THE SOURING OF BARK LIQUOR. 269 



(reckoned as acetic acid) per litre, and is, as the previously men- 

 tioned researches of Wladika have shown, 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 understand 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 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 

 composition from our indigenous tanning barks, &c., and conse- 

 quently 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. 



