CO VEB-SLIP PREPARA TIONS. 141 



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 distilled water or physiological salt-solution. 

 With a platinum needle, which has been sterilized in 

 the gas-flame jMsi 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 

 the forceps), at some distance above the gas-flame, 

 until it is quite dry. If held with the forceps over 

 the flame at this stage, too much heat may be un- 

 consciously applied, and the morphology of the organ- 

 isms 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 the forceps, and, holding the side upon which 

 the bacteria are deposited away from the direct action 

 of the flame, 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 

 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 tempera- 

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



7* 



