'BACTERIA IN ARTS AND INDUSTRIES 93 



by a special form of putrefaction or by chemical means. They 

 are then steeped in a fluid which is concocted from the manure 

 of poultry, dogs, and similar peculiar sources, bran, etc., in which 

 they are allowed to ferment, and then transferred to the bark- 

 or tan-pit, where a further process of fermentation takes place. 



Sewage- Works and Ensilage. In ordinary sewage there are 

 enormous numbers of bacteria present from one to ten or 

 twelve million per cubic centimetre, in comparison with which 

 numbers it may be of interest to refer to the table on pp. 53-4, 

 from which it will be seen that milk as purveyed to the public 

 often contains as many organisms as liquid sewage ! 



In order to get rid of the solid and semi-solid constituents of 

 sewage, it is often allowed to putrefy and become liquefied so 

 as to render its disposal easier. The old-fashioned cesspool and 

 cesspit owed their use to this object. On a larger scale, sewage 

 may be stored in covered tanks or reservoirs and allowed to 

 undergo anaerobic putrefaction for a day or two. The 

 "sludge" so obtained is passed through coarse filters in which 

 further putrefaction and liquefaction in presence of air occurs, 

 and the effluent is passed into a river or the sea, or utilised as 

 manure on an ensilage farm. 



The Curing of Tobacco. The ripening of the dried leaves 

 of the tobacco-plant, during which process they are fermented 

 in heaps or piles, is probably to some extent brought about by 

 the action of bacteria, the varying characters of which help, 

 along with other factors, to give the various tobaccos their 

 special flavour and aroma. 



Fermentations in Baking. Wheat-flour and water, in the 

 form of dough, forms a very easily fermentable substance, and 

 if this fermentation is allowed to occur all through the mass, 

 innumerable little cavities are formed by the liberation of gas, 

 usually carbon dioxide (C0 2 ), which give the characteristic 

 sponginess to the bread or other products of the bakery. This 

 fermentation is usually started by adding some brewer's yeast, 

 or if preferred, pure cultures of yeasts, or by mixing in some 

 leaven or dough already fermented at a previous baking. The 

 mass is allowed to stand for several hours at a suitable tempe- 

 rature, during which time the fermentation andgas-production 

 occurs. The fermented dough is then divided up and baked in 

 the oven, the imprisoned gas expanding and making the bread 

 light and spongy. Changes of a very complicated nature take 

 place, both during the fermentation and the heating, such as 

 the production of a certain proportion of sugar from the starch. 



