This process of impregnation, therefore, does not afford us sufficient safeguards, but by means of the following device we shall render it more satisfactory. Let us take a series of flasks shaped as in Fig. 17, containing an organic liquid suitable to the development of fungoid growths, that is to say, slightly acid—yeast-water, plain or sugared, the wort of beer, or Raulin’s fluid[52] will answer our purpose; let us boil the liquid, and having previously drawn out the necks, let us close the ends in the flame of a lamp whilst the steam is escaping, as soon as we judge that the air has been nearly all expelled. Having prepared ten or twenty of these flasks in this manner, when they are cold we may break their points in any place we may be in. The air will rush into the flasks, and we must then seal them up again in the flame of the lamp, and put them aside for future observations. In a certain number of these flasks, as we have already explained in our experiments carried on after this fashion, we shall see some fungoid growths appear, first in the shape of flakes of _mycelium_ floating in the liquid, and afterwards coming to the surface to fructify. Now, it often happens that _penicillium glaucum_ appears alone, so numerous are the spores of this fungus floating in the air. Under such conditions, we shall evidently obtain a field of sporanges quite free from the presence of other organisms. If we now take off the neck of one of the flasks containing the pure _penicillium_, and take out some of the germs with our platinum wire,[53] we shall thus obtain with most certainty spores of _penicillium_ free from impurities. Our readers will excuse the length of these details and the minutiæ of our precautions, but we shall again and again see that to neglect them, or any part of them, is to expose ourselves to hazard in drawing sure conclusions from facts which come under our observation. [Illustration: Fig. 18.] On June 17th, 1872, we placed some pure spores of _penicillium_ in a series of three flasks containing wort (Fig. 18), observing all the precautions that we have indicated. We shall designate these flasks by the letters A, B, C. On the following day the spores germinated, and the liquid became full of flakes of mycelium, some of which came to the surface to fructify. The temperature varied between 25° C. and 30° C. (77° to 86° F.). On June 22nd, small patches, with whitish borders and green centres, developed on the liquids. We then shook up the flask A, in order to submerge the plant and the spores.[54] We also shook the flask B, after observing the precaution of sealing up the slender bent tube.[55] The flask C was attached on one side to an aspirator, on the other to a tube filled with cotton, and every day we renewed the air in it.