!6 LECTURES ON IMMUNITY 



the poison (at 37 C. it is reached in about 40 minutes with 

 sodium hydrate, in about 100 minutes with ammonia, and 

 not fully in 200 minutes with tetanolysin as the haemolytic 

 agent). After this time the tubes are placed on ice during 

 about twenty-four hours, when the unattacked erythrocytes 

 fall to the bottom, leaving over them a clear haemoglobin- 

 coloured fluid. Generally the intensity of the colour is pro- 

 portional to the quantity of poison added. The strength of 

 the haemolysis and the colour proportional to it is measured 

 by common colorimetric methods. It is supposed that the 

 haemolysis is dependent (under similar external conditions) 

 only on the quantity of free lysin present, and not to a 

 sensible degree on the quantity of bound lysin or antitoxin 

 present in the mixture. By an operation analogous to 

 that described under the measurement of diphtheria-toxin, 

 it is possible when the haemolysis lies between certain 

 limits (below total haemolysis and above a certain observ- 

 able minimum), to use not only a certain strength of colour, 

 but the whole material of research, for the comparison. 



The experiments "in vitro" may be carried out through 

 quite definite intervals of time and at any fixed temperature 

 (between o and 1 00 C. ). Further, to the fluid examined, we 

 may add large quantities of any foreign body and, for in- 

 stance, investigate the influence of different salts by using 

 a physiological solution of cane-sugar (about 7.2 per cent) 

 for the suspension. It is evident how much more diversified 

 knowledge we may obtain by these researches " in vitro " 

 than by those on living animals, which are also expensive, 

 and hence can only be carried out in a relatively small 

 number and at the uncommon institutions equipped with 

 well-filled quarters for animals. 



