*Drench Damage.*--Drenching is a very important and useful process in leather manufacture, but, like other fermentive processes, requires great care and experience in its application. Although not so risky as puering, a good deal of serious damage may occur in the drench. The writer knows of no fuller discussion of all the various forms of drench damage than the articles of Eitner and others which appeared in “Der Gerber” during a long series of years (1882–1900). In these articles perhaps every known form of drench damage is described. Procter[152] also gives a concise account of drenching. It will suffice to mention briefly the chief accidents to which the drench is liable; these are generally known as the “turning” of the drench. [152] Principles, pp. 166–170. 1. _Acid “turning.”_--This usually occurs when the atmosphere is charged with electricity--during thunderstorms. It appears to be an extremely rapid form of butyric fermentation, which has not been fully investigated. The skins become swollen, transparent and tender, and, unless the fermentation is stopped, soon dissolve to a jelly. The only way to save the skins from destruction is to add salt to the drench; this reduces the swelling and, in fact, pickles the skin. I have pointed out elsewhere that this is really the origin of the modern pickling process, which is still known by the name of “rising.” A very acid drench will swell the skins almost the same as dilute sulphuric acid, and if they are then placed in a salt solution, or salt is added to the sour liquor, it is absorbed by the skins, and they are preserved almost as effectually as by the use of mineral acids. 2. _Putrid “turning.”_--This occurs under very similar atmospheric conditions to 1, and may be almost as rapid. Instead of becoming acid, the liquor turns slightly alkaline, frequently becoming bluish black, due to the presence of certain chromogenic bacteria. The goods fall as in a puer. Both the chromogenic and other bacteria present are peptonizing organisms, which obtain the upper hand owing to favourable conditions, and rapidly digest the skin unless the action is stopped. The best remedy is to remove the skins from the drench and pickle them without delay by the usual acid and salt process. 3. _“Pinholey” drench._--This is the same as the Germans call “pikiren,” and is generally due to a too rapid evolution of gas both in the drench and in the capillary spaces of the skin itself. The gases form under the hyaline layer, and finally burst through in small holes. A damage very similar in appearance is also caused by colonies of gelatin liquefying bacteria developing on the grain. Each colony forms a small hole. The trouble is usually due to the setting of the drench at too high a temperature. 4. _Spongy leather_ may be produced by leaving the goods too long in a sound drench, if they are allowed to rise too often. In this case experience alone can determine the proper duration of the drenching process.