218 Scientific Proceedings, Roijal Dublin Society. 



turn blue spontaneously, the change usually coming on after about a minute. 

 In judging the result it is therefore advisable to have regard only to decided 

 tinges of blue coming on at once on addition of the suspected fluid or substance, 

 and, in case of doubt, to make up double the above specified amount of reagent, 

 and dividing it between two test-tubes, observe the reaction in one, keeping 

 the other as control. The test-tubes employed should be absolutely clean, as 

 the minutest traces of oxidizing agents (bleaohing-lime, plumbic oxide, 

 permanganate of potash) suffice to bring about the colour change. Later 

 on I shall have more to say regarding the fallacies of the test and the 

 mode of guarding against them. 



2. Delicacy. — In a series of experiments, with accurately made-up decimal 

 dilutions of sheep's blood, I found that a specimen of benzidine from 

 Kalilbaum gave a distinct bluish-green reaction in a few seconds, with so 

 great a dilution as 1 in 500,000, whilst a control, put up with tap-water 

 instead of the blood-solution, did not begin to turn blue till two minutes 

 after admixture. At the end of five minutes there was hardly any difference 

 between the two tubes, both being a decided blue. I find, however, as the 

 result of a number of tests made with different preparations of benzidine, 

 and different test-tubes, that one cannot always be sure of avoiding pseudo- 

 reactions at so great a dilution, and I am in agreement with Schumm and 

 Westphal (5) that 1 in 200,000 is the greatest dilution, the recognition of 

 which by this test can be safely depended on. By comparative work with 

 freshly prepared guaiacum tincture and hydrogen peroxide, I found, using 

 the same solutions of blood, that the guaiacum ceased to react at 1 in 20,000. 

 Moreover, I find that the most distinct reaction with extremely dilute blood- 

 solutions can be had by superposition of the suspected solution on the surface 

 of the reagent. The blue zone which forms on the line of contact is quite 

 unmistakable. 



3. Use in medieo-kcjal investujations. — But the real value of benzidine in 

 medico-legal work is its power of reacting, not with very dilute solutions 

 of blood, but with minute traces of blood in particulate form ; and this is 

 a point on which I desire to lay special emphasis, as I have not seen it 

 referred to in the literature. The material submitted in medico-legal work 

 consists practically always of suspected stains tliat have long since become 

 quite dry. The matter of which the stain is composed can usually be 

 obtained in particulate form by a process of scraping, and if the fine dust 

 so produced be allowed to fall on the surface of benzidine sol. + H2O2 freshly 

 mixed in a clean watch-glass, each particle, if composed of or containing 

 blood, at once betrays its nature by turning a vivid blue, and, as it goes 

 to solution, yielding a blue streak when the fluid is gently stirred. Should 



