E. A. Watson 
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is under liis perfect control and that the least disturbing factor will 
be known to him. In the actual test only one complement unit is 
employed (the minimal amount necessary for the completion of the 
haemolytic system) so that only one of the reaction groups can come 
into operation; it is according to whether the complement unit is 
attracted and affixed to the antibody-combining group or to the 
haemolytic group that we obtain a positive or a negative reaction. 
The former will always be effected when the antigen and antibody 
correspond, that is, when the serum tested contains the specific reaction 
products of dourine infection even though in minutest quantity, so 
delicate is the reaction. Neither the antigen alone nor the serum alone, 
when properly prepared, can take up the complement unit; to do so, 
all three factors must be brought into intimate contact, and when the 
test serum does not contain specific dourine antibodies the complement 
is not fixed to this group but remains free to join with and complete 
the two factors of the haemolytic system, so that the red cells undergo 
haemolysis and a negative reaction is indicated. 
Technique and Procedure recommended. 
Apparatus required. Heavy glass tubes without lip, 5 ins. by f in., 
and racks to hold twenty-four tubes in a double row, one above the 
other. Small test tubes, 4 ins. by § in., for serum inactivation. Finely 
graduated measuring pipettes of 0-1, 1-0, 5-0 and 10-0 c.c. capacities. 
Graduated cylinders of 50 and 100 c.c. capacities. Erlenmeyer flasks of 
heavy glass, standard sizes up to 500 c.c. capacity. Large centrifuge 
cups and small centrifuge tubes. Ampoules and vials. A high power 
centrifuge machine, large water bath, and incubator room. 
All glassware is sterilized by dry heat. 
Diluting, washing and preserving fluids. 
(1) Normal salt solution—0-85 per cent, pure sodium chloride in freshly 
distilled water. A large quantity should be made up (5000 c.c.) and 
sterilized in flasks having a siphon attachment. 
(2) Citrated salt solution: 
Normal salt solution .... 100-0 
Sodium citrate . . . . . 1-5 
(3) Preserving fluid for trypanosomes: 
Normal salt solution .... 90-0 
Pure neutral glycerine .... 10-0 
Formalin (Scherings) . . . . 0-1 
(4) Preserving fluid for serum: 
Glycerine ...... 95-0 
Phenol ...... 5-0 
