Ae ae 
1892.] Some Uses of Bacteria. 993 
them. I have stated that nature is perpetual because the 
same material can be used over and over again. That is true 
in a sense, but not true completely, for you will see with a 
little thought that little by little the soil is being drained of 
its food, little by little the materials in the soil are being 
turned into the ocean. A tree grows, takes out of the soil its 
food, and finally dies. If it falls on to the ground, as I have 
described, the bacteria get at it and grow there until the tree 
eventually becomes wholly incorporated into soil so that it 
can be used once more as plant food. But it may be that the 
tree instead of falling in the forest falls into a river, drifts 
down the river, begins to decay, and eventually goes into the 
ocean. After the products of decomposition are passed into 
the ocean, there is no getting them back tothe soil. “The sea 
will not give up its dead,” and the ocean does not give up the 
nitrogen and the other salts that are gradually being carried 
to it by this process. Or, again, a plant grows and produces 
wheat, produces fruit, produces nuts, and the grain, the fruit, 
and the nuts are taken to the city to be used as food for men. 
The food is used by men, and most of it eventually gets into the 
sewage of the city, is carried down to the river, and from the river 
itis carried into the ocean. So here again through the sewage 
of our cities, the foods which are supplied to our cities are 
being thrown into the ocean and thus the soil is being 
drained of its foods. This process is nota rapid one. It is 
only slowly that the foods are being taken out of the soil and 
carried to the ocean. Nevertheless, it is the constant dropping 
that wears away the rock, and it is easy for us to see that if 
this process goes on age after age, our souls are inevitably 
doomed to exhaustion. You know that’ many fields have 
become sterile, that many farms have been worn out, that 
many gardens are becoming infertile. You cannot cultivate 
your fields as you used to without furnishing them food. In the 
old world this is quite noticeable. Although the constant 
drainage of the soil by these agencies is a slow one, it is a sure 
one, and if there be no way of getting nitrogen and other salts 
back from the ocean to the soil, it would seem that the life of 
all vegetation is inevitably doomed to exhaustion, and with 
