COVER-SLIP PREPARATIONS. 169 



in water, after which they are kept in a mixture of 

 equal parts of alcohol and ammonia. They are to be 

 dried on a cloth from which all fat has been extracted. 



Steps in making the preparations. Place upon the 

 centre of one of the clean dry cover-slips a very 

 small drop of water or physiological salt-solution. 

 With a platinum needle, 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 ap- 

 plied, and the morphology of the organisms in the prep- 

 aration 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 bac- 

 teria are deposited away from the direct action of the 

 flame, it is to be passed through the flame three times, 

 a little more than one second being allowed for each 

 transit. Unless the preliminary drying at the low tem- 

 perature 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 tempera- 

 tures, and in doing so the normal outline of the cells is 



