OF MICR O- OR GANISMS. 33 



Analogous facts have been observed under circum- 

 stances somewhat different. In a preparation from 

 the intestines of a silk-worm, M. Balbiani has seen 

 Bacteria which were uniformly distributed throughout 

 all points of the preparation, gather about the green 

 and undigested cellules of the leaves contained in the 

 intestines, and bury themselves in them as if to par- 

 take of them. In other instances, the same naturalist 

 has observed that Bacteria developed in a drop of 

 silk-worm's blood, would gather, after a while, about 

 the globules of the blood; undoubtedly for the purpose 

 of seizing the oxygen being absorbed by them. 



Upon the basis of these facts M. Engelmann has 

 established the method called the Bacteria method. 

 He regards bacteria as a living reagent which enable 

 us to reveal the trillionth part of a milligram of oxy- 

 gen, that is to say, a quantity scarcely greater, accor- 

 ding to the calculations of physicists, than a molecule. 

 This curious method enables us to explain biological 

 problems which had hitherto remained unsolved. 

 Before this, it was not known whether the colorless 

 protoplasm of green plants could or could not disen- 

 gage oxygen. It is now known, thanks to the bacte- 

 ria, that grains of chlorophyl are the only points about 

 which the liberation of oxygen takes place. The same 

 method has enabled us to prove, in the variegated 

 plants, that the maximum liberation of oxygen coin- 

 cides with the maximum absorption of light. Thus, 

 in the case of green algae, the red and the violet colors 

 of the spectrum are the spots where the bacteria ac- 

 cumulate the thickest: consequently here is where the 

 liberation of oxygen is greatest. Now, these colors 

 correspond to the lines of greatest absorption" in the 

 spectrum of chlorophyl. In the case of brownish yel- 



