192 PHYSIOLOGICAL CHEMISTRY. 



fibrin may be removed and the remaining* fluid is termed defibrin- 

 ated blood. The above theory of the coagulation of the blood may 

 be stated briefly as follows: 



I. Prothrombin + Calcium Salts = Thrombin (or Fibrin- 

 ferment). 



II. Thrombin (or Fibrin- ferment) + Fibrinogen = Fibrin. 

 Among the medico-legal tests for blood are the following: 



(i) Microscopical identification of the erythrocytes, (2) spectro- 

 scopic identification of blood solutions, (3) the guaiac test, (4) 

 the benzidine reaction, (5) preparation of haemin crystals. Of 

 these five tests the two last named are generally considered to be the 

 most satisfactory. They give equally reliable results with fresh 

 blood and with blood from clots or stains of long standing, pro- 

 vided the latter have not been exposed to a high temperature, 

 or to the rays of the sun for a long period. The technique of the 

 tests is simple and the formation of the dark brown or chocolate 

 colored crystals of haemin or the production of the green or 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 196), although generally considered 

 less accurate than the haemin test, is really a more delicate test 

 than the haemin 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 ob- 

 scure the blue coloration : 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 there- 

 fore be considered a specific test for blood and is of value only in 

 a negative sense. We have demonstrated to our own satisfaction, 

 however, that milk many times gives the blue color upon the addi- 

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

 hydrogen peroxide or old turpentine. Buckmaster has very recently 

 advocated the use of an alcoholic solution of guaiaconic acid instead 

 of an alcoholic solution of guaiac resin. He claims that he was 



