MICROORGANISMS AND FERMENTATIONS 



23 



development are studied. In this way the moulds may readily 

 be identified. Greater difficulties are encountered in dealing 

 with the two other groups, which are much simpler in morpho- 

 logical structure ; in these groups cultural and especially bio- 

 chemical characteristics are more important as means of identifica- 

 tion. It has already been mentioned that the appearance of a 

 stab culture will reveal the character of the organism with respect 

 to atmospheric oxygen, and as to whether it produces proteolytic 

 enzymes or coloured products. Sometimes even the shape and 

 general macroscopic appearance of the colony may be so charac- 

 teristic as to afford in itself a means of identification. By varying 

 the composition of the nutrient medium characteristic preferences 

 may be discovered for example, an investigation may be made 

 to determine which sugars are fermented with the production of 

 acid or acid and gas. Satisfactory results, however, can only be 



FIG. 23. Jorgensen's Moist Chamber. 



got by finding out what fermentation products are formed under 

 certain conditions. 



For detailed descriptions of the microscope, works on micro- 

 scopy or optics may be consulted. It need only be mentioned 

 here that high magnification is achieved as the product of the 

 magnifications due to the two systems of lenses, the eye-piece 

 (ocular) and the objective. The object to be examined is placed 

 on a glass slide and covered with a square or round cover slip of 

 thin glass. For high magnifications, a drop of cedar- wood oil of 

 the same refractive index as glass is placed between the cover 

 slip and the special immersion objective, an arrangement which 

 entails less loss of light than when air intervenes, and which is 

 essential when magnifications of 1,000 diameters are required. 



In order to determine whether an organism is motile or non- 

 motile, it is observed in a hanging drop adhering to the under side 

 of a cover slip placed over a hollow slide ; in this way the growth 

 and development of the organism may also be observed. In 

 examining moulds, which require a plentiful supply of oxygen, 

 the cover slip is separated from the slide by a glass ring, and the 

 whole arrangement, known as a moist chamber, is sealed together 



