258 PHYSIOLOGICAL CHEMISTRY 



blue color with benzidine is indisputable proof of the presence of blood 

 in the fluid, clot or stain examined. The weak point of the tests, 

 medico-legally, lies in the fact that they do not differentiate between 

 human blood and that of certain other species of animal. 



The guaiac test (see page 262), although generally considered less 

 accurate than the hemin test, is held by some to be a more delicate test 

 than the hemin test if properly performed. One of the most common 

 mistakes in the manipulation of this test is the use of a guaiac solution 

 which is too concentrated and which, when brought into contact with 

 the aqueous blood solution, causes the separation of a voluminous 

 precipitate of a resinous material which may obscure the blue colora- 

 tion; this is particularly true of the test when used for the examination 

 of blood stains. A solution of guaiac made by dissolving i gram of the 

 resin in 60 c.c. of 95 per cent alcohol is very satisfactory for general use. 

 The test is frequently objected to upon the ground that various other 

 substances, e.g., milk, pus, saliva, etc., respond to the test and that it 

 cannot therefore be considered a specific test for blood and is of value 

 only in a negative sense. We have demonstrated to our own satisfac- 

 tion, however, that many samples of milk give the blue color upon the 

 addition of an alcoholic solution of guaiac resin without the addition of 

 hydrogen peroxide or old turpentine. It has also been shown 1 that 

 those milks which respond positively, fail to do so after boiling. In the 

 case of blood the test is positive both before and after boiling the blood 

 for 15-20 seconds. Pus does not respond after boiling. Old, partly 

 putrified pus gives the test even without the addition of hydrogen 

 peroxide or old turpentine, whereas fresh pus responds upon the addi- 

 tion of hydrogen peroxide. Saliva gives a positive reaction only in 

 case blood or pus is present. Certain plant extracts give the test before 

 but not after boiling for 15-20 seconds. Buckmaster has advocated 

 the use of an alcoholic solution of guaiaconic acid instead of an alcoholic 

 solution of guaiac resin. He claims that he was able to produce the 

 blue color upon the addition of the guaiaconic acid to milk only when 

 the sample of milk tested was brought from the country in sterile bottles, 

 and further, that no sample of London milk which he examined responded 

 to the test. In the application of the guaiac test to the detection of 

 blood, he states that he was able to detect laked blood when present in 

 the ratio i : 5,000,000 and unlaked blood when present in the ratio 

 1:1,000,000. This author considers the guaiac test to be far more 

 trustworthy than is generally believed. 



Up to within recent times it has been impossible to make an absolute 

 differentiation of human blood. The so-called " biological" blood test 



x Leary: Private communication. 



