Expertmenfs in th Tanning Principle nnd Jcid of Galh. 339 



Afbumlnous liquors are precipitated by the tanning folmion, but the refult is not a mag- 

 ma fulccptiblc of the fame concentration. 



The green fulphate of iron is no more altered by the tanning principle than by the acid 

 of galls *. 



7 he red fulphate, on the contrary, is precipitated in a fomewhat tarnifhed blue depofi- 

 tion, which is abundant, coarfe, and of a black colour when dry. If the precipitate which 

 this fulphate aflbrds with the gallic acid be compared with this, it will be feen that they 

 greatly differ. The former is of an extreme fubtlety, and remains long fufpended in 

 water : it is perfedlly black, &c. 



The gallate of iron is foluble in acids ; the tannate of iron is decompofed by thofe falts. 

 It abandons its iron, and the tanning part falls down. 



If th*; red fulphate be poured, rather in excefs, into a folution of the tanning principle, the 

 difengaged fulphuric acid re-diflblves the precipitate, and affords a black fluid in its greatefl 

 jntenfity, which is blue when much diluted. To feparate the precipitate, without depriving 

 it of the tanning principle. It is neceffary that the excefs of acid in the fluid (hould be gra- 

 dually faturated by pot-afh. With a little attention, the operator may fucceed in rendering 

 the liquor colourlefs, without affecting the fuperabundant portion of fulphate; or other- 

 wife. If a fmall portion of oxyde of Iron has been thrown down along with the black pre- 

 cipitate, which may be known by the rult which is formed upon the filter, a few drops of 

 acid will re diffolve it. 



But it is a remarkable fa£l, that all the red fulphate remaining in the fluid is brought 

 back to the ilate of green fulphate. The quantity of oxygen which conftitutes the diffe- 

 rence between the one and the other, is feized by a portion of the tanning principle. 

 The latter thus oxyded, and by that means rendered incapable of precipitating the red ful- 

 phate, remains in folution. This oxydation of the tanning principle is direftly and eafily 

 obtained by the oxygenated muriatic acid. The folution of the tanning principle, inflead of 

 becoming clearer, acquiies a deeper colour, and lofes all its peculiar characters in its 

 tranfition to a new Itate, which Mr. Proud has not yet examinedf. 



The gallic acid undergoes .the fame alterations by the oxygenated muriatic acid, and, like 

 the folution of the tanning matter, becomes incapable of precipitating the red fulphate. 

 This is the effeft which age produces upon ink. The gallic acid is deftroyed, the red oxyde 

 remains, and may eafily be. rendered blue by the method of Blagden. 



As the aftringent juices cannot afford indeflru£lible inks, we mud therefore return to the 

 method of the antients, who ufed coal for this purpofe. Perhaps, fays Mr. Proufl, there is 

 no fubftance fo proper as Spanifti chalk. This Hone, which is neither bituminous nor am- 

 pelite, is found with the amianthus, and is compofed of alumine with fix or feven per cent, 

 of pure charcoal. 



The fame phenomenon of the oxydation of the blackening principle at the expence of the 

 red fulphate. Is alfo obferved in black dyes made with fumac and this metallic fait. Mr. 

 Proud had occafion to obferve it in a oath of this kind, which no longer added to the black co- 

 lour of the piece goods. The liquor was greenifli; the red fulphate, or the oxygenated muriatic 



• On tljis fubjedl, fee the interefting paper of this author, in our Journal^ I. 453. N, 

 t See the obfervations of Seguin, Philof. Journal, I. 475. 



acid^ 



