_Industrial Application of Bacterial Ferments._ From what has been said we trust it has been made evident that bacteriology has a place of ever-increasing importance in regard to fermentative processes. Not only have the causal agents of various fermentations been isolated and studied, but from their study practical results follow. The question of pure cultures alone is one of practical importance; the recognition of the causes of "diseases" of beer is another. We cannot enter into a full discussion of the rôle of bacteria in industrial processes, but several of the chief directions may be pointed out. Without exception, bacteria have a part in them on account of their powers of fermentation. In securing their food, bacteria break down material, and bring about chemical and physical change. The power which organisms have of chemically destroying compounds is in itself of little importance, but the products which arise as a result are of an importance in the world which has not hitherto been recognised. We have used bacteria abundantly in the past, but we have not perceived that we were doing so. _The maceration industries_ may be mentioned as illustrative of this use without acknowledgment. The flax stem is made up of cellular substance, flax fibres, and wood fibres; the later are of no service in the making of linen, but the whole is bound together by a gummy, resinous substance. Now this connective element is got rid of in the process of _retting_. There is dew-retting and water-retting. The former is practised in Russia, and consists in spreading the flax on the grass and exposing it to the influence of dew, rain, air, and light. The result is a soft and silky fibre. Water-retting is accomplished by means of steeping the flax in bundles, roots downwards, in tanks or ponds. In ten to fourteen days, according to the weather, fermentation sets in, and breaks the "shore" or "shive" from the fibre, and the process is complete. This is always done by the aid of bacteria, which, under the favourable circumstances, multiply rapidly, and cause decomposition of the pectin resinous matter. The same operation occurs in _jute_ and _hemp_. _Sponges_, too, are cleared in this manner by the rotting of the organic matter in their interstices. The preparation of _indigo_ from the indigo plant is brought about by a special bacterium found on the leaves. If the leaves are sterilised, no fermentation occurs, and no indigo is formed. _Tobacco-curing_ is also in part due to decomposition bacteria, and several bacteriologists have experimented independently in fermenting tobacco leaves by the action of pure cultures obtained from tobacco of the finest quality. In all these applications we have advanced only the first stage of the journey. Nevertheless, here, as in nature on a big scale in the formation of fertile soils and coal-measures, we find bacteria silently at work, achieving great ends by co-operating in countless hordes. CHAPTER V