250 SCIENCE PROGRESS 



The citrate prevents clotting of the blood and the isotonic salt 

 solution does not allow haemolysis to take place. About twenty 

 drops of blood are thus prepared. The mixture is then centri- 

 fugalised, when it will be found that the corpuscles have been 

 driven to the bottom whilst the plasma remains in the super- 

 natant fluid. The latter is syphoned off and the corpuscles 

 are again washed in physiological salt solution. A second 

 centrifuge again drives the corpuscles to the bottom and leaves 

 the clear salt solution above. If this is syphoned off the tube 

 only contains washed human corpuscles, and it is found that 

 the white corpuscles occupy the upper layers whilst the majority 

 of the reds are at the bottom ; so that if the tube is well sloped 

 on its side, and some of the "cream" be drawn up into a 

 pipette, a large number of white corpuscles are available. 

 The bacteria are obtained by taking a loopful of growth off an 

 agar slope and mixing it up in salt solution (in some cases a '85 

 per cent., in others a 1*5 per cent., solution is used). An 

 emulsion of bacteria is thus obtained. 



The blood serum is readily obtained by pricking the finger 

 and drawing a small quantity into a capillary pipette or a small 

 capsule. The blood thus obtained is allowed to clot, and as 

 the clot contracts the clear serum is expressed. An equal 

 volume of the blood-cream, the bacterial emulsion, and the 

 serum are drawn up into a capillary pipette and intimately 

 mixed together. The pipette is sealed off in the flame and the 

 mixture is placed in an incubator, kept at body temperature 

 (37 C.) for fifteen minutes. At the end of this time the pipette 

 is taken out, the end broken off, and the mixture blown on to a 

 clean slide, where it is again well mixed. A sample of the 

 mixture is then placed on another slide and a blood film pre- 

 pared. This is stained with Leishman's dye and is ready for 

 microscopic examination. We will suppose that this experiment 

 has been conducted with the serum obtained from a healthy man; 

 in actual practice the serum obtained from a number of normal 

 men is " pooled" together, equal quantities of each contributing 

 to the whole. The experiment is now repeated in exactly the 

 same way, but the serum used in this case is obtained from the 

 patient under treatment. We now have two blood films to 

 examine. 



Using a y^th oil-immersion lens for the purpose it is seen 

 that the phagocytic white cells have eaten up a certain number 



