566 Proceedings of the Royal Irish Academy. 



earthy salts that occur in natural waters, as ■well as by the organic 

 matters that may be there occasionally. It might be naturally 

 supposed that soluble salts of iron (which are sometimes present to 

 some extent in certain waters), producing as they do the well-known 

 black or ink-like reaction with gallic acid, would preclude its employ- 

 ment as a means of estimating nitrites, where the former salts were 

 present ; but this is not the case ; for the iron may be separated by the 

 additioE of ammonia and filtration, after which I have found that the 

 filtrate, having been acidified, may be treated with gallic acid, for the 

 estimation of nitrites. It appears, therefore, that none of the sub- 

 stances which would be likely to occur in natural waters interfere 

 with the employment of this test. 



As to what may be the exact limits of its indications, I have not 

 yet been able to determine ; but I have readily detected, by its use, an 

 amount of nitrite in water equivalent to one part of nitrous acid 

 in about twenty millions parts of water. 



I have made a number of comparative experiments with this test 

 of mine and those hitherto proposed for nitrites, but chiefly with that 

 of P. Griess, already referred to, as Dr. Prankland (who is one of the 

 first chemists of the day) has stated in his " Water Analysis," that it 

 is the only trustworthy means we have for the estimation of nitrites. 



This test, I may briefly say, depends on the reaction of nitrous 

 acid on metaphenylene diamine, or meta-diamido-benzol, a derivative 

 of benzol, whereby an orange-coloured compound is produced, by the 

 oxidation of this complex basic substance. This reaction is one of 

 extreme delicacy, and the test is carried out pretty much in the same 

 manner as the well-known Nessler's method for the determination of 

 ammonia ; or of mine, just described, for that of nitrites ; the depth of 

 colour produced by the test solution, with the water under examination, 

 being compared with that of one containing a known quantity of 

 nitrite ; the details, however, of the method will be found fully stated 

 in Dr. Frankland's " Water Analysis." 



From several comparative experiments I have made with Griess's 

 method and that of my own, I have come to the conclusion that the 

 latter is almost, if not quite, as delicate a test for the nitrites as the 

 former. I have, however, observed this difference between them, that 

 when the proportion of nitrites present was considerable, that then 

 Griess's test gave a more decided reaction, or that the colour produced 

 was of greater intensity than in the case of the gallic acid test ; but 

 that when the amount of nitrite was exceedingly minute, that then 

 there was but little or no difference in the delicacy of their indications. 

 In some other respects, however, the test which I have proposed 

 possesses advantages over that of Griess ; thus the metaphenylene 

 diamine is at present a compound very difficult to be procured ; so 

 much so that though I applied twice, lately, to one of the best known 

 firms in London for the manufacture of chemicals (Messrs. Hopkin & 

 Williams), they were unable to procure me a little of that substance ; 

 and that which I operated on was kindly given to me by my friend 



