Il8 LABORATORY GUIDE IN BACTERIOLOGY 



slide to one of the cover slips and from this to each of 

 the other cover slips. 



g) Allow to dry in the air. 



h) Cover with the mordant. 



i) Heat over a small flame for i minutes while 

 steam rises, or better, heat on a water bath for 5 minutes. 

 Replace the evaporated mordant to prevent its drying 

 on the cover slip. 



j) Wash in water. 



k) Drain the water off with blotting paper. 



/) Cover with anilin gentian violet or carbol fuchsin. 



m) Heat as before over a small flame for ij minutes, 

 or better, on a water bath for 5 minutes. 



n) Wash in water. 



0) Dry and mount in balsam. 



4. Special study. Agglutination. Dried-blood 

 method of Johnston: A drop of blood of a typhoid 

 fever patient is obtained by pricking the lobe of the ear, 

 previously cleaned and washed with alcohol. The 

 blood is taken up by a piece of sterile non-absorbent 

 paper or on a sterile aluminum slide. This is sent to a 

 laboratory, where the blood is dissolved in physiologi- 

 cal salt solution in such a manner as to obtain an 

 approximate dilution of 1:25. This solution is then 

 tested with a suspension of typhoid bacilli, a young 

 culture of which is constantly kept on hand for this 

 purpose. A loopfai of the diluted blood is mixed with 

 a loopful of the suspension on a cover glass, this making 

 a dilution of i : 50. The cover glass is then inverted over 

 a hollow slide like a hanging drop and observation made 

 after two hours' incubation at 37 C. For laboratory 

 tests the serum of an animal (either a rabbit or a guinea- 

 pig) which has been injected with cultures of B. typho- 



