McWeeney — Vcdiie of Benzidine for Minute Traces of Blood. 223 



I will now formulate the conclusions at which I liave arrived as the 

 result of my three years' experience of benzidine : — 



1. In tliis substance we have the most delicate reagent for blood hitherto 

 discovered. 



2. Ou contact with benzidine in acetic acid solution + H2O2, dried blood 

 in particulate form from stains, &c., at once assumes a deep blue colour vi'hich 

 is quite distinctive. 



3. None of the secretions and excretions hitherto tested, and none of the 

 substances more frequently occurring in medico-legal practice, have been found 

 to behave towards benzidine in the same way as blood. 



4. Precisely similar reactions are given by many freshly cut vegetables 

 and fruits ; but these are eliminated by boiling, whereas the reaction given 

 by blood-solutions is unimpaired by boiling for ten minutes. 



5. When testing very dilute solutions of blood (weaker tiian 1 : 50,000), 

 tlie test fluid should be divided and one-half kept as a control. No regard 

 should be paid to colour-changes coming on after tiie lapse of one minute 

 from the time of admixture. 



6. Whilst emphasizing the fact that I have hitherto met with no substance 

 reacting witii benzidine in the same way as blood, I do not suggest (as does 

 Asearelli [loc. cit. sub fin.'], that positive results with benzidine should be 

 looked upon as absohifehj conclusice proof oi the presence of blood. Negative 

 results may, however, safely be taken as proving its absence. For the 

 present, until our experience has matured, I am of opinion that positive 

 results with benzidine require confirmation by otlier methods (e.g., microscopic 

 demonstration of the corpuscles, htemin crystals, the spectroscope). 



7. Benzidine is much superior to guaiacum as a preliminary test, and 

 forms an important and most useful addition to the armamentarium of the 

 medical jurist. 



I desire, in conclusion, to gratefully acknowledge the assistance I have 

 received from Prof. H. E.yan, d.sc, f.h.u.i,, on the chemical side of this 

 paper. 



[During and after the reading of the paper, the effect of the benzidine 

 test on dilute solutions of blood, as well as on fresh vegetables and fruits, 

 was demonstrated.] 



SCIENT. PEOC. E.D.S., VOL, XII., NO. XXII. 2o 



