COVER-SLIP PREPARATIONS. 125 



peratures, and in doing so the normal outline of the 

 cells is altered. If carefully dried before fixing, this 

 does not occur and the morphology of the organism 

 remains unchanged. A better plan for the process of 

 fixing is to employ a copper 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 temperature, beginning at the 

 point of the plate above the gas-flame where it is hot- 

 test, and becoming gradually cooler toward the other 

 end of the plate, which may be of a very low tempera- 

 ture. 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 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. This plan is to 

 be preferred to the process 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 appli- 

 cation of high temperatures may in part 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. 

 When fixed, the staining is usually a simple matter. 

 The majority of bacteria with which the beginner will 

 have to deal stain readily with solutions of any of the 

 basic aniline dyes. 



