2G6 Crace Calvert on the 



rite of lime to the above solution of gallate of protoxide of 

 iron, which not only precipitated a certain quantity of the 

 black gallate of iron, but the liquor gave a permanent black 

 on a fresh piece of iron-mordanted calico, leaving no doubt 

 that the hypochlorite had maintained the iron of the mordant 

 in the state of peroxide. A very important question now pre- 

 sented itself, viz., will the presence of a free acid increase the 

 reducing power of gallic acid? To determine this point, a 

 weak solution of persulphate of iron was mixed with some 

 gallic acid, and it was found that in proportion to the excess 

 of acid, so did the blue precipitate first formed rapidly disap- 

 pear, leaving in the glass vessel a brown-tinted liquor, con- 

 taining a salt of proto and peroxide of iron. It was also ascer- 

 tained that the addition of a small quantity of weak hydro- 

 chloric, sulphuric, or oxalic acids, greatly increased the redu- 

 cing action. If, on the contrary, an excess of pure hydrate of 

 peroxide of iron was added to a solution of gallic acid, even 

 after several days, the dark-blue precipitate at first produced 

 remained permanent, and no protoxide of iron was produced 

 in the solution. If heat, however, was applied to the mixture, 

 protoxide of iron might be detected in the liquor. 



These facts clearly show, that gallic acid cannot be em- 

 ployed as a dye when used in excess, or in presence of any 

 other acid. Whilst tannic acid placed in similar circum- 

 stances to the above described with gallic acid, does not reduce 

 the peroxide of iron, either at natural temperatures, or under 

 the influence of heat. The only circumstances in which the 

 conversion of the hydrate of peroxide of iron into protoxide 

 was remarked, were on the addition of large excesses of hy- 

 drochloric, sulphuric, or oxalic acids. I am inclined, therefore, 

 to believe, that, under the influence of a great excess of mine- 

 ral acid, the tannic acid splits up into sugar and gallic acid, 

 and the latter substance produces the reducing effect above de- 

 scribed. 



These results seem to afford an explanation of the fact observed 

 some years ago by M. J. Girardin of Rouen, that, to obtain 

 good blacks, a calcareous water is advantageous, a result 

 which is probably due to the lime of the carbonate neutralizing 

 the gallic acid existing in the tanning matter, and so prevent- 



