C MICRO-ORGANISMS AND FERMENTATION. 



water, and then placed in an aqueous haematoxylin solution. 

 After a further 24 hours the cells, as seen under the micro- 

 scope, are stained deep black. They are then treated for a 

 few minutes with a 0-25 per cent, solution of iron alum, after 

 which the yeast cells appear colourless with violet or greyish- 

 black cell nuclei. The preparation is mounted in undiluted 

 glycerine. One of the more advanced of recent workers 

 in this field, A. Guilliermond, makes use of the method of 

 staining just described to prove the presence of the cell- nucleus 

 in the yeast cells, but for fixing he prefers picroformol (Bouiri). 

 To prove the presence of the fine hairs, which serve the 

 bacteria as organs of movement, the flagella or cilia, which can 

 seldom be detected by direct microscopical examination of the 

 living bacteria, the following method (Ldffler) is adopted : 

 A small quantity of a very young growth of bacteria 

 (developed for five to eight hours in an incubator) is placed in 

 a drop of water the ordinary supply is preferable to distilled 

 water and the contents of this drop are divided amongst a 

 number of drops of water, placed on a series of carefully 

 cleaned cover-glasses. They are air-dried, and are then passed 

 through a flame in order to fix the bacteria. Care must be 

 taken that the preparation is not heated too strongly. The 

 simplest means of avoiding this is to hold the cover-glass 

 between the fingers, and not to heat it more strongly than 

 they can bear. A large drop of a mordant is now spread 

 over the heated cover-glass. The mordant, which is applied 

 to render the bacteria absorbent to the actual stain, consists 

 of 10 c.c. of tannic acid solution (20 per cent.) mixed with 

 5 c.c. of a cold saturated ferrous sulphate solution and 1 c.c. 

 of a saturated aqueous or alcoholic fuchsine solution. The 

 cover-glass is warmed for about half a minute until steam is 

 given off, but violent boiling must be avoided. The prepara- 

 tion is washed with a powerful stream of distilled water, and 

 afterwards with absolute alcohol until the cover-glass is clear, 

 and only the spot on which the water drop has been evaporated 

 appears cloudy. The staining fluid is now poured over the 

 surface of the cover-glass. It consists of a neutral saturated 

 fuchsine solution in aniline. The preparation is warmed again 

 for a minute until steam rises, washed with a stream of water, 



