TEXT-BOOK OF BACTERIOLOGY. 91 



sterilized test-tubes about 10 c.cm. in each. In these it is made to 

 consolidate by heating, and it is generally desirable, as in the case 

 of agar, to heat the fluid in an oblique position, so as to obtain a 

 large surface. 



For this heating, double-cased tin boxes are used whose bottom 

 is moderately inclined. Between its walls there is water, which is 

 heated from below by a gas-burner. Into this apparatus the filled 

 test-tubes are placed, and with them a thermometer, to give at any 

 time the temperature of the interior space. Warm slowly up to 

 about 68 C. and take care that the limit of 70 C. be not exceeded. 



The serum consolidates more or less quickly. Those tubes in 

 which it is fully coagulated are at once removed, for as we must 

 not employ more than 70 of heat, so also we must not expose 

 albumin too long to the action of heat after it has once coagulated 

 in both cases the liquid would harden into a dirty-gray opaque 

 mass. 



To be faultless the prepared serum should, as already stated, 

 be transparent, yellowish, and of a jelly-like consistency. The 

 serum in the tube has well-defined, smooth edges; at the bottom 

 of the tube a little clear water always condenses, and for months 

 prevents the substance from drying up. 



Were the serum to be used at once, a grave fault would be com- 

 mitted : the substance has not as yet been sterilized. 



This cannot be done afterward in the usual way, by heat in the 

 steam generator, for that would cause a complete loss of trans- 

 parency and render the serum useless.^ It must, therefore, be ster- 

 ilized before its consolidation, by Tyndall's method of discontinuous 

 sterilization, the principles of which have been mentioned. The 

 liquid serum must be warmed for about eight days, each day for 

 about two hours, to 54 or 56 C., and then brought to consolidation 

 at 68 C. 



It is better still to avoid this troublesome and tedious process 

 altogether by taking all possible care in obtaining the blood, in 

 pouring it from vessel to vessel, and to so manage that no germs 

 get into the serum, in which case sterilization is superfluous. If 

 done in this manner, it will be found that the greater part of the 

 tubes remain unaltered. To make sure, keep the consolidated 

 serum for three or four days at breeding temperature. Any 

 germs of bacteria that may have been present will, by the end of 

 that time, have arrived at a distinctly- visible point of development, 

 and the tubes containing them can be removed. The others may 

 be regarded as germ-free and can be used. 



With the other kinds of food media this very convenient method 



