H 



grown in a solid medium they may appear as different as 

 chalk from cheese. 



The most useful culture medium is a "meat jelly," 

 composed of a pound of lean beef made into a broth, with a 

 litre of water, to which is added 5 per cent, of gelatine, 0.5 

 per cent, of peptone, and 0.25 per cent, of common salt. 

 This is cooked and filtered, according to directions which 

 may be found in any of the undermentioned works on Bac- 

 teriology, and is transferred to sterilised test tubes, plugged 

 with sterilised cotton wool. The filtration, which is 

 absolutely necessary to secure a perfectly clear medium, is a 

 most tedious process, so that I should advise anyone who 

 wishes to experiment with cultivations of bacteria, to buy the 

 tubes of nutrient material ready made. 



Most important is the sterilisation of the jelly in the 

 test tubes. They are placed in iron crates in a steaming 

 apparatus, like that in the room, and kept at a temperature 

 of loo'C. for 15 or 20 minutes. This kills all micro- 

 organisms, but not their spores. To get rid of them the 

 tubes are kept at a temperature of from 80° to go°F. for 

 24 hours, which causes them to germinate ; and then the 

 steaming process is repeated for 10 minutes or so, to kill the 

 fresh growths. The next day the process should be repeated ; 

 and afterwards the tubes should be kept at the same tem- 

 perature for three weeks, when, if they remain perfectly 

 clear, they may be pronounced sterile. The steaming 

 should not be carried on for more than 20 minutes at the 

 outside, or the jelly loses its consistency, less than 10 

 minutes will not kill the bacteria. Some of the tubes, on 

 the table were prepared in this way more than a year ago, 

 and you will see that the jelly is still perfectly clear. 



This jelly is suitable for the cultivation of those organisms 

 which will grow at the ordinary temperature of the air in 

 summer ; but many flourish only at 90° to ioo°F., at which 

 it melts. To get a mixture which will remain solid at this 

 higher temperature, Agar Agar, or Japanese isinglass, is 

 used instead of the gelatine. The drawback to this, 



