Action of Gallic and Tannic Acids in Dyeing. 269 



cifie gravity of 1*090 ; and observed, that although gallic acid 

 acted in the way above described, tannic acid on the contrary, 

 under the influence of the sugar, attacked the iron and gave a 

 bulky dirty purple precipitate. I regret that I had not time 

 to examine the nature of this action, neither the peculiar com- 

 pound formed by the oxidation of the gallic acid, when brought 

 in contact with an acid persalt of iron ; but if circumstances 

 permit, I propose to return to this subject. 



In conclusion, from the facts contained in this paper, there 

 can be no doubt, that tannic acid is the constituent of tanning 

 substances, which produces blacks with iron mordants. Second- 

 ly, That the reason why gallic acid produces no black dye is, 

 that it reduces the peroxide of iron of the mordant, forming a 

 colourless and soluble gallate of protoxide of iron. Thirdly, 

 That gallic acid has the property of dissolving hydrate of alu- 

 mina, and also of separating alumina mordant, from the cloth 

 on which it is fixed. Fourthly, That the reason why extracts 

 of tanning matters lose their dyeing properties is, that the 

 tannin is transformed into gallic acid. Fifthly, That gallic 

 acid possesses the property of dissolving iron, and thus lays 

 claim to the character of a true acid, whilst tannin not having 

 this action, appears to me to be in reality a neutral substance. 

 Sixthly, That carbolic acid possesses the property of prevent- 

 ing the tannin-fermentation, or the conversion of tannin into 

 gallic acid and sugar, or a similar substance, under the influ- 

 ence of a peculiar ferment called pectase. 



On the Geologic Range of the Pterygotus problematicus. 

 By the Rev. W. S. Symonds, F.G.S. 



" One of the strangest organisms of the formation," says Mr 

 Hugh Miller, in " The Old Red Sandstone," " is a fossil lob- 

 ster of such huge proportions that one of the average-sized 

 lobsters, common in our markets, might stretch its entire 

 length across the continuous tail flap in which the creature 

 terminated.'' 



This crustacean is the " Seraphim" of the Forfarshire 

 quarrymen, and was for a long time supposed by Agassiz to 

 be a " fish." Mr Hugh Miller gives an interesting account of 



NEW SERIES. VOL. T. NO. IT. APRIL 1855. T 



