On March 9th, that is, after having been subjected to a heat of 20° C. (68° F.) in the oven for four days, fermentation began to manifest itself by the appearance of patches of froth on the surface of the wort. From this it was evident that the yeast had not been destroyed, but only retarded in its revival. On July 25th, 1873—that is, after a lapse of seven months and a half—we resumed our experiments, and sowed some more of the yeast-and-plaster powder in another flask of wort. On August 2nd, eight days from the time of our sowing, the little islets of froth appeared on the surface of the liquid. Observed under the microscope, the yeast still seemed pure, and resembled the original yeast; we append a sketch, which will give an idea of its shape (Fig. 15). [Illustration: Fig. 15.] On November 7th, 1873, we once more sowed some of the powder. This time the yeast was dead; we observed the flask which contained it, day by day, until February 1st, 1874, without detecting the slightest sign of fermentation or development of the yeast that we had sown. On February 1st, we made a microscopical examination of the yeast, and found it mixed with the plaster and absolutely inert at the bottom of the saccharine liquid; its cells were isolated, very old-looking and granulated, without any appearance that might denote the possibility of their ever budding again. Thus we determined that alcoholic ferment may be dried at the ordinary temperature of the atmosphere, and preserved, in the form of dust, for a period of seven months or longer, without losing its faculty of reproduction. This faculty evidently diminishes in the course of time, for our dried yeast, after having been kept for seven months and a half—all the other conditions of the two experiments having been precisely the same—required about eight days to develop sufficiently to reveal fermentation, whilst, immediately after the drying, it only required three or four days to accomplish the same thing. [Illustration: Fig. 16.] Side by side with these experiments on alcoholic ferment, we carried on exactly similar ones with yeast obtained from “high fermentation” breweries. On December 16th, 1872, we prepared a powder of this yeast and plaster as before. Our last sowing took place on July 25th, 1873, in a flask of pure wort, which showed signs of fermentation on July 27th. We append a sketch (Fig. 16) which gives the general aspect of this “high ferment,” when revived after such a lapse of time; it had preserved the distinctive features of the cells of “high ferment.” These facts can leave no doubt whatever as to the possibility of cells of yeast existing, in a state of suspension in the air, in the form of fine dust, particularly in a laboratory where researches on alcoholic fermentation are pursued. Footnote 21: We have heard of liquids even less sensitive than these, which required a temperature of 120° C. (248° F.) or more, but we have had no opportunity of studying them.