APPENDIX. 369 



the number in some small divisions is counted, if less in 

 some large ones, and an average is obtained from which 

 the number of colonies on the entire surface is calculated. 

 A separate calculation of the liquefying colonies should 

 be also made, and their number, as well as the total 

 number of colonies present in I ccm. of the sample, re- 

 corded. Any peculiar macroscopical appearances, colour, 

 etc., should be noted, and then the microscopical appear- 

 ances of the colonies studied. Lastly, examination of 

 the individual organisms should be made by cover-glass- 

 preparations, and by inoculation of nutrient gelatine, pota- 

 toes, and other media. 



Examination by Test-Tube Cultivation. A 

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

 nutrient gelatine in a tube, the organisms distributed as 

 already explained (p. 75,) 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 in the way 

 described under drop-cultivations (p. 94), or a drop is 

 allowed to evaporate on a cover-glass placed under a 

 bell-glass. This is then passed three times through the 

 flame, and stained in the usual manner. The examina- 

 tion of rain water, drinking water, tap water, sea water, 

 various liquids and infusions, etc., by these methods opens 

 up a wide field for research. Pettenkofer has shown that 

 impregnation with carbonic acid of water containing many 

 bacteria diminishes the number of the latter. The ex- 

 amination of waters before and after filtration, or after 

 addition of chemical substances, are matters which require 

 further investigation. 



