170 BACTERIOLOGY. 



altered. If carefully dried before fixing, this does not 

 occur and the morphology of the organism remains un- 

 changed. 



A better plan for the process of fixing is to employ 

 a copper plate about 35 cm. long by 10 cm. wide by 

 0.3 cm. thick. This plate is laid upon an iron tripod 

 and a small gas-flame is placed beneath one of its 

 extremities. By this arrangement one can get a gradu- 

 ated temperature, beginning at the part of the plate 

 above the gas-flame where it is hottest, and becom- 

 ing gradually cooler toward the other end of the plate, 

 which may be of a very low temperature. By dropping 

 water upon the plate, beginning at the hottest point and 

 proceeding toward the cooler end, it is easy to determine 

 the point at which the water just boils ; it is at a little 

 below this point that the cover-slips are to be placed, 

 bacteria side up, and allowed to remain about ten min- 

 utes, when the fixing will be complete. The same may 

 be accomplished in a small copper drying-oven, which 

 is regulated to remain at a temperature of from 95 to 

 98 C. In very particular comparative studies this plan 

 is to be preferred to the process of passing the cover- 

 slips through a flame, as the organisms are always sub- 

 jected to the same degree of heat, and the distortions 

 which sometimes occur from too great and irregular ap- 

 plication of high temperatures may be eliminated. The 

 fixing consists in drying or coagulating the gelatinous en- 

 velope surrounding the organisms, by which means they 

 are caused to adhere to the surface of the cover-slip. It 

 is sometimes desirable to fix the preparations without the 

 use of heat, as in the case of pus or other exudates. In 

 this event, after drying the thinly spread material care- 

 fully in the air, the cover-slip on which it is placed is 



