McWki'INioy — Value of Benzidine for Minute Traces of Blood. 319 



it be fovind impracticable to obtain a scraping, tbe fabric can be teased in 

 a drop of normal saline, tlie stained fibres removed to another slide, and 

 treated between slide and eoverglass with a drop of the reagent, when the 

 presence of blood at once reveals itself by the brilliant blue coloration of 

 the affected fibre. This is the most delicate test possible for minute 

 quantities of dried blood, and groups of a dozen or fewer shrivelled 

 and deformed red corpuscles can readily be picked out by its means. 

 Should there be available a little more of the stained material, the positive 

 benzidine test may be most readily corroborated by treating between slide 

 and eoverglass with 32 per cent, solution caustic potash or soda. This 

 renders visible tlie outlines of the individual red cells, and causes them to 

 separate from each other without producing distortion, so that they can be 

 readily recognized and measured. Moreover, the strong alkali makes the 

 little mass translucent, brings out its reddish colour, and transforms the 

 blood-pigment into hsemochromogen, which can be easily identified as such 

 by means of the micro-spectroscope. Should all the suspicious matter go to 

 solution during the initial microscopic examination, tlie best way to proceed 

 is to take it up in a fine glass tube, and then allow some of tlie benzidine 

 reagent to run up by capillarity, whereupon the blue colour, developing at 

 the line of junction between the fluids, at once reveals the presence of blood. 



4. Sources of error. — We have first of all to guard against pseudo- 

 reactions due to the presence of oxydizing substances in the test-tubes and 

 other glass-ware used. I need hardly say that test-tubes that have been 

 used for urine-testing and have traces of copper solutions adherent to 

 their walls are quite unsuitable. Even the use of perfectly new test-tubes 

 does not always get rid of this source of error. I have occasionally met 

 with new test-tubes in which the benzidine + acetic acid at once struck 

 a most brilliant blue without addition of hydrogen peroxide — a phenomenon 

 due, no doubt, to the presence of plumbic oxide in or on the glass. Of course 

 such tubes must not be used. In general 1 think a few preliminary experi- 

 ments with the batch of test-tubes about to be used are advisable, and in 

 doubtful cases one-half of the quantity of reagent (acetic benzidine + H3O2 

 kept over, without addition, for comparison. 



Coming now to the reactions given by benzidine with the substances most 

 likely to occur in the examination of specimens for forensic purposes, I find 

 that mliva does not give a typical reaction either when directly added 

 to the reagent or dried on filter-paper. Tiie same remark applies to naml 

 mucus and mucoid sjnituin, if free from blood. I have on several occasions 

 tested spots of nasal mucus dried on a pocket-handkerchief without obtaining 

 more than a dirty greenish-blue tinge remaining localized, and without that 



