TYPHOID 181 



of the typhoid bacillus. The culture should preferably be 

 from a specimen of low virulence, but it is not an essential 

 point. It is also better (but again not essential, if free from 

 clumps) for the culture not to be over twenty-four hours 

 old. If no broth culture is available, an agar culture rubbed 

 up in broth or salt solution will do as well — and for compara- 

 tive experiments, even better. A control cover-glass 

 preparation of the culture should always be made and 

 examined. 



The mixed drop of diluted serum and of culture should 

 have very little depth, as the ' clumps ' have a tendency tO' 

 sink and get beyond the focal distance of the lens. The 

 cover-glass is now placed on a glass slide having a central 

 hollow, which is surrounded by vaseline to prevent evapora- 

 tion, or on an ordinary glass slide covered with a piece of 

 moistened blotting-paper with the central portion cut out, 

 and examined with an immersion lens. At first the bacillii 

 may be moving actively about, but if the case is one of 

 enteric fever they soon slow down or stop, then gradually 

 groups of two or three form ; these groups soon hang on to 

 each other until nearly all are collected into various crowds 

 or 'clumps' with very few isolated bacilli left. If the 

 reaction is complete within thirty minutes the case is 

 certainly one of enteric fever. Generally it is not complete, 

 and there may be groups of only ten or twenty, but the 

 occurrence of grouping and loss of movement are in them- 

 selves almost as decisive. 



The dilution of the serum is necessary because the serum 

 of normal individuals will often, undiluted, evince an 

 agglutinative power, but not, so far as my experience goes, 

 in a total dilution of less than one to sixteen. Therefore 

 neglect of sufficient dilution may lead, and has already led, 

 into error. 



The time limit is necessary for a similar reason. Very 



