60 dlservatlom on the Processes of Ta/ini/ig. 



infusions are used for tanning, a larger proportion of tan- 

 nin is conibiiietl with the matter of skin. 



For calf skins, and light cow skins, which arc usually 

 prepared in the grainer, weak lixiviums are used in the first 

 part of the process ; but thick ox hides, for the purpose of 

 stout sole leather, are generally kept in a strong ooze, pre- 

 served constantly in a state approaching to saturation, by 

 means of strata of bark. 



Calf skins, and light cow skins, in the usual process, 

 require for their full impregnation with tannin from two 

 \o four months ; but thick ox hides demand from ten to 

 eighteen months. 



Jn any case the state of the skin with regard to impreg- 

 nation with tannin may be easily judged of, if it be cut 

 transversely w ith a sharp knile : in this case the tanned 

 part appears of a nutmeg colour ; but the unimpregnated 

 skin retains its whiteness. 



The tanned hides designed for sole leather, are, while 

 drying, generally smoothed with a stout steel pin, and beat 

 with a mallet. By this process they are rendered denser, 

 firmer, and less permeable to water: calf skins are not sub- 

 jected to the operation of beating; and they are treated in 

 different ways by the currier, according as they are needed 

 for difi'erent purposes. 



III. General Remarks relating to the Processes of Tar?ning. 



A very great number of vegetable productions, besides 

 oak bark, contain the principle essenlial to the conversion of 

 skin into leather: galls, sumach, the bark of the Spanish 

 chesnut, of the elm, of the common willow, and of the Lei- 

 cester willow, the branches of the myrtle, tormentil, and 

 heath, have all been used in the processes of tanning. 



Ditferent methods have been proposed for estimating the 

 fjuantity of tannin in different vegetable productions. Tan- 

 nin, by being dissolved in water, increases its spccifjc gravi- 

 ty, and the hydrometer has been used for estimating the 

 strength of the tarming ooze. 'J"he results given by this in- 

 stnnnent are, however, often fallacious in comparative ex- 

 periments, in consc(|uence of the presence of extractive 

 matter, and of saline substances ; and the action of the so- 

 lution of gelatine affords the best indication of the quantity 

 of the tanning principle. 



The solution of gelatine most proper for the general pur- 

 poses of experiments, is made by dissolving an ounce of 

 glue, or of isindass, in three pints of boiling water. 



The 



