COVER-SLIP PREPARATIONS 173 



which has been sterilized in a gas-flame just before using and 

 allowed to cool, take up a very small portion of the colony 

 to be examined and mix it carefully with the drop on the 

 slip until there exists a very thin homogeneous film over 

 the larger part of the surface. This is to be dried upon the 

 slip by either allowing it to remain upon the table in the 

 horizontal position under a cover, to protect it from dust, 

 or by holding it between the fingers (not with forceps} , at some 

 distance above a gas-flame, until it is quite dry. If held 

 with the forceps over the flame at this stage, too much heat 

 may be unconsciously applied, and the morphology of the 

 organisms in the preparation distorted. When held between 

 the fingers with the thin layer of bacteria away from the flame 

 no such accident is likely to occur. When the whole pellicle 

 is completely dried the slip is to be taken up with forceps, 

 and, holding the side upon which the bacteria are deposited 

 away from the direct action of the flame, it is to be passed 

 through the flame three times, about a second being allowed 

 for each transit. Unless the preliminary drying at the low 

 temperature has been complete, the preparation will be 

 rendered worthless by the subsequent "fixing" at the higher 

 temperature, for the reason that the protoplasm of bacteria 

 when moist coagulates at these temperatures, and in doing 

 so the normal outline of the cells is distorted. 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 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 part of the plate above the gas-flame where it is 



