PASTEUR, KOCH, AND OTHERS 303 



researches 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 hrst 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, i.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 



