124 BACTERIOLOGY. 



centrated sulphuric acid for a time, then riuse them 

 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 the 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 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 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 uncon- 

 sciously applied, and the morphology of the organisms in 

 the preparation distorted. When held between the fiu- 

 gers with the 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 proto- 

 plasm of bacteria when moist coagulates at these tern- 



