564 Proceedings of the Royal Irish Academy. 



whereby the compound known under the name of tanno-melanic acid 

 seems to be formed, whilst nitric oxide and carbonic acid gases are 

 evolved. The following equation represents the changes which occur 

 in the reaction : — 



Gallic Acid. Nitr°- ^^"Tcfd!^^"^" "^S" Se. W^*- 



CvHeOs + 2HNO2 = C6H4O3 + CO2 + 2K0 + 2H2O. 



These changes, with the development of colour, take place in neutral 

 as well as in acid solutions, but more readily in the latter, and when 

 they are heated, than at the ordinary temperature, as already observed. 

 The colouring principle which is so produced seems to be the same 

 substance that is formed by the gradual oxidation of gallic acid in 

 aqueous solution by exposure to the air ; or when this takes place 

 more rapidly, by the solution being rendered alkaline by the addition 

 of one of the alkalies before exposing it to its influence. The colour- 

 ing matter so formed is unaffected by diluted acids — at least diluted 

 sulphuric, nitric, and hydrochloric acids had no apparent effect on it ; 

 and the organic acids, acetic, oxalic, and tartaric, even in a concentrated 

 condition, did not seem to produce any change. It is also very per- 

 manent, and does not appear to be affected by exposure to air and 

 light, even after being a long time subjected to their influence. 



The depth or intensity of the colour produced being in direct 

 proportion to the amount of nitrite reacting on the gallic acid, a ready 

 means is afforded for the quantitative determination of the nitrites. 

 Thus, if a standard solution be prepared, containing a known quan- 

 tity of nitrite, and if a given amount of water or solution under exa- 

 mination yielded with gallic acid a certain shade or depth of colour, 

 and if the same bulk of the standard solution, or of a mixture of it 

 with distilled water in known proportion, developed the same tint, 

 the former would be considered to contain the same amount of nitrite 

 as the latter, and by thus comparing the tints produced by the waters 

 under examination with those caused by solutions containing known 

 quantities of nitrite, the quantitative estimation of such may be 

 quickly accomplished, just as in J^essler's process (now so much 

 employed by chemists) the determination of ammonia is so readily 

 effected. Indeed the colour which is developed by the action of the 

 nitrites on gallic acid most closely resembles that produced by am- 

 monia on Nessler's reagent. The process, too, is conducted pretty 

 much in the same way, except that we have a standard solution of an 

 alkaline nitrite, instead of one of ammonia ; and the test reagent is 

 one containing gallic acid, instead of !Nessler's solution; and finally, 

 that the water or mixture, after the addition of the gallic acid solu- 

 tion, and a few drops of either sulphuric or hydrochloric acid, is 

 heated to boiling in a test tube and allowed to cool ; after which it is 

 placed in a cylindrical flat-bottomed glass, to compare more accurately 

 the degree of colour produced by the water under examination with 

 that containing some known quantity of nitrite. 



