Filfous Matter. — Operation of Tanning. 277 



. But if the temperature be raifed to ebullition the fibrous texture is altered, becomes foft, 

 and fucceffively diffolves. It aflumes the charafter of gelatin, and may be entirely converted 

 into a vifcid and tenacious glue. The fibre which forms the folid ftruflure of the Ikin does 

 not therefore eflentially differ from gelatin, but in its texture, its concretion, and its infolu- 

 bility in cold water ; and as it is obferved that fubftances capable of abforbing oxygene de- 

 prive the fibrous matter of its folidity, and haften and determine its converfion into gelatin, 

 we are authorifed to conclude that thefe diftin6l quahties of the fibre "depend only upon a 

 proportion of oxygene which the vital aftion and progrefs of life combine with the gelatin. 

 Multiplied experiments appear to leave no doubt on this point. " They have proved to me 

 (fays Seguin) that the fibre is oxygenated glue, which in that ftate cannot combine with the 

 tanning principle, but which acquires that property by paffing to the ftate of gelatin in con- 

 fequence of the lofs of a portion of oxygene." 



The elTential point in the operation of tanning is therefore to afcertain, and to direft in a 

 precife and invariable manner, the circumftances and conditions which determine the tran- 

 fition of fibre to the ftate of gelatin, and to feize the inftant in the procefs to effeit a pro- 

 per combination with the tanning principle. This objeft is too important not to recall fome 

 of the obfervations made by Seguin. 



Glue, as has before been remarked, pofleffes a great attra£lion for the tanning principle. 

 It immediately forms with this fubftancc an infoluble matter, which is not fubjedl to putre- 

 faflion ; but it muft be well remarked that the precipitate is dry and brittle. 



Hence it is evident that a fkin, the pores of which might contain gelatin ready formed, or 

 of which the fibrous texture {hould have been altered and converted by certain preparations 

 to the gelatinous ftate, would tan very fpeedily, but would afford a kind of leather which 

 would be harfh, brittle, and difpofed to crack or peel in the wear. On the contrary, the 

 operation will be long, incomplete, and wUl afford only a foft, fpongy, and putrefcible leather, 

 if the fibre preferves its ftate of oxygenation, if it do not pafs by fuccelTive degrees into the 

 ftate of gelatin, or if the tanning principle do not penetrate its thicknefs in proportion to the 

 converfion of the fibre into glue, 



Thefe inconveniences are avoided, and the propofed obje£l is obtained with precifion, by 

 the ufe of the lixivium of tan. This fluid has been fhewn to contain two very diiTerent 

 principles, of which the union and adion are alike necelTary for the fuccefs of the operation. 

 On tlie one fide the gallic principle, which, as is known, readily feizes the oxygene of me- 

 tallic folutions, and reduces them or brings them nearer to the metallic ftate, a£ts nearly in 

 the fame manner upon the fibre ; it unburns it, or deprives it of oxygene, and converts it into 

 die ftate of gelatin. On the other hand, the tanning principle, which alfo exifts in the fo-- 

 lution, exercifes its ailion as foon as the fibre is fulficiently reduced to the ftate of gelatin. 

 The tanning procefs is not, therefore, inftantly cffcfted, like the precipitation of animal glue j 

 but it operates gradually, and in fuccefTion, by ftrata from the furfaces of the fkin to the 

 centre ; and as the adion of the tanning principle immediately follows that of the gallic 

 acid, the fibre i* furprifed in its pofition, the felted texture of the fkin is totally preferve;}, 

 and its compofition alone is changed. Accordingly, the leather prepared by (his procefs has 

 tlie advantage of being fupple, flexible, infoluble, imputrefcible, more durable, aiid Icfs dif- 

 pofed than any other to imbibe humidity. 



The ncvy method of Scguiu for the preparation of leather is founded upon this fciics of 



obfervations 



