32 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



placed. A filter-paper is kept permanently in this funnel, and the 

 stain is filtered directly on to the slide or cover-glass. 



Gram's iodine solution may be mentioned here, though it is not 

 a stain. It consists of a solution of iodine, i part ; iodide of 

 potassium, 2 parts ; water, 300 parts. It keeps indefinitely. 



CLEANING SLIDES AND COVER-GLASSES 



Slides and cover-glasses must be absolutely clean when used in 

 the bacteriological laboratory ; it is especially necessary that they 

 should be free from the slightest trace of grease, for this will 

 prevent fluid from spreading out into a thin and uniform film. 

 As a rule, new slides and cover-glasses are not greasy, and require 

 no elaborate cleaning. It is usually sufficient to rub the former 

 with an old handkerchief or linen rag, whilst cover-glasses should 

 be dropped into absolute alcohol first. The use of new slides and 

 cover-glasses facilitates the task of the bacteriologist to an extent 

 which more than compensates their expense. Old slides and 

 cover-glasses may be used for sections. 



Slides are best cleansed by dropping them one at a time into 

 strong nitric or sulphuric acid, and allowing them to soak for an 

 hour or more. They are then washed in running water for 

 another hour, soaked in strong ammonia for an hour, and kept in 

 alcohol. As methylated spirit is not so good for this purpose, the 

 use of alcohol is rather expensive ; but it is not absolutely 

 necessary, and the slides may be stored in a solution of ammonia 

 (about I in 10) until required. When about to be used, they are 

 to be wiped dry with an old linen handkerchief kept specially for 

 the purpose. This handkerchief should be as old as possible, and 

 should have been washed until it has begun to fall to pieces. 



Another and more rapid method is to place the slides in a thin 

 glass or earthenware vessel and moisten them with methylated 

 spirit, and then to cover them completely with strong commercial 

 nitric acid, placing the vessel in the open air. In a little while 

 the acid will become very hot and emit copious fumes. When 

 the ebullition has ceased, any fat which may be present will have 

 melted, and will form a pellicle on the surface, whilst other organic 

 materials will have been destroyed. The acid is then to be poured 

 off, taking care to remove the pellicle of grease with it, and a 

 stream of water allowed to fall into the vessel until the acid has 



