172 BACTERIOLOGY 



COVER-SLIP PREPARATIONS. 



In order that the distribution of the organisms upon the 

 cover-slips may be uniform and in as thin a layer as possible 

 it is essential that the slips should be clean and free from 

 grease. For cleansing the slips several methods may be 

 employed. 



The simplest plan with new cover-slips is to immerse 

 them for a few hours in strong nitric acid, after which they 

 are rinsed in water, then in alcohol, then ether, and, finally, 

 they may be kept in alcohol to which a little ammonia has 

 been added. When about to be used they should be wiped 

 dry with a clean cotton or silk handkerchief. 



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 in water, 

 and treated as above. Knauer suggests 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 rinsed carefully 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 concentrated 

 sulphuric acid for a time and 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 all fat has been extracted. 



Steps in Making the Preparations. Place upon the center 

 of one of the clean dry cover-slips a very small drop of water 

 or physiological salt-solution. With a platinum needle, 



