v] METHODS OF INOCULATION 65 



them prevented, by devitalising the microbes, and by sterilis- 

 ing the medium on and in which the growth has been taking 

 place. This is done by the fumes of formalin (J strength); 

 commercial formalin is a 40 per cent, solution of formal- 

 dehide. A tube, or a number of culture-tubes, in which the 

 further growth of the microbe is to be arrested, are placed 

 best without their cotton-wool plugs into a wide-mouthed 

 bottle or glass cylinder, into which a small quantity of for- 

 malin (J strength) has been poured, then close the cylinder 

 air-tight and let it stand. The vapours of formalin penetrat- 

 ing the tubes even through the wool plugs do their work in a 

 day or two. If a plate- cultivation is to be fixed, a few drops 

 of formalin are placed on the middle of the cover-dish, the 

 plate-dish is now inverted, and allowed to stand for from some 

 hours to a day or two. The formalin vapours fix thereby 

 permanently and kill the colonies, and no further growth 

 either of the colonies already formed or of new contaminating 

 colonies occurs. 



4. Hanging drop cultures. — In order to study microbes 

 in the living state as for motiUty, growth, multiplication, 

 and spore formation, the methods used are practically those 

 known as "hanging drop preparations " first used by Koch. 

 An object-glass slide, possessing a shallow circular pit, is 

 covered, over the pit, with a cover-glass in the centre of which 

 a drop of the fluid suspension of bacteria, or of serum, blood, 

 &c. , is deposited, the drop facing the pit ; the edge of the 

 cover-glass can be fixed around the pit by paraffin or oil or 

 cement, and the observation can be carried out either at 

 the temperature of the laboratory or by placing the object on 

 a warm stage at any desired temperature. The droplet being 

 small the examination can be carried out even with high 

 powers as easy as an ordinary fresh preparation. Motility, 

 the elongation of bacilli, their division, germination of spores, 



