156 Proceedings of the Royal Irish Academy. 



lain capsule, furnished •with a handle, which wiU admit of the applica- 

 tion of heat when it may be desirable to hasten the reaction by that 

 agent. 



This test is one of great delicacy, for I have found that one small 

 di'op of an aqueous solution of carbolic acid, containing a thousandth 

 part of its weight of that acid, and only absolutely about the one- 

 seventy-thousandth part of a grain, when mixed with three or four 

 drops of the molybdic solution, produced immediately the yellowish- 

 brown eifect, which, after a few minutes, passed into a very distinct 

 and beautiful purple colouration, and this colour remained quite per- 

 ceptible on the fourth clay afterwards, though it had each day become 

 fainter from exposure to the air, and its consequent absorption of 

 moisture. But this is not the limit of its deKcacy, for I have detected 

 by its means the carbolic acid in one drop of an aqueous solution five 

 times more dilute, or -where it contained the one-five-thousandth part 

 of its weight of that acid, and in which there was only about the one- 

 three-hundi'ed-and-fifty -thousandth part of a grain present. 



Por the success of this test, it is necessary to attend to a few parti- 

 culars, one of the most important being, that only a drop or two of the 

 liquid under examination should be employed, for if much more be 

 used the reagent will be diluted too much, and the characteristic re- 

 action will not take place : for so great an effect has water on it, that 

 even when the pui'ple colouration is fully developed, the addition of 

 that substance will cause either the coloui'ation to disappear almost 

 entirely, if the quantity of carbolic acid present be exceedingly 

 minute, or if more abundant it first changes the purple to red, and 

 then into a light reddish-brown, which becomes more and more faint 

 on further dilution ; but the addition of a few drops of the test solu- 

 tion, or even of strong sulphuric acid, again reproduces the purple, 

 though of course fainter in its colour in proportion to the previous de- 

 gree of dilution ; thus showing that the mixtui'e must be very strongly 

 acid for the production and continuance of this purple effect. Another 

 point to bear in mind is this, that when carbolic acid itself, and not an 

 aqueous solution of it, is acted on by the molybdic reagent, a dark 

 olive, quickly changing to a very deep blue, will be produced, but not 

 the pui'ple colouration ; a cold saturated aqueous solution, however, of 

 carbolic acid when so treated will yield the purple reaction ; but even 

 here there will be a tendency to develop the olive or blue effect, 

 especially where the reagent employed contains a large proportion of 

 molybdic acid ; and I may observe that weaker solutions of carbolic 

 acid give more satisfactory results, as the action appears to be too 

 energetic when the acid itself or very strong solutions are employed. 



The last precaution I wish to direct attention to, for the successful 

 performance of the test, is this, that in applying heat to hasten the 

 reaction, it should be limited to a gentle heat that the hand can bear 

 when applied to the bottom of the capsule, this being about from 

 120° to 130° F., which is quite sufficient for this purpose, besides 

 not exercising any destructive effect on the purple reaction ; for I 

 may observe that if the heat be raised even to 212° F., and con- 



