COVER-SLTP PREPARATIONS. 127 



normal outline of the cells is altered. If carefully 

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

 phology of the organism remains unchanged. A better 

 plan for the process of fixing is to employ a copjier 

 plate of 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 graduated tempera- 

 ture, beginning at the point of the plate above the gas- 

 flame where it is hottest, and becoming 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 to- 

 ward 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 minutes, when the 

 fixing will be complete. The same may be accomplished 

 in a small copper drying oven, which is regulated to 

 remain at the temperature of 95° to 98° C. In very 

 jiarticular work this plan is to be preferred to the pro- 

 cess of passing the cover-slips through the flame, as the 

 organisms are always subjected to the same degree of 

 heat, and the distortions which sometimes occur from 

 the too great and irregular application of high tem- 

 peratures may in part be eliminated, or if not, will be 

 more nearly constant. 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. When fixed, the stain- 

 ing is usually a simple matter. The majority of bac- 

 teria with which the beginner will have to deal stain 

 readily with solutions of any of the basic aniline dyes. 



