We have shown in the first place, that the changes which take place in the ferment, the wort, and the beer itself, are due to the presence of microscopic organisms of an entirely different character to that of the ferment-cells properly so called, which organisms, by simultaneously giving rise, in the course of their multiplication in the wort, ferment, or beer, to other products, make the materials difficult to keep or effect their deterioration. Again, we have seen that these change-producing organisms, the ferments of disease, never arise spontaneously in the wort or beer, but, whenever they make their appearance in these fluids, have been imported from without, either in company with the yeast, or from accession of atmospheric dust, or from contact with the vessels, or from the materials themselves which the brewer uses in his manufacture. Moreover, we know that these disease-ferments, or their germs, are destroyed when the wort has its temperature raised to the boiling-point. And, following up the inferences from such facts, we have seen that wort exposed to pure air, after having been heated to boiling, remains absolutely free of any sort of fermentation. Inasmuch then as the disease-germs of wort and beer are destroyed in the copper in which the wort is boiled, and as, by employing a perfectly pure ferment, we guard against the admission of any foreign ferment of an evil character, we have it in our power to prepare a beer which shall be incapable of undergoing any pernicious fermentation whatsoever. This we shall have effected provided we can take the wort as run off from the coppers, cool and manipulate it out of contact with ordinary air or in contact with pure air, charge it with a pure yeast, and, lastly, store the beer when the fermentation is complete in vessels thoroughly purified from disease-ferments.[162] § I.—Preliminary Experiments. We may readily satisfy ourselves as to the truth of these inferences. The following is one of the earliest experiments which I devised with a view to establish their certainty. Into a flask with a straight neck of about a litre (1-3/4 pints) capacity, a quantity of wort from a brewery was introduced and there raised to boiling, and whilst the vapour still issued from the neck of the flask, connection was made with a two-necked flask in which the cultivation of pure yeast had been carried on. The cork and glass tube used for this purpose had previously been treated with boiling water. [Illustration: Fig. 75.]