LEATHER 81 



The salts of lead, beyond those already named, which enter into any of our manu- 

 factures, aro few and unimportant. Watts'* Dictionary of Chemistry should be con- 

 suited for them. 



XiEAX>, RED. See BED LEAD. 



ZiEAX>, WHITE. See WHITE LEAD. 



X.E ABH ZZiIiZTE . A sulphato-carbonat o of lead found at Leadhills, in Lanarkshire, 

 Scotland. 



LEATHER (Cuir, Fr. ; Leder, Ger. ; Leer, Dutch; Lader, Danish; L'ddtr, 

 Swedish ; Cuojo, Italian ; Cuero, Spanish ; Kusha, Eussian). This substance con- 

 sists of the skins of animals, chemically changed by the process called tanning. 

 Throughout the civilised world, and from the most ancient times this substance has 

 been employed by man for a variety of purposes. Barbarous and savage tribes use 

 the skins of beasts as skins ; civilised man renders the same substance unalterable by 

 the external agents which tend to decompose it in its natural state, and by a variety 

 of peculiar manipulations prepares it for almost innumerable applications. 



Although the preparation of this valuable substance in a rude manner has been 

 known from the most ancient times, it was not until the end of the last, and the 

 beginning of the present century (1800) that it began to be manufactured upon right 

 principles, in consequence of the researches of Macbride, Deyeux, Seguin, and Davy. 



Skins may be converted into leather either with or without their hair ; generally, 

 however, the hair is removed. 



The most important and costly kinds are comprised under sole leather and upper 

 leather, to which may be added harness leather, belts used in machinery, leather 

 hose, &c., but as far as the tanner is concerned, these are comprehended almost en- 

 tirely in the kinds known as upper leather. 



The active principle by which the skins of animals are prevented from putrefying, 

 and at the same time, under some modes of preparation, rendered comparatively im- 

 pervious to water, is called tannin, or tannic acid, a property found in the bark of the 

 various species of Quercus, but especially plentiful in the gall-nut. When obtained 

 pure, as it may easily be from the gall-nut, by chemical means, tannic acid appears as 

 a slightly yellowish, almost a colourless mass, readily soluble in water ; it precipitates 

 gelatin from solution, forming what has been called tannogelatin. Tannic acid also 

 precipitates albumen and starch. There can be little difficulty, after knowing the 

 chemical combination just alluded to, in understanding the peculiar and striking 

 change produced on animal substance in the formation of leather. The hide or skin 

 consists principally of gelatin, for which the vegetable astringent tannin has an 

 affinity, and the chemical union of these substances in the process of tanning pro- 

 duces the useful article of which we are treating. 



Before entering upon the various processes by which the changes are effected on 

 the animal fibre, it may not be uninteresting to speak of some of the principal as- 

 tringents used for the purpose of. producing these effects. 



Bark obtained from the oak-tree is the most valuable and the most extensively used 

 ingredient in tanning, and for a long time no other substance was used in England 

 for the purpose. In consequence of the demand having become very much greater 

 than the supply, and the consequent increase in the price of the article, it became 

 necessary to investigate its properties, in order, if possible, to furnish the required 

 quantity of tanning matter from other sources. Among other substitutes which 

 were tried with some success in other countries may be mentioned heath, myrtle 

 leaves, wild laurel leaves, birch-tree bark, and (according to the ' Penny Cyclopaedia ') 

 in 1765 oak-sawdust was applied in England, and has since been used in Germany 

 for this purpose. 



Investigation proved that the tanning power of oak-bark consisted in a peculiar 

 astringent property, to which the name of tannin has been given, and this discovery 

 suggested that other bodies possessing this property would be suitable substitutes. 



According to Sir H. Davy the following proportions of tannin in the different sub- 

 stances mentioned will be found : ' 8 Ibs. of oak-bark are equal to 2J Ibs. of galls, 

 to 3 Ibs. of sumach, to 7 Ibs. of bark of Leicester willow, to 1 1 Ibs. of the bark of the 

 Spanish chestnut, to 18 Ibs. of elm-bark, and to 21 Ibs. of common willow-bark.' 

 Penny Cyclopedia. 



OAK-BARK contains more tannin when cut in spring by four and a half times 

 than when cut in winter ; it is also more plentiful in young trees than in old ones. 

 About 40,000 tons of oak-bark are said to be imported into this country annually, 

 from the Netherlands, Germany, and ports in the Mediterranean. The quantity of 

 English oak-bark used we have no means of ascertaining. It is prepared for use by 

 grinding it to a coarse powder between cast-iron cylinders, and laid into the tan-pits 

 alternately with the skins to be tanned. Sometimes, however, as will be hereafter 

 noticed, an infusion of the bark in water is employed with better effect. See OAK.-BABK. 



VOL. III. G 



