the danger. It is thus that a part of the sulphur, phosphorus, and nitrogen of the sulphates, phosphates, and nitrogenized substances, return to the atmosphere in the form of gaseous products, but they will not long remain deleterious, for the oxygen of the air decomposes them incessantly, and transforms them into gaseous products useful to vegetation.
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 aërobic, that is, they 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 were held in solution by the liquid.
While the vibrios transform a large part of the solid materials macerating in our liquid into the gaseous products 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 oxygen of the air; then appear, here and there, on the 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 differently-colored moulds. Like the bacteria and the mycodermic 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 gaseous and mineral. But during the continuance of all these phenomena, which last for months or for years according to the quantity of original material and the state of the atmosphere, 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 we 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-
