and with a see-saw movement. The objection to this method is that the hides do not receive equal treatment, those in the centre receiving hardly any movement, while others at the ends of the beam are raised too far out of the liquor. Under these conditions, the colouring of the leather must be irregular. Another method, invented in England, consists in suspending the hides from a wooden frame which can be mechanically moved on wheels. As the contact of tanning liquors with iron must be avoided to prevent discoloration (iron and tannin form the basis of inks), the wheels should be galvanized. This mechanical method is very satisfactory, for the hides are moved gently and kept in the liquors. As some tannins oxidise rapidly (_i.e._, darken in colour by exposure to the air), any process which exposes the hides at this stage for lengthy periods should be avoided. [Illustration: FIG. 24 BARKOMETER] The theory of the vegetable tanning process is not perfectly understood, and the principal trade chemists are not yet agreed as to whether the changes are chemical, physical, or both. Many are inclined to believe that both changes take place, and this view is probably correct, as a pure tannic acid produces a thin, empty leather, while a tanning material which contains a fair proportion of non-tannin matters, yields a full and firm leather. The practical tanner who adapts his work to the theory that tanning is a process of feeding the hides with gradually increasing strengths of tannin, is, in any event, on the right track. The suspender liquors are always weak in tannin, and are pumped from "handler" liquors through which packs of hides have already passed. It is important that these liquors possess a certain degree of acidity, and, if the natural acids of the tan liquor are too weak, or neutralised completely by the lime in a pack of hides that has been treated, a small quantity of acetic, formic, or lactic acid is added. Formic acid has the additional advantage of making the liquor antiseptic. It is absolutely necessary that the liquors have acid properties, otherwise tanning cannot proceed. Acid assists penetration of tannin. Hyposulphite of soda, formaldehyde, or synthetic tannin may be used for the same purpose, while these also help to keep the leather light in colour, owing to their bleaching properties. After passing through three or four suspender pits, in which the tan liquor is gradually strengthened, the hides are often rounded and the butts (Fig. 23) are transferred to the handlers. The first handler liquor should register about 25 degrees by the barkometer (Fig. 24), an instrument which registers the density of tan liquors. The handlers consist of a series of six or eight, and the strength of the liquors should be increased gradually. After a day's immersion in the first handler liquor, the hides are hauled out with a sharp two-pronged hook fixed to the end of a long wooden pole, similar to that used in the liming process.