7i8 THE POPULAR SCIENCE MONTHLY. 



the danger. It is thus that a jDart of the sulphur, phosphorus, and ni- 

 trogen of the sulphates, phosphates, and nitrogenized substances, return 

 to the atmosphere in the form of gaseous products, hut they will not 

 long remain deleterious, for the oxygen of the air decomposes them 

 incessantly, and transforms them into gaseous products useful to vege- 

 tation. 



I spoke of a fatty, humid pellicle, formed on the surface, and of a 

 milky cloudiness, occupying the whole mass : examine the pellicle 

 under the microscope ; the smallest fragment of it shows us millions 

 of bacteria ; below it, even in the deepest layers, a drop of the liquid 

 presents the vibrios of putrefaction without a trace of bacteria. The 

 bacteria are only on the surface, because they are aerobic, that is, thev 

 need air to live ; the vibrios are below, because they do not need air 

 to live, and, indeed, the layer of bacteria protects them against the 

 approach of the oxygen gas which would be fatal to them if too much 

 of it w^ere held in solution by the liquid. 



While the vibrios transform a large part of the solid materials 

 macerating in our liquid into the gaseous j^roducts of putrefaction, the 

 bacteria of the surface fix a considerable amount of the oxygen of the 

 air upon the carbonic substances held in solution, and now again large 

 quantities of carbonic-acid gas are set free. But, little by little, the 

 medium which at first was so suitable for the nourishment of bacteria 

 and vibrios becomes less so, a change which is announced, especially 

 as to the bacteria, by a steadily-decreasing appropriation of the oxy- 

 gen of the air; then appear, here and there, on tlie surface of the fatty 

 pellicle, greenish, glaucous spots which increase as life becomes less 

 active in the bacteria. These spots are spots of mould, new aerobic 

 beings like the bacteria, and which find in the latter an appropriate 

 food ; little by little, the whole surface of the liquid will thus become 

 covered by difierently-colored moulds. Like the bacteria and themy- 

 codermic pellicles, these moulds also cause much oxygen to combine 

 with the subjacent substances which serve as their food, and again 

 carbonic-acid gas is set free. 



You see, then, that the substances dissolved in the liquid of our 

 reservoir and deposited upon its bottom are constantly becoming gas- 

 eous and mineral. But during the continuance of all these phenomena, 

 which last for months or for years according to the quantity of origi- 

 nal material and the state of the atmospliere, an incessant evaporation, 

 much increased by the heat of the successive combustions of which the 

 surface of the liquid has been the theatre, has removed most of the 

 water contained in the reservoir, and the latter dries, leaving on the 

 bottom an insignificant quantity of each of the substances originally 

 deposited there ; but the combustions go on : here by the moulds, there 

 by bacteria, monads, kolpods. When a mould has exhausted, if w^e may 

 so call it, the appropriation for its life furnished by that portion of the 

 surface or of the material upon which it has lived, it is replaced by an- 



