BLOOD-SERUM. 73 



jarred with the hand to see if solidification, as indi- 

 cated by the disappearance of tremors from the serum, is 

 beginning. If the temperature gets too high, or the ex- 

 posure is too long, an opaque medium results. The tem- 

 perature to be observed is that of the air inside the cham- 

 ber, and also that of the water surrounding it. The latter 

 is usually a degree or two higher than the former. The 

 tubes should not rest directly upon the heated bottom or 

 against the heated sides of the chamber, but should lie 

 upon racks of wood or wire, and be protected from the 

 sides by a wire screen of gauze ; in this way the tubes 

 are all exposed to about the same 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 the large form of apparatus is 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-fashioned apparatus of Koch is employed. 

 The latter form is preferable, as it is more easily man- 

 aged. 



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 dry- 

 ing. The superfluous ends of the cotton plugs should be 

 ^ burned oif, and the mouths of the tubes should then be 

 covered by sterilized rubber caps. Even with the greatest 

 care, it not uncommonly happens that one or two of the 

 lot of tubes thus prepared and protected will become con- 

 taminated. 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 hyphen downward through 



