EXAMINATION OF AIR, SOIL, AND WATER. 147 



of moderate temperature. In about two or three days the cultivation 

 may be examined. In some cases the colonies may be counted at 

 once ; more frequently they are so numerous that the plate must 

 be placed on a dark background, and a special process resorted to. 

 A glass plate, ruled by horizontal and vertical lines into centimetre 

 squares, some of which are again subdivided into ninths, is so 

 arranged on a wooden frame that it can cover the nutrient -gelatine 

 plate without touching it (Fig. 74). A lens is used to assist in dis- 

 covering minute colonies. If then the colonies are very numerous, 

 the number in some small division is counted, if less in some large 

 one, and an average is obtained from which the number of colonies 

 on the entire surface is calculated. A separate calculation of the 

 liquefied colonies should be also made, and their number, as well 

 as the total number of colonies present in 1 cc. of the sample, 

 recorded. Any peculiar macroscopical appearances, colours, etc., 

 should be noted, and then the microscopical appearances of the 

 colonies studied. Lastly, examination of the individual organisms 

 should be made by cover-glass preparations, and by inoculation of 

 nutrient gelatine, potatoes, and other media. 



Instead of plates, Petri's dishes may be used both for gelatine 

 and agar-agar cultivations. 



FIG. 75. ESM ARCH'S ROLL-CULTURE. 



a., India-rubber caps; b b b, longitudinal line drawn on the tube; c,c,c, transverse 

 lines for counting colonies (FRANKLAXD). 



Another plan is to take a measured quantity of the sample of 

 water and prepare a roll-culture, using a large-sized test-tube 

 (Fig. 75). The colonies can be counted with the aid of a lens (Fig. 76). 

 Microscopical preparations and sub-cultures can be made from the 

 colonies, and the anaerobic bacteria can be examined by Frankel's 

 modification of this method (p. 131). 



A drop of the sample of water may also be added to liquefied 

 nutrient gelatine in a test-tube, the organisms distributed, and the 

 gelatine allowed to solidify in the tube. A rough comparison of 

 water samples may be made in this way. 



Microscopic Examination. A drop of the water may be mounted 

 and examined without staining; or allowed to evaporate on a cover- 



