8 TJ. S. BUREAU OF PISHERIES. 



Then the extract, catechu, came into use. This extract is not so 

 bulky, is readily sohible in water, and has been used successfully. The 

 methods as practiced varied greatly, the variations being apparently 

 not based on any particular reason. In general, the extract is dis- 

 solved in water — 1 pound of catechu to from 1 to 3 gallons of hot 

 water — and applied to the nets. 



Nets preserved with tanning extracts are brown or tan in color and 

 -are more resistant to decay than untreated nets, as will be seen in 

 figures to be given later. The tan, however, washes out, and it is 

 necessary to repeat the treatment often. 



CHEMISTRY OF TANNING MATERIALS. 



Before going further with the methods of curing nets with tanning 

 materials, it will be necessary to consider the chemistry of the latter. 



It is not improbable that the treatment of nets by tanning was in- 

 duced by the success of the treatment of animal skins by such ma- 

 terials to make leather. Tanning materials are bark or other parts 

 of various trees which produce a reddish or brownish solution when 

 steeped in cold water. There is scarcely a tree or shrub in some 

 part of which tanning substances are not found. 



The action of those materials may be illustrated by their effect on 

 leather. Untanned hide when boiled for some time is nearly all 

 dissolved; after it has been treated with tan liquor it is scarcely 

 affected by boiling. When untanned hide is left in a moist condi- 

 tion for some time it putrefies; tanned hide does not. Thus a hide 

 that is soluble in boiling water and putrefies in cold water is con- 

 verted by tanning into a hide which is neither dissolved in hot water 

 nor putrescible in cold. Just how these great differences are brought 

 about can be illustrated by the effect of tanning materials on gelatin 

 or glue. If a tanning extract be added to a solution of gelatin, the 

 latter is coagulated and precipitated and is made permanently in- 

 soluble. Pure untreated gelatin readily swells in cold and dissolves 

 in warm water and easily decomposes if left wet. But this tanned 

 precipitate is resistant to the action of water and does not decom- 

 pose. How is this important change brought about? The answer 

 is given by a pair of balances. If we weigh a dry piece of gelatin, 

 dissolve it, precipitate or curdle it with tan liquor, collect the pre- 

 cipitate or curd, wash, dry, and weigh it, we find that it has increased 

 in weight. It can also be shown that the tan liquor has lost as much 

 weight as the gelatin has gained — something from the tan liquor 

 combined permanently with the gelatin to make the latter insoluble 

 and resistant to change. This something is called tannin. It would 

 be more correct to say these somethings, for there are several tannins, 

 all closely akin, all affecting gelatin in the same way. It is they 

 that, acting on liides, convert the soluble, putrescible gelatins in 

 hides into insoluble substance, causing the hides to become leather. 



The earlier mt^thod of analyzing tanning materials was to treat 

 the solution with gelatin. This was done by taking first a definite 

 volume of the solution and evaporating it to dryness and weighing 

 the dry residue; then a like volume was taken from a sample that 

 had been treated with gelatin, evaporated to dryness, and weighed. 

 The sample that had been treated with gelatin had lost Aveight. The 



