COVER-SLIP PREPARATIONS. 135 



centrated sulphuric acid for a time, then rinse 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 unconsciously ap- 

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

 aration distorted. When held between the fingers with 

 the layer of bacteria away from the flame no such acci- 

 dent is likely to occur. When the whole pellicle is com- 

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

 and, holding the side upon which the bacteria are de- 

 posited 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 temperatures, and in doing so the 



