160 THE AMERICAN MONTHLY [May 



Various other substances may be added to the gelatin : of 

 these glucose and litmus are the most important. For 

 surface cultures 2 per cent agar (a dried sea-plant closely 

 related to Irish moss, and found otf the coast of East 

 Asia) is added to the beef bouillon. The nearly transpar- 

 ent jelly formed by this mixture remains solid at all tem- 

 peratures required for bacterial growth ; consequently it 

 is used verj^ largely in propagating pathogenic germs 

 that rec^uire a high temperature for their development. 

 Glucose, glycerin and many other substances may be 

 added to the plain agar, as desired by the experimenter. 

 The glycerin-agar is perhaps the most important, and it 

 is used very extensively for growing the bacillus of tub- 

 erculosis. 



One of the most important points to be determined in 

 making up all kinds of culture media is the amount of 

 alkali to be added. For ordinary work 1 cc. sliould re- 

 quire about 0.18 cc. of N-20 sodium-hydrate solution to 

 make it neutral when phenolphtalein is used as an indi- 

 cator, and will be slightly alkaline when tested with litmus. 



All artificial and most natural culture media, after 

 being tilled into the sterilized test-tubes (which are then 

 plugged with cotton), must undergo fractional steriliza- 

 tion — that is, be heated for about thirty minutes on sev- 

 eral successive days in live, flowing steam, which destroys 

 all forms of life. If the media is to be used at once, the 

 cotton plugs which prevent gei'ms from passing into 

 the tube will be sufficient protection, but if the tubes are 

 to be kept for any time, or placed on the market, the pro- 

 truding portion of the plug must be cut off, and the tubes 

 capped with some preparation, as rubber, sealing-wax, 

 etc., to prevent evaporation. In this work extreme care 

 must be taken, else many of the tubes will be found in- 

 fected within a few days. Even when the greatest pains 

 have been taken, an occasional tube will show develop- 

 ment. On on account should the tubes, after they have 



