PASTEUR, KOCH, AND OTHERS 303, 
rescarches of Pasteur that the septic property of the atmos- 
phere depended, not on oxygen or any gaseous constituent, 
but on minute organisms suspended in it, which owed their 
energy to their vitality, it occurred to me that decomposition 
in the injured part might be avoided without excluding the 
air, by applying as a dressing some material capable of de- 
stroying the life of the floating particles.” At first he used 
carbolic acid for this purpose. ‘‘The wards of which he had 
charge in the Glasgow Infirmary were especially affected by 
gangrene, but in a short time became the healthiest in the 
world; while other wards separated by a passageway retained 
their infection.” The method of Lister has been universally 
adopted, and at the same time has been greatly extended and 
improved. 
The question of immunity, 7.e., the reason why after 
having had certain contagious diseases one is rendered 
immune, is of very great interest, but is of medical bearing, 
and therefore is not dealt with here. 
Bacteria and Nitrates.—One further illustration of the 
connection between bacteria and practical affairs may be 
mentioned. It is well known that animals are dependent 
upon plants, and that plants in the manufacture of protoplasm 
make use of certain nitrites and nitrates which they obtain 
from the soil. Now, the source of these nitrites and nitrates 
is very interesting. In animals the final products of broken- 
down protoplasm are carbon dioxide, water, and a nitrog- 
enous substance called urea. These products are called 
excretory products. The animal machine is unable to utilize 
the energy which exists in the form of potential energy in 
these substances, and they are removed from the body. 
The history of nitrogenous substance is the one which at 
" present interests us the most. Entering the soil, it is there 
acted upon by bacteria residing in the soil, these bacteria 
possessing the power of making use of the lowest residuum 
