STERILISED BLOOD SERUM. 



IOI 



lost. At the end of this process the serum is sterile, but still liquid. 

 In order to solidify it, without at the same time destroying its clear- 

 ness and transparency, the tubes containing it are laid on their sides 



\TT. 



Fig. 54. — Serum inspissator or thermostat, with two test-tubes in 

 position. One-half of glass lid and felt cover are not shown in 

 drawing. The apparatus consists of a double-walled box, covered 

 with felt, and heated from below ; the temperature being maintained 

 by warm water occupying the interspace between the two walls. 



on the sloping floor of a thermostat (see Fig. 54), which is so tilted 

 that the plane of the surface of the serum cuts that of the lumen of 

 the test-tube obliquely, and the surface of the serum is thus largely 

 increased, and offers a wider area for the cultivation of micro- 

 organisms. This is of the greater importance, from the fact that 

 in the case of most organisms which can be advantageously culti- 

 vated in this medium it is found that growth only occurs on the free 

 surface of the serum, and not at all in its deeper layers. 



The next part of the process requires the most careful and con- 

 stant attention of the operator. It consists in raising the temperature 

 in the thermostat to from 68' to 8o° C, which causes a more or less 

 rapid solidification of the serum. The serum first becomes solid 

 merely, retaining its transparency, but if the process be allowed to 



