472 THE BLOOD [CH. XXIX. 



Fill with a pipette the compartment a' over the wedge with distilled water. 

 Fill about a quarter of the other compartment (a) with distilled water. 



Prick the finger and fill the short capillary pipette provided with the instru- 

 ment with blood. Dissolve this in the water in compartment a, and fill it up with 

 distilled water. 



Having arranged the reflector (S) to throw artificial light vertically through 

 both compartments, look down through them, and move the wedge of glass by the 

 milled head (T) until the colour of the two is identical. Read off the scale, which is 

 so constructed as to give the percentage of haemoglobin. 



Dr George Oliver's Method consists in comparing a specimen of blood 

 suitably diluted in a shallow white palette with a number of standard tests very 

 carefully prepared by the use of Lovibond's coloured glasses. These standards are 

 much better matches for blood in various degrees of dilution than in most colori- 

 metric methods. The yellow tint of diluted haemoglobin is very successfully 

 imitated. 



Tests for Blood. These may be gathered from preceding descrip- 

 tions. Briefly, they are microscopic, spectroscopic, and chemical. 

 The best chemical test is the formation of haemin crystals. The old 

 test with tincture of guaiacum and hydrogen peroxide, the blood 

 causing the tincture to become bluish green, is very untrustworthy, 

 as it is also given by many other organic substances. The test, 

 for instance, is given by milk, and is there due to the presence 

 of an enzyme called a peroxidase, which is destroyed by boiling. 

 Boiled blood, however, gives the test as well as fresh blood, and the 

 reaction is due to the presence of the iron-containing radical of 

 haemoglobin. 



In medico-legal cases it is often necessary to ascertain whether or 

 not a red fluid or stain upon clothing is of blood. In any such 

 case it is advisable not to rely upon one test only, but to try every 

 means of detection at one's disposal. To discover whether it is blood 

 or not is by no means a difficult problem, but to distinguish human 

 blood from that of the common mammals is possible only by the 

 " biological " test described at the end of the next section. 



Immunity. 



The chemical defences of the body against injury and disease are 

 numerous. The property that the blood possesses of coagulating is 

 a defence against haemorrhage ; the acid of the gastric juice is a great 

 protection against harmful bacteria introduced with food. Bacterial 

 activity in urine is inhibited by the acidity of that secretion. 



Far more important and widespread in its effects than any of the 

 foregoing is the bactericidal (i.e. bacteria -killing) action of the blood 

 and lymph; a study of this question has led to many interesting 

 results^ especially in connection with the problem of immunity. 



It is a familiar fact that one attack of many infective maladies 

 protects us against another attack of the same disease. The person 

 is said to be immune either partially or completely against that 



