part Second 



MICROCHEMISTRY. 

 A. Inorganic Compounds. 



i. Oxygen, O 2 . 



63. For the microchemical recognition of oxygen, the 

 method with Bacteria devised by Engelmann (I) may often 

 be used with success. This depends upon the fact that 

 moving Bacteria at once cease their motion if oxygen is 

 withdrawn from them, and immediately resume it on the 

 subsequent renewal of the oxygen supply. Oxygen also 

 affects the direction of motion of Bacteria, since they move 

 toward the fluid which is richest in oxygen. 



It is easy to satisfy one's self of this by placing a drop of 

 fluid containing moving Bacteria on a slide, and covering it 

 with a large cover-glass. The oxygen of the fluid is soon 

 exhausted and the motion continues only at the edges of 

 the cover, or around included air-bubbles, which are espe- 

 cially instructive. It may also soon be seen that the Bacteria 

 group themselves in heaps at these places. 



The sensitiveness of this reaction, which shows very small 

 quantities of oxygen, is naturally dependent in some degree 

 upon the choice of Bacteria. Those which are obtained by 

 letting split peas decay in water are very useful. After a 

 few days innumerable Bacteria appear, which are commonly 

 called Bacterium termo. 



It may be added, with reference to the management of 

 the reaction, that it is usually desirable to use large cover- 

 glasses, whose edges may be sealed with cacao-butter, wax, 



44 



