METHODS OF CULTIVATION. 167 



Artificial Culture -Fluids. The culture -fluids 

 which have been most extensively used in in- 

 vestigations relating to the physiology and life- 

 histories of the various species of bacteria are 

 infusions of animal and vegetable substances, such 

 as beef, mutton, chicken, fish, gelatine, turnip, 

 potato, cucumber, hay, malt, etc., etc. These in- 

 fusions, as a rule, do not require to be very con- 

 centrated, and they should be as transparent as 

 possible, as the slightest opacity from suspended 

 particles, albuminoid or inorganic, may interfere 

 with the detection by the naked eye of changes 

 in the fluid due to the development of bacteria, 

 and with the recognition of these organisms upon 

 microscopical examination. It sometimes occurs 

 that an infusion of beef or of chicken, which has 

 been carefully filtered and is quite transparent, 

 becomes opalescent from the coagulation of a 

 minute quantity of albuminoid material as the 

 result of the operation of sterilization. I have 

 found this opalescence difficult to remove by fil- 

 tration. It is objectionable, but could hardly be 

 mistaken for the opalescence, or milky opacity, 

 which results from the breaking-down of an infu- 

 sion of this kind, and with due care the experi- 

 menter is not likely to be deceived, especially if 

 he retains a portion of the sterilized fluid for 

 comparison with that used in his culture experi- 

 ments. 



Nitrogen, which is an essential element of the 

 protoplasm of bacterial organisms, is supplied by 



