From what has been said, it is obvious that, with regard to fermentations, a double problem is presented to the leather manufacturer, since he desires to utilise those which make for his advantage, while controlling or destroying those which are injurious. The first step to a solution of these problems is a more complete knowledge of the organisms which serve or injure us, that we may, as it were, discriminate friends and enemies. We may then approach the question in two ways. Taking the drenching process as an example, we may on the one hand introduce a “pure cultivation” of the right ferment into a sterilised bran infusion, and so induce only the one fermentation which we require; or, on the other hand, as different ferments are affected in varying degrees by antiseptics, we may perhaps choose such as permit the growth of the organism we want, while killing or discouraging the rest. We may also arrange the nutriment, temperature, degree of acidity and other conditions, so as to favour one organism rather than another. All three methods have been applied in brewing with good results. CHAPTER V. _ANTISEPTICS AND DISINFECTANTS._ “Antiseptics” are often defined as substances which check putrefaction without necessarily destroying bacteria and their spores, while “disinfectants” are poisonous to ferment-organisms, and actually destroy them; great differences exist in the extent of their sterilising power, and the whole distinction is one rather of degree than of kind, and has little practical value. Thus common salt is incapable of _killing_ most bacteria, even in concentrated solution, though it holds putrefaction in check both by withdrawing water from the hide and by directly preventing the multiplication of bacteria. If the salt be washed out of the hide, putrefaction is at once resumed by the organisms present. Hides, on the other hand, which have once been sterilised by powerful disinfectants, such as phenol (“carbolic acid”) or mercuric chloride, do not again putrefy till the organisms which are killed are replaced by fresh ones from outside. The action of sodium sulphate, and many other salts, is similar to common salt in this respect, while a large proportion of the aromatic compounds are permanently disinfectant, though their efficiency varies with the species of bacteria involved. Biernacki and others have shown that some disinfectants when extremely diluted actually stimulate alcoholic fermentation, and probably the growth of other ferments, e.g. mercuric chloride 1 in 300,000, salicylic acid 1 in 6000, and boric acid 1 in 8000, and in many cases organisms become habituated to antiseptics in doses which would at first have proved fatal. The number of antiseptics available is now so great that it is impossible to give a detailed account of all, but the following are among those which are best known and have been practically employed.