COVER-SLIP PREPARATIONS. 125 



their color, are employed. The ordinary method of 

 cover-slip examination of bacteria, constantly in use in 

 these studies, is performed in the following way : 



CovEE-SLip Preparations. — In order that the dis- 

 tribution of the organisms upon the cover slips may be 

 uniform and in as thin a layer as possible, it is essential 

 that the slip should be clean and free from grease. 

 For cleansing the slips several methods may be em- 

 ployed. 



The simplest plan with new cover-slips is to im- 

 merse them for a few hours in strong nitric acid, after 

 which they are rinsed in water, then in alcohol, ether, 

 and, finally, they may be kept in alcohol to which a 

 little ammonia has been added. When they are to be 

 used they should be wiped dry with a clean cotton or 

 silk handkerchief. 



If the slips have been previously used, boiling in 

 strong soap solution, followed by rinsing in clean warm 

 water, then treated as above, renders them clean enough 

 for ordinary purposes. 



A method commonly employed is to remove all 

 coarse adherent matter from slips and slides by allowing 

 them to remain for a time in strong nitric or sulphuric 

 acid. They are removed from the acid after several 

 days, rinsed off in water, and treated as above. Knauer 

 has recently suggested the boiling of soiled cover- slips 

 and slides for from twenty to thirty minutes in a 10 per 

 cent, watery solution of lysol, after which they are to be 

 carefully rinsed in water until all trace of the lysol has 

 disappeared. They are then to be wiped dry with a 

 clean handkerchief. 



Loffler's method, which provides for the complete 

 removal of all grease, is to warm the cover-slips in con- 



