CH. XXVI.] TESTS FOR BLOOD 439 



bearing a white reflecting surface (S) and a platform. Under the platform is a slot 

 carrying a glass wedge stained red (K) and moved by a wheel (R). On the platform 

 is a small cylindrical vessel divided vertically into two compartments, a and a'. 



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 (SJ 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 hsemin crystals. The old 

 test with tincture of guaiacuni and hydrogen peroxide, the blood 

 causing the red tincture to become green, is very untrustworthy, as it 

 is also given by many other organic substances. 



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

 not a red fluid or stain upon clothing is or is not 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 coagu- 

 lating 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. This 

 subject is one of great importance. 



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 

 disease. Vaccination produces in a patient an attack of cowpox or 



