40 AIDS TO BACTERIOLOGY 



which will not grow on an organic medium The gelat- 

 inous consistency is obtained by means of dialysed silicic 

 acid. 



Irish Moss Jelly is used for the culture of the thermo- 

 philic organisms, and various other media are employed, 

 the composition of which is given in different monographs. 



To prevent evaporation, and assist exclusion of aerial 

 organisms, culture-tubes may be covered with rubber 

 caps, or after the last steaming their mouths may be pro- 

 tected with gutta-percha tissue. Media should be kept 

 in a cool, dark place, such as in suitable- sized tins with 

 lids. A piece of filter-paper may be placed at the bottom 

 of the tin, and a few drops of clove oil sprinkled thereon. 

 In this way the percentage of tubes spoiling on keeping 

 is reduced considerably. 



Desiccated media may be purchased, some of which 

 answer very well. 



The Cultivation and Isolation of Bacteria. The ubiqui- 

 tous character of bacteria renders it necessary that 

 those under examination should not become contaminated 

 with extraneous organisms. To preclude such con- 

 tingency, resort is had to certain devices for protection 

 against pro temjwre aliens. In the absence of draughts, 

 aerial bacteria do not move in a horizontal direction, 

 but merely drop. Consequently, tubes of media are not 

 held with the mouths up during manipulation, but in a 

 more or less horizontal position. Dry cotton-wool is an 

 effectual bacterial filter, and is used for plugging vessels. 

 The part of the plug entering the test-tube is never touched 

 with the hand by conservative workers, who leave an 

 ample portion of plug outside the tube for this purpose. 

 Many, however, regard the large plug as an archaic fetish, 

 and practise economy in cotton- wool by using a loosely fit- 

 ting plug about an inch long, which all goes in the mouth 

 of the test-tube, and is removed by sterile forceps and held 

 by the top part. It was formerly the custom to always 

 ' flame ' a plug before reinserting it, but except when the 

 plug has been dropped, or otherwise exposed to con- 

 tamination, this ritual is not generally honoured in routine 

 work. 



Gelatin Plate Cultures. Three test-tubes contain- 

 ing nutrient gelatin are placed in warm water at about 

 40 C. until the contents are liquid. This temperature is 



