819 
its Relations with Agriculture. 
link which connects us with our food-producers belongs to chemical, 
and only incidentally to mechanical, science. 
As is well understood, failure would attend any attempt to pro- 
duce a crop from the moistened mineral matter which enters into the 
composition of the soil. For the purposes of fertility the presence of 
organic matter is necessary, the main object of which is to assist in 
the assimilation of the earthy substances which enter into the compo- 
sition of what are known as the ashes of the plant. These are small 
in quantity, but are indispensable for the existence of vegetable life. 
Not the least remarkable phenomenon in connection with the 
production of what may be regarded as the origin of all food is the 
enormous area from which plants have to gather their nutrition 
and, therefore, their substance. Towards this the soil contributes 
little or nothing, and it is from our vast atmosphere itself that all 
the carbon which enters, to the extent of 3-5 to 50 per cent., into 
most vegetable matter is derived. This element, in the form of 
carbonic acid gas, is found in the air we breathe to the extent of 
only 4 volumes in 10,000. Notwithstanding this mere trace, it has 
been estimated that in our atmosphere there is stored up more 
carbon than is contained collectively in the earth’s surface in the 
solid form, in the bodies of plants and animals, and under the earth’s 
solid crust in the coal formation. 
Before I proceed to the more immediate objects of this paper, I 
will say a word or two on another constituent of vegetable life, which 
indeed may perhaps be regarded as covered by the title. I refer to 
the metal iron. U sually, the ash of plants doos not exceed 2^ per cent, 
of their weight, that of wheat, according to Boussingault, ranging 
from 2'34 to 2'43 per cent. Of this, possibly 2^ per cent, only is 
oxide of iron. Out of this small beginning, I remember Hope, of 
the Edinburgh University, or Thenard, of the Sorbonne, telling us 
nearly sixty years ago that an adult human body extracts from the food 
only as much iron as would make a wedding ring. In McKendrick’s 
Physiology the actual quantity is stated to be 3 grammes, or say 
46| grains. The metal in question is chiefly to be found in the 
blood, and upon the proper changes in point of oxidation of this 
minute quantity is animal life entirely dependent. Introduce a 
substance sufficient in quantity to interfere with this series of alter- 
nations, and death may be almost instantaneous. As an example, a 
very small volume of sulphuretted hydrogen — probably one in 300 
or 400 of air — would vitiate all the blood in a human being in the 
space of 25 seconds. 
The two ingredients which enter into the composition of all 
animals, and therefore into that of the vegetables upon which 
animals feed, and which concern us at present, are nitrogen and 
phosphorus, and it is to these that the remainder of this paper is 
exclusively directed. 
Nitrogen . — Although nitrogen constitutes fully three-fourths 
of the air we breathe, it was long considered that neither vege- 
table nor animal was able to assimilate, by direct absorption, any 
