THE BACTERIA IN GENERAL 27 



manure, especially horse manure, in the construction of hot beds for 

 the cultivation and forcing of young plants. In silos, the highest 

 temperature recorded during the fermentation of the ensiled material 

 was 7o°C. but the best silage is secured by keeping the temperature 

 below so°C. Sometimes this spontaneous heating increases to the point 

 of actual ignition (spontaneous combustion) and it may occasionally 

 happen that such substances, as baled cotton, may be set on fire in this 

 way, for Cohn found in damp cotton waste a Micrococcus which, when 

 furnished with a plentiful supply of air, raised the temperature of the 

 decaying mass to 6'j°C. 



Aerobic and Anaerobic Organisms. — Another useful division of 

 bacteria is into those which are aerobic, requiring oxygen for their 

 growth, and anaerobic, those which are indifiEerent to the presence of 

 oxygen. The process of respiration in the aerobes is the same as in 

 all ordinary organisms. Contrasted with the obligatory aerobes, we 

 have those which thrive only in the absence of oxygen (obligatory 

 anaerobes). The growth of some of the latter is inhibited by small 

 traces of oxygen {Bacillus tetani and some butyric organisms). One 

 of the classic experiments in biology was devised by Engelmann 

 (Botanische Zeitung, 1881 and 1882) to detect minute traces of free 

 oxygen. It is a well-known fact that in the process of photosynthesis, 

 or carbon fixation, by green plants that free oxygen is formed. Experi- 

 ments have shown that not all the rays of the spectrum are equally 

 effective in causing this chemic change. The red rays between Fraun- 

 hofer's lines B and C are most effective and after them those just 

 beyond the F line. It is these rays that are most active in the evolution 

 of oxygen. Engelmann reasoned, that if a green alga was placed under 

 the microscope and illuminated from below by a spectrum, so that the 

 algal filament paralleled the band of spectrum colors, that if aerobic 

 organisms were introduced into water beneath the cover glass, these 

 aerobic organisms would congregate in greatest numbers along the 

 green alga at those points illuminated by the rays most effective in 

 oxygen evolution by the plant. His anticipations were realized for 

 he found a grouping of the aerobic bacteria in the neighborhood of the 

 B and C Fraunhofer lines and beyond the F line, where theory told him 

 to expect the greatest photosynthetic activity. Such minute quan- 

 tities of oxygen must be formed by a filamentous green alga, that this 

 experiment becomes a microchemic test for the gas. 



