THE SOURING OF BARK LIQUOR 207 



yhown, mainly due to lactic acid, the presence of which acid has a favourable 

 influence on the quality of the leather. According to J. PASSLER (I.) a leather 

 tanned with pure tannin handles poor and hard, but is rendered soft and supple 

 by the acid gradually formed in the bark liquor. It is therefore easy to under- 

 stand why the tanner likes to see his bark liquor turn sour, and even attempts 

 to favour and accelerate this state of things by mixing with the fresh liquor 

 some of an older, already soured batch ; thus unwittingly inoculating it with a 

 culture (in any case impure) of acid bacilli. Whether this microbe also acts in 

 other ways is at present unknown, but it is a subject worthy of investigation. 

 This applies particularly to the liberation of gas in bark liquors, which, according 

 to Haenlein's researches, amounts to 1-2 c.c. per gram of pine-bark. Important 

 researches on the dependence of the normal progress of this souring operation 

 on external conditions were made by F. ANDREASCH (I.). 



Although Bacillus corticalis is by no means the only species found on pine- 

 bark, still the composition of the bark liquor is specially favourable to its 

 development. It is therefore probable that in the liquors prepared from other 

 (and especially tropical) tanning materials, other (but allied) species will be 

 found, not only because the bacterial flora of these materials (grown under 

 other conditions) is different, but because these latter differ in chemical com- 

 position from our indigenous tanning barks, &c., and consequently favour the 

 development of other species of bacteria. J. T. WOOD (III.) has described 

 the micro-organisms present in sumach infusions. In conclusion, it may be 

 remarked that the process of tanning still presents a very profitable field of 

 research to the Technical Mycologist. 



