine decompose water, uniting with 
