124 BACTERIOLOGY. 



temperature. The thermometer which indicates the 

 temperature inside the chamber should not touch the 

 surfaces, but should either be suspended free from 

 above through a cork in the top of the apparatus, if 

 a large form of apparatus be used ; or should lie upon 

 a rack of cork or wood, its bulb being free and a little 

 lower than the other extremity, if the small, old-fash- 

 ioned apparatus of Koch be employed. The latter form 

 is preferable, as it is more easily managed. 



When solidification is complete the tubes are to be 

 retained in the erect position, and, unless they are 

 intended for immediate use, must be prevented from 

 drying. The superfluous ends of the cotton plugs 

 should be burned off, and the mouths of the tubes may 

 then be covered by sterilized rubber caps, or, as Ghris- 

 key suggests, they may be closed with sterilized corks 

 pushed in on top of the cotton plugs. Even with the 

 greatest care it not uncommonly happens that one or 

 two of a lot of tubes thus prepared and protected will 

 become contaminated. This is usually due to spores of 

 moulds that have fallen into the rubber caps or on the 

 cotton plugs during manipulation, and, finding no 

 means of outward growth, have thrown their hyphse 

 downward through the cotton into the tube, and their 

 spores have fallen on the surface of the serum and 

 developed there. 



The foregoing is, in the main, the plan originally 

 recommended by Koch for the preparation of this 

 medium. In recent times, however, particularly since 

 the diagnosis of diphtheria by the method of Loffler has 

 become so general, and large quantities of serum-tubes 

 have been found necessary, modifications have been sug- 

 gested that have, in this country at least, almost entirely 



