CHAPTER XXVII. 



THE PART PLAYED BY BACTERIA IN TANNING. 



157. The Fermentation of the Plumping Soak. 



THE preservation of animal hides by simple drying is feasible 

 only when they are to be stored for some time and brought on 

 the market in a dry state. To accelerate this drying, the fleshy 

 side is in many instances rubbed over with arsenic or with common 

 salt. A few investigations into the utility of the last-named sub- 

 stance were made by F. .HAENLEIN (I.). The brittle solidity and 

 fragility acquired by the hides in drying prevent their utilisation 

 for practical purposes, and the more so because putrefaction sets 

 in directly they are wetted. To remedy this defect is the object 

 of tanning, i.e. the conversion of hides into leather. Bacterial 

 activity here comes into play at the outset of this operation, micro- 

 organisms being necessary even in the preliminary treatment known 

 as unhairing. As is well known, leather consists neither of the 

 outer skin (epidermis) nor of the next layer of mucous cells 

 (mucous membrane), but of the third layer (composed of closely 

 interwoven cells), which, on this account, has been termed the 

 leather (or true) skin (corium). Several methods of exposing this 

 layer and simultaneously removing the hair exist, one of them 

 being the so-called sweating process. In this operation the 

 cleansed, soft hides are maintained at a moderate warmth in a 

 chamber saturated with moisture the sweating pit. Putrefaction 

 quickly sets in, but is only allowed to proceed so far as to loosen 

 the hairs and enable them to be scraped off, along with the epi- 

 dermis and mucous membrane. A second method of unhairing is 

 by liming or slackening, the hides being repeatedly steeped in an 

 initially weak, but progressively stronger milk of lime. In the 

 absence of investigation on the subject, it is still uncertain whether 

 an active part is played in this operation as is undoubtedly the 

 case in the sweating process by bacteria, certain species of which, 

 capable of resisting the action of the milk of lime, have been dis- 

 covered in the liquor by J. VON SCHRODER and W. SCHMITZ- 

 DUMONT (I.). The result of both operations is substantially the 

 same, viz., the hair, epidermis, and mucous membrane are loosened, 

 and can then be removed. 



In one respect, however, limed hides differ from those slackened 

 by sweating. In the former case, calcium carbonate has been 



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