MICROBES, MOULDS, AND YEAST. 205 



dried under pressure in specially built silos. The 

 decomposition is carried on to a certain point by 

 bacteria, and stopped before decay takes place. Salt is 

 used with the grass, and the result is a brown, 

 pleasant-smelling substance of which cattle are very fond. 

 (c) The souring of milk : This is due to the bacteria 

 of lactic acid fermentation acting on the sugar of the 

 milk. The lactic acid so formed is, however, 

 necessary for the coagulation of casein, which is 

 essential in butter-making. {d) Another bacterium 

 produces the butyric acid fermentation, which has the 

 effect of destroying both butter and other food 

 materials. This same species is, however, necessary for 

 the formation of cheese, {e) The ^nanufacture of leather : 

 This is a very complex process, depending for its 

 success on two different processes, both of which are 

 connected with bacteria. The hide, as brought to the 

 tannery, contains many very perishable substances, 

 which putrefy easily, and would very soon rot, and 

 destroy the firmer and stronger material which becomes 

 leather. By operations of a peculiarly horrible nature, 

 bacteria are made to destroy all these perishable 

 matters as quickly as possible. After this, the hides 

 are saturated with tannin substance, such as extracts of 

 the bark of oak, wattle, sumach, or other tannins. These 

 astringent " tannins " are strong antiseptics, and the 

 leather is kept from bacterial decay by their action. 



There are so many other commercial and agricultural 

 processes which depend upon bacteria, that it is 

 impossible even to mention more than a few. (i) 

 Linen Manufacture : The separation of the tough 

 fibre of flax bark from the softer cortex is carried out 

 by the assistance of certain special bacteria, which 

 '^rett" the bark. (2) The common and important dye, 



