This method consists in proving that beer never possesses any unpleasant flavour, so long as the alcoholic ferment, properly so called, is not associated with foreign ferments; that this also holds good in the case of wort, and that wort, liable to change as it is, may be preserved in a state of purity, if it is kept under conditions that protect it from the invasion of microscopic parasites, to which it presents not only favourable nutriment, but also a field for development. By employing this second method we shall, moreover, have the advantage of proving with certainty a proposition that we just advanced, and showing that the germs of these organisms proceed from the particles of dust which the common air wafts about and deposits on every object, or which are spread over the utensils and materials used in a brewery, materials that are naturally charged with microscopic germs, which various changes in the store-houses and maltings may multiply to an indefinite extent.[19] [Illustration: Fig. 3.] Let us take a glass flask having a long neck (Fig. 3 A), and holding from 250 c.c. to 300 c.c. (_i.e._, about 9 or 10 fl. oz.); let us put into it some wort, hopped or not, and then draw out the neck of the flask in the flame of a lamp, so as to give to it the shape B (Fig. 3); let us next heat the liquid to the boiling point, when the steam will rush with a hissing sound out of the curved end. We may then, without further precaution, permit our flask to cool, or, as an additional safeguard, we may introduce a small quantity of asbestos into the open extremity, at the very moment when the flame is taken away from the flask. Before introducing the asbestos, we may pass it through the flame, and we may repeat this after it has been placed in the end of the tube.[20] The air which first re-enters the flask must come in contact with the heated glass and the hot liquid, and these will destroy the vitality of any germs existing in such particles of dust as this air may introduce. The re-entrance of the air will be effected very gradually—sufficiently so to enable the drop of water which the air, as it enters, forces up the curved tube, to catch all particles of dust. Ultimately, the tube will become dry, but then the passage of the air will proceed so slowly that every foreign particle will get deposited on its interior sides. Experience tends to prove that external particles of dust cannot find their way into flasks of this pattern, having free communication with the air, at all events within ten or twelve years—the longest time that has been devoted to experiments of this kind; the liquid in the flasks, if originally clear, will not become in the least degree contaminated, either upon its surface or throughout its bulk, although the outside of the flasks may be covered with a thick coat of dust. This is an undeniable proof of the impossibility of particles of dust finding their way inside such flasks.