CHAPTER XIV 

 THE TESTS FOR BLOOD 



FROM what has gone before we may now group the chief tests for 

 blood. These may be divided into ( 1 ) microscopical, (2) spectroscopiea^ 

 (3) chemical, (4) biological. 



Microscopical. By the use of the microscope the size and shape 

 of the corpuscles can be ascertained (see p. 86). Reptilian, birds', or 

 camel's blood can be distinguished from that of the domestic animals 

 or man. The method is of no service in distnguishing between the 

 commoner mammals. 



Spectroscopical. The preparation of spectra of haemochrornogen, 

 haematoporphyrin, and acid haematin, are useful in indicating the 

 presence of blood (see p. 94). In old blood-stains the haemoglobin 

 is broken down to hsematin. 



Chemical. Under this heading we may include (a) the preparation 

 of haemin crystals, (b) the guaiacum test for blood. 



Preparation of Hcemin Crystals. Some of the suspected deposit is 

 taken and placed upon a slide with a crystal of common salt or sodium 

 iodide. Acetic acid is added sufficiently to float the cover-slip. 

 Warmth is then applied until bubbles begin to rise beneath the cover- 

 slip. The slide is then removed from the flame to cool, and the pro- 

 cess is repeated three or four times. Great heat must not be used. 

 Upon examination beneath the microscope, chocolate rhombic crystals 

 of haemin (haematin chloride or haematin iodide) will be seen (Fig. 25). 



The Guaiacum Test for Blood is usually employed in testing 

 for blood in urine, stomach contents, and other body fluids. If to 

 the boiled suspected solution a drop of tincture of guaiacum be added, 

 then a few drops of ozonic ether, and the reddish guaiacum turns to 

 a blue colour, it signifies blood. The reaction depends on the iron 

 combined in the haemoglobin. The enzymes known as oxidases give 

 the test, and therefore a positive result is sometimes obtained with 

 such body fluids as milk and saliva, and with the juices of vegetables, 

 apple, pineapple, potato, which sometimes leave a brown stain re- 

 sembling, stale blood. As the oxidases are destroyed by heat a solu- 

 tion suspected to contain blood should be boiled before it is tested. 

 If this condition is complied with, a positive reaction may be taken 

 to indicate blood. If the test is negative, blood is certainly absent. 

 Instead of ozonic ether, hydrogen peroxide or old oil of turpentine 

 can be used. Various tests have been devised using bodies other 

 than guaiacum resin. Such bodies are aloin, bcnzidin, and the leuco 



112 



