THE SOURING OF BARK LIQUOR. 267 



consequently can only come into play when the cerealin has com- 

 pleted 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 fer- 

 menting 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 fer- 

 mentation for this soak. It should be mentioned, 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 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 stand- 

 point. 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 

 filled up with water. At the end of eight or ten weeks the hides 

 are taken out and relaid in fresh strata of bark, and this operation 

 is repeated (three or five times) until the leather is impregnated 

 with tan. There is no doubt at all but that the activity of 

 microbes is also manifested in the tanpit, but at present no 

 hypothesis can be formulated on this point, there being no scien- 

 tific foundations to build upon. 



Our knowledge of the reactions occurring in the bark liquor 

 is, however, in a more satisfactory state. All the thinner hides, 

 unsuitable for sole-leathers, are steeped, not in the tanpit, but in 

 an aqueous (cold-draw r n) extract of tanning substances, known as 

 bark liquor or ooze (Fr. jusee, Ger. Lokbruhe). The actual pro- 

 cess of tanning, i.e. the combination of tannic acid and phlobaphene 

 with the fibres of the skin, does not concern us at present, this 

 being a purely chemico-physical operation. One circumstance, 

 however, deserves brief mention here, and that is the subsidiary 

 phenomenon of the gradual souring of the bark liquor. This 



