466 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



to remove the salt with which they have been preserved. They are then 

 sweated in a warm moist chamber where a commencement of putrefaction 

 occurs, or steeped in milk of lime. Next they are steeped in a "pickle" 

 containing bran and animal excrements. A lactic fermentation takes 

 place which removes the calcium carbonate by changing it into soluble 

 calcium lactate and causes the hide to swell or "plump" to twice its 

 original thickness. 



By these operations the hair is removed and the epidermis loosened 

 from the dermis from which alone the leather is made. 



These operations are preliminary and required to prepare the hides 

 for the tanning proper of which there are several methods. In the oldest 

 method, that by the use of the tannin of oak bark or other sources, micro- 

 organisms take some part not yet completely understood. There are 

 several variations of the method but they all consist in bringing all parts 

 of the hide into different baths containing gradually increasing quantities 

 of tannin. These baths undergo changes due to bacterial action, the 

 principal of which is the production of acid. This increase of acidity 

 is necessary to prevent the action of putrefactive bacteria which would 

 destroy the hides. 



