In deliming light leathers the process of puering is widely used. 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 phenolphthalein 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 mechanism 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.), the fowl-dung bate involves two fermentations, in each of which (ærobic and anærobic) 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 deliming 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 fairly successfully by artificial methods. "Erodin" (Wood, Popp and Becker) involves the use of _B. erodiens_ and a suitable culture medium including organic deliming salts: "Oropon," "Pancreol" and others involve the use of ammonium chloride and trypsin, together with some inert matter.