24 NOTES ON MICRO-ORGANISMS. 
matters, thus bringing them into a form in which they can be 
assimilated by plants, and so enter again into the round of life. 
Hydrogen, nitrogen, sulphuretted hydrogen, and carbonic acid 
gas are also products of decomposition by the intervention of 
organisms, and some other bodies not very easily identified are 
those of the phenol class. Niageli has advanced a theory that the 
decompositions are set up by molecular vibrations within a certain 
radius of the organism, and proceed partly internally, partly 
externally. In the case of all these decompositions, a point is 
reached when the action of the organisms is arrested or checked 
by the nature of their products, which act towards them as 
poisons, whether they be acids, alcohols, or other of the substances 
mentioned. It is the same with the Saccharomycetes or alcoholic 
ferments, and is a very important fact, as will be more apparent in 
its relation to the pathogenic or disease organisms. 
Bacteria thrive best in weakly alkaline solutions, containing 
carbon and nitrogen in the form of carbohydrates and albuminoids. 
After the Bacteria have ceased growing, Saccharomycetes 
forms and moulds may appear. In an acid solution, such as 
grape juice, the sequence is different, the Saccharomycetes de 
veloping first, followed by Bacteria, aerobic ferments, and lastly 
moulds, 
Bacteria in the motile state have an extraordinary affinity for 
oxygen gas. This is shown in an interesting way when a cover- 
glass is placed over a drop of liquid, containing them on a micro- 
scopic slide, especially if air bubbles be enclosed. The moving 
Bacteria flock to the edges of the bubbles, and also to the edge of 
the cover-glass If Alge giving off oxygen are introduced into 
the same drop the Bacteria will collect about them ; so that these 
motile forms constitute rather a delicate test for oxygen. One 
authority, Engelmann, says that the motile condition ceases in 
darkness. It also appears to do so in a very strong light, for if 
flashes be sent through a solution containing the motile forms they 
are seen to start as if they had received a sudden shock, the 
movement ceasing for a few seconds and then recommencing. 
This is certainly extraordinary, and tends to raise the Bacteria in 
