108 THE LIFE OF A TREE, 
This may not be thought to be very important 
for us to consider; but what would the reader 
say if we were to cut off all his supply of milk, 
of cheese, and of fresh meat,—and how would he 
thrive without them? Lach of these articles of 
our food is produced actually from the ammonia 
of the air in the following manner :—First, grass 
and other plants receive ammonia by means of rain 
as food for them, and make use of the nitrogen it 
contains for their purposes. Second, the cow and 
the sheep eat up the grass and other plants, and 
they make use of the nitrogen they find in this 
sort of food for their purposes; for example, to pro- 
duce milk, and the ingredient which forms cheese 
contained in milk, and, finally, to nourish their 
flesh. Lastly, man consumes the milk, and the 
cheese, and indeed the animals which form them 
for iis food; and the nitrogen they contained 
goes to strengthen and support his body. There- 
fore we might actually say, that we ourselves 
partly dive upon air; for one of its ingredients, 
nitrogen, by means of these different channels, 
comes to us as the most important element for 
our nourishment. This is a beautiful link of 
chemical processes, and well deserves the reader’s 
