68 
The four latter foods (being the ingredients of the primitive 
rock) are abundant in most soils, and as the supply ot Carbon, 
Hydrogen and Oxygen is usually fully provided for by means 
of rain and the atmosphere, the chief question affecting the well 
being of a plant, once the soil has been depleted of its natural 
humus, is a supply of available Nitrogen. This element con- 
stitutes the chief bulk of our atmosphere, but plants are unable 
to assimilate it in a free state, that is, unless it is first 1 chemically 
combined with another element. Although surrounded by an 
inexhaustible supply of this necessary food, vegetation therefore 
will languish and die unless some means is at hand to render it 
available. This is generally accomplished by the chemical com- 
bination of Nitrogen with Hydrogen in the form of Ammonia, 
or with Potash or Soda m the form of one of the well known 
"Nitrates," so largely employed as artificial fertilizers. 
Nitrogen is a necessary constituent of every organic body. 
Although it plays so important a part in the composition of liv- 
ing matter and exists in such inexhaustible quantity in the at- 
mosphere, the free Nitrogen of the air is not drawn upon for 
plant and animal food because it possesses the property of re- 
fusing, under most circumstances, to combine with other sub- 
stances. This peculiarity of Nitrogen is remarkable when we 
consider the readiness with which some other elements combine 
with one another as seen, for instance, in the case of Oxygen. 
The slow combination of this latter gas with some metals is ex- 
emplified in the corrosion or rust of iron and in the tarnishing of 
silver. The more rapid and' energetic combination of Oxygen 
with other substances, produces, as is well known, the phenomenon 
of fire. 
So noteworthy is the inertness of Nitrogen, that Lavoisier, the 
eminent French chemist, in reference to this quality termed it 
Azote, a name signifying "without life," and which is still in 
general use by the French. On account of this property the 
element was long regarded as a more or less useless gas whose 
chief function lay in diluting the atmospheric Oxygen and thus 
rendering it suitable for animal respiration. 
How comes it then, in view of the refusal of Nitrogen to enter 
into combinations with other elements, that this element, so ex- 
tremely insoluble in water, plays such an important part in the 
economy of plant life? What subtle force is at work which 
overcomes its inert quality and renders it suitable for absorption 
1 The distinction between a mechanical and a chemical mixture is 
most important. In a mechanical mixture the ingredients are simply 
mixed together and no new body is formed, while in a chemical mixture 
an entirely new body is produced. The mechanical mixture of Hydrogen 
and Oxygen is an invisible gas, but their chemical mixture produces 
water. In the air, Oxygen and Nitrogen are mixed mechanically, while 
if they were in chemical combination they would produce suffocating am- 
monia and nitric acid gases in which nothing now living could exist. 
