THE SOURING OF BARK LIQUOR 205 



causing it to swell up (" plump ") to almost double its former thickness, thus 

 loosening the cellular tissue and facilitating the subsequent penetration of the 

 tannin. In order to make the most of this advantage, it is generally the custom 

 to pickle even the hides that have been unhaired by the sweating process, and 

 which consequently do not contain any lime that needs to be removed. The 

 object in this case is to plump the hides, i.e. cause them to swell up, and this 

 explains the term " plumping soak " applied to this acid liquor. This expansion 

 of the skin is due to the action of the gases liberated in the fermenting liquor. 

 The reactions occurring in this process are very diversified ; as might be 

 expected from the large number of bacterial species present in the bran itself 

 and in the added frecal matter. Of these organisms, such as attack the carbo- 

 hydrates develop most vigorously, these substances being present in large 

 amount. 



J. T. WOOD and W. WILLCOX (I.) described one species of this kind and gave 

 it the name of Bacterium furfur is. The starch in the bran ishydrolised and con- 

 verted into glucoses by an enzyme, cerealin isolated from bran extract by J. T. 

 WOOD (I.) and these sugars are then acted upon by the bacteria in question, 

 organic acids and considerable quantities of gas being produced. In a sample 

 examined by WOOD (II.), i litre of the fermented bate contained 0.8 gram of 

 lactic acid, 0.2 gram of acetic acid, 0.03 gram of formic acid, and o.oi gram of 

 butyric acid. The disengaged gas was found, in different experiments, to consist 

 of 22-42 per cent, of C0 g , 28-53 P er cent, of H, 24-26 per cent, of N, and 1-4 

 per cent, of 0, the percentage of carbon dioxide increasing as fermentation 

 progresses. This fermentation is attributed by Wood and Willcox to the said 

 bacterium, which they describe as short rods 0.7 p, long and 1.3/1 broad, united 

 to form chains, but not, so far as they could ascertain, producing endospores. 

 The cell-walls exhibit a tendency to swell up. The organism is incapable of 

 attacking solid or dissolved starch, and consequently can only come into play 

 when the cerealin has completed its diastatic action and converted the starch of 

 the bran, &c., into glucose. 



These discoveries must be regarded as a preliminary step to be followed by 

 many others. The unappetising and (hygienically considered) objectionable use 

 of faeces is shown to be unnecessary ; and it would be advisable to endeavour to 

 artificially " sour " the plumping soak by means of a sufficient quantity of actively 

 fermenting leaven prepared from pure bran. 



The next step in bacteriological investigation in this matter should be to 

 ascertain whether as the figures above given lead one to suppose pure lactic 

 fermentation is the best form of fermentation for this soak. It should be men- 

 tioned, moreover, that the removal of the lime from the slackened hides by the 

 aid of dilute mineral acids is impracticable, since it seriously impairs the quality 

 of the resulting leather. 



158. The Souring 1 of Bark Liquor. 



For the actual tanning of the plumped hides there are, as is well known, 

 three different systems available, viz., alum- or white-tawing ; oil- tawing, or 

 shamoying ; and bark-tanning the last of which will now be briefly considered 

 from a bacteriological standpoint. The chief tanning material employed in this 

 case is the bark of various trees (oak, pine, &c.), in addition to which gall-nuts, 

 myrobolams, sumach, &c., are also used. 



Hides intended for sole-leather are placed in a tanpit in such a manner that 

 each hide is separated from its neighbour by a layer (a couple of fingers in 

 thickness) of coarsely-broken fresh bark, mixed with powdered gall-nuts, &c. The 

 empty corners and vacant spaces are packed with old spent tan, and the pit is then 



