84 APPLIED BACTERIOLOGY 



various reagents, such as acids, iodine, alcohol, etc. The 

 great importance of this fact will be seen when the staining 

 of bacteria in the tissues is dealt with. 



Cover-glass Preparations. — A chemically clean cover-glass* 

 is taken up with a pair of forceps and drawn through the 

 flame of a Bunsen burner or spirit-lamp to remove any 

 faint trace of grease remaining on the cover-glass. A small 

 droplet of water is then placed in the centre of the cover by 

 means of a clean glass rod. A trace of the culture or other 

 material to be examined is taken on the end of a sterile 

 platinum wire and mixed with the drop of water, and the 

 mixture spread out as evenly as possible over the cover- 

 glass. Care should be taken not to convey particles of 

 nutrient medium with the bacterial material on to the cover- 

 glass, as in the subsequent treatment these are apt to become 

 detached and carry the bacteria off the cover-glass with 

 them. Only the smallest possible quantity of material 

 should be used, otherwise the bacteria will be found to be 

 too crowded ; the right quantity to use is soon found by a 

 little practice. The cover-glass is now allowed to dry 

 either spontaneously, taking care to protect from dust, 

 or by holding between the fingers some distance over a 

 flame. 



Now, in order to prevent the bacteria from becoming 

 detached from the cover-glass during the subsequent wash- 

 ing, it is necessary to ' fix ' the layer containing the bacteria. 



* Mere rubbing with a cloth is not sufficient to clean cover-glasses. 

 They are best cleaned as follows : The cover-glasses are first cleaned 

 by rinsing in water, and wiped with a clean rag, after which they are 

 heated for ten minutes in a mixture, of strong sulphuric acid and 

 bichromate of potash. After rinsing them in distilled water, they are 

 immersed in dilute ammonia, and after that polished on a clean linen 

 rag which is quite free from grease. The clean cover-glasses are best 

 preserved in strong alcohol, or in a clean stoppered bottle, so as to be 

 kept free from dust. 



