232 OUTLINES OF BACTERIOLOGY 



of a more important kind takes place, viz. a swelling of the hides to 

 almost double their former thickness. The swelling must also probably 

 be attributed to the lactic-acid bacteria, for some of them produce gas 

 copiously as well as lactic acid, and it is the expansion of this gas which 

 causes the swelling. 



With regard to the hides that have been unhaired by the sweating 

 process, these are also placed in the bate, not because lactic acid is 

 necessary in their case, but because of the swelling that takes place as a 

 result of the evolution of gas. 



It seems strange that in such an important industry these crude 

 methods should still be adopted to effect the swelling of the hides. 

 The introduction of lactic acid must be effected by fermentation, and 

 not by purely chemical means, because apparently the latter method 

 does not turn out a durable form of leather. Now bran and faecal 

 matter, when undergoing putrefaction, contain many varieties of 

 bacteria, and the industry would gain considerably if the manufacturers 

 could ascertain which kinds liberate lactic acid and produce gas. If 

 this were done, they would be able to dispense with the objectionable 

 material with which they work, and by using pure cultures would 

 very probably be able to expedite the operation of liberating the lime 

 and swelling the hide. One step has already been made in this 

 direction by the isolation of one of these bacteria, to which the name 

 Bacterium furfuris has been given. This was isolated from a sample of 

 bran in which putrefaction had already partially set in. There is a 

 large quantity of starch in bran, and likewise a ferment called cerealin, 

 which changes this starch into a kind of sugar. Bacterium furfuris 

 was found to thrive on this sugar, changing it into organic acids and 

 at the same time causing the liberation of large quantities of the gases, 

 carbon dioxide, oxygen, nitrogen, and hydrogen. The individuals of 

 this species are described as consisting of short rods, 0'7 f. long and 

 1'3/i broad: they are often united to form chains, and do not form 

 spores. 



III. After the hides have been sufficiently "plumped," they are 

 ready for the final phase, viz. the introduction of tannin. In the 

 bark-tanning method, the tannin is derived from the bark of various 

 trees, from gall-nuts, myrobalans (dried fruit from East Indies), 

 sumach (dried leaves containing much tannin), etc. The hides intended 

 for sole leather are placed in the tan-pit in such a way that each hide 

 has above and below it a layer of broken bark, powdered gall-nuts, etc., 

 and at the same time a portion of spent tan from the previous tanning. 

 After filling the pit with water, the hides are allowed to remain in it 



