MICRO-ORGANISMS. 143 
The Tanning of Leather. 
In this industry the raw hide, which has been preserved 
from putrefaction by the use of disinfectants, is cleaned, and 
then either ‘“‘ sweated’ or “ limed,” in order to take off the 
hair with ‘the adjacent mucous layer, and leave the corium 
or true fibrous skin. One of these processes, the “ sweat- 
ing,’ consists in exposing the hide in pits saturated with 
moisture, when a rapid putrefaction fermentation sets in. 
In the other process, the hides are steeped successively in 
stronger and stronger solutions of milk of lime. After these 
treatments the hair and outer layers of skin are easily 
removed. The hide is next immersed in the “ bates,’ which 
consist of fermentable solutions of dog or pigeon dung and 
of bran. The bates, by their fermentation, furnish lactic 
acid, which acts upon the dehaired hides in two ways. Ii 
eliminates the lime salts, and causes the hides to swell up 
and become porous or “plump,” in which condition they 
are quickly and uniformly tanned. The remainder of the 
outer skin is removed by the tryptic ferments of the manure. 
The lactic acid is formed by lactic bacteria from the starch 
of the bran, which is saccarified by the bran diastase cerealin. 
Thus several processes go on in the “bates.’”” There is also 
a production of fermentation gases which were supposed 
to be the chief agent in causing the plumping, by blowing 
up the substance of the hide. Lactic acid is being used in 
place of the bran-drench, and it is claimed for this improve 
ment that the process is more under control ; and it certainly 
seems more feasible to employ the product of the bacteria 
than the irresponsible organisms themselves. It has been 
* suggested that pure cultures of bacteria could be used to 
replace the dung; but, as the latter contains something 
more than pure bacteria, the suggestion has apparently not 
been put into practice. 
In the tan liquors, where the prepared hide is converted 
into leather, quite a number of fermentations go on, There 
is the alcoholic, the acetic, the lactic, and sometimes the 
viscous. These are to be traced to micro-organisms 
introduced in the hides, the water, the air, and the bark. 
In these fermentations the lactic is the chief, and by its 
plumping action the tanning material is assisted into the 
interior of the hide. 
The lactic fermentation is found in the distillery, where it 
plays a not unimportant part in acidifying the wort, and 
thus preventing the growth of putrefactive bacteria, which 
prefer a neutral or slightly alkaline medium. It is also 
