After some weeks, or perhaps months, a white ring may show itself on the surface of the liquid on the glass. This is a crown formed by a mass of young yeast globules, which grow there like a mould, by absorbing the oxygen of the air as it enters the flask. The bulk of the yeast which has been fermenting remains at the bottom of the liquid in the form of an inert deposit. This inertness, however, is apparent rather than real; the globules may be internally active, without any development of new buds, and the effects of this working may cause them to become more and more languid, and in the course of time may even destroy them. The case is quite different when the yeast with which our wort is impregnated, instead of being pure, is mixed to any extent whatsoever with diseased ferments. Should there be any of these in the yeast with which we impregnate our wort, even though their quantity were so infinitesimal that the most skilful observer could scarcely discover them with the microscope, they would multiply in the flask after the beer had been finished, especially if the beer were left for a short time on a stove. In this manner we may secure an excellent test of the original purity of the yeast which we employ in the impregnation of our wort. Thus it may be seen that the absence of microscopic organisms that are foreign to the nature of pure yeast may invariably be noticed in the case of a beer which is sound, and which will remain sound for any length of time, when in contact with pure air, at any temperature. We may see, too, that the presence of these organisms may invariably be detected in an unsound beer, the peculiar unsoundness of which depends upon the peculiar species of the organisms contained in it. It would be difficult to adduce clearer proofs than those which we have given as to the intimate relation existing between these organisms and the deterioration of beer. The relation between cause and effect, in the succession of physical phenomena in general, is established by proofs that are by no means more decisive. Footnote 16: A statement of this proposition, as far as it concerns beer, appeared first in outline in the author’s _Etudes sur le vin_, published in 1866. Footnote 17: As the deposit in the heated bottles is, as a rule, inconsiderable, it is necessary to exercise some precaution in collecting it. The bottles are taken up; after some days’ rest they are decanted very carefully, with as little shaking as possible, until not more than one or two cubic centimetres (about a tea-spoonful) of the liquid remains at the bottom. The bottles are then shaken vigorously, with the object of collecting the whole of the deposit from the bottom and the sides into this small quantity of liquid; a drop of this is then examined under the microscope. Footnote 18: