94 ANIMAL PROTEINS 



very special methods of treatment (sweating and painting) 

 on account of the importance and value of the wool, the 

 quality and value of which would be impaired by putting 

 the skins through ordinary lime liquors. The pelts, how- 

 ever, are limed after unwoolling. 



In deliming light leathers the process of puering is widely 

 used (see p. 25). This consists in immersing the skins after 

 depilation in a warm fermenting infusion of dog-dung. In 

 principle this disgusting process presents a close analogy with 

 bating, and indeed the two terms are both used somewhat 

 loosely, but there are nevertheless several points in which the 

 two processes are radically different. The dog-dung puer is 

 a process carried out at a higher temperature than the fowl- 

 dung bate ; it is also a much quicker process, and the 

 infusion employed is generally more concentrated. Whilst 

 the fowl-dung bate is always slightly alkaline to phenol 

 phthalein the dog-dung puer is always acid to this indicator, 

 and the course of the puering may be conveniently followed 

 by testing the pelts with it. The mechanism of the two 

 processes is also probably somewhat different. The mechan- 

 ism of the dog-dung puer has been largely made clear by the 

 researches of Wood and others, and been found due partly 

 to a deliming action by the amine salts of weak organic 

 acids and partly to the action of enzymes from a bacillus 

 of the coli class, which received the name of B. erodiens, 

 and which effects a solvent action on the interfibrillar 

 substance. As we have noted (Part I., Section II., p. 24), 

 the fowl-dung bate involves two fermentations, in each of 

 which (aerobic and anaerobic) several species of bacteria 

 are probably active. Wood found the bacteria of the bate 

 to be chiefly cocci, and ascribed part of the difference in 

 mechanism by the nature of the media, which in the bate 

 includes also the urinary products. In the dog-dung puer, 

 also, a lipolytic action is probably an essential part of the 

 total effect. The puer gives a much more complete de- 

 liming and a much softer and more relaxed pelt than the 

 bate, it is therefore particularly suited to the needs of light 

 leather manufacture. The puering action has been imitated 



