154 BACTERIOLOGY. 



dried before fixing, this does not occur and the morpho- 

 logy of the organism remains unchanged. 



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. In very particu- 

 lar comparative studies this plan is to be preferred to 

 the process of passing the cover-slips through a flame, 

 as the organisms are always subjected to the same degree 

 of heat, and the distortions which sometimes occur from 

 too great and irregular application of high temperatures 

 may be eliminated. The fixing consists in drying or 

 coagulating the gelatinous envelope surrounding the 

 organisms, by which means they are caused to adhere 

 to the surface of the cover-slip. It is sometimes desira- 

 ble 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 carefully in the air, 

 the cover-slip on which it is placed is immersed in a 

 mixture of equal parts of absolute alcohol and ether for 

 about 15 minutes. At the end of this time it may be 

 removed and stained. The advantage of this method is 



