206 THE PART PLAYED BY BACTERIA IN TANNING 



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

 and relaid in fresh strata of hark, 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 scientific foundations 

 to build upon. 



Our knowledge of the reactions occurring in the bark liqour 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-drawn) extract of tanning 

 substances, known as bark liquor or ooze (Fr.jusee, Ger. Lohbriihe). The actual 

 process 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 change, well known to all tanners, was first chemically investigated by 

 H. BRACONNOT (I.), who, in 183 2, discovered in soured bark liquor an acid which 

 as little was then known of the properties of lactic acid he considered to 

 be a new, hitherto undiscovered acid, and therefore named it " acide nanceique," 

 after the town of Nancy, where the discovery was made. This acid was, how- 

 ever, quickly recognised by L. Gmelin as lactic acid. J. WLADIKA (I.) in 1890 

 showed that volatile, as well as non-volatile, acids are present in old spent bark 

 liquors, the former consisting chiefly of acetic acid and the latter of lactic acid. 

 The reader will have no doubt that the reduction of these acids (which are not 

 present in the "sweet" liquor) is attributable to the activity of micro-organisms, 

 which are here present in abundance, HAENLEIN (II.) having found no less than 

 60,000 bacteria (along with a few budding fungi) in the liquor prepared from, 

 and corresponding to i mgrm. of, Silesian oak-bark. The acids are produced 

 from the saccharine constituents of the bark, a fact already evidenced by the 

 researches of B. KOHNSTEIN (I.) and J. VON SCHROEDER (I.), who showed that 

 in proportion as the sweet liquor becomes sour, so the percentage of these sugary 

 constituents decreases. In practical working their initial quantity is found to 

 be from 0.3 to 0.8 gram per litre. 



A fission fungus named Bacillus corticalis, and recognised as an active partici- 

 pant in the souring of bark liquor, was discovered by HAENLEIN (III.) in large 

 quantities in sour pine-bark liquor, as also in the fresh pine-bark. This microbe 

 appears in the form of short rods 0.7-1 /* broad and one and a half to twice these 

 dimensions in length ; at the period of active reproduction the individual cells 

 are connected into many-jointed chains. The bacillus thrives on nutrient gela- 

 tin, nutrient agar-agar, and slices of potato, and requires but very little nutri- 

 ment. Being able to withstand desiccation, it remains alive in bark, where it is 

 found in large numbers. It has a great affinity for light, and grows more freely 

 when illuminated than in the dark, though it is still capable of performing its 

 vital functions under the latter condition. It thrives most vigorously at 30- 

 40 C., and ceases to develop below 5 C. ; in habit it is facultatively anaerobic, 

 i.e. oxygen is not essential to its growth. Its most important property, so far as 

 we are now concerned, is its behaviour towards sugars, of which it acts upon 

 dextrose as well as saccharose and lactose, and produces, in addition to acids, a 

 considerable quantity of gas, composed (in one case investigated) of 95 per cent, 

 of hydrogen and 5 per cent, of carbon dioxide. Tannin is not attacked by this 

 microbe, a fact confirmed by the observation made by J. VON SCHROEDER and A. 

 BARTEL (I.), that the percentage of tannin in the liquors undergoes no alteration 

 on standing or during storage. 



The acidity of tannin bark liquors amounts to 0.25 gram (reckoned as acetic 

 acid) per litre and is, as the previously mentioned researches of Wladika have 



