224 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



place, it is obvious that the paraffin, which permeates all 

 parts of the section and surrounds it on all sides, must be 

 removed, and this is done by pouring xylol, benzine, or tur- 

 pentine upon it. At least two supplies of the fluid should be 

 used, and it should be allowed to act for at least two minutes, 

 the slide being" rocked all the time. We have now removed 

 the paraffin, and the next step is to remove the xylol or 

 other solvent; this is done by means of absolute alcohol. 

 At least two lots should be used, and it. should be allowed to 

 act for two minutes. The slide is then washed in water, and 

 is ready for staining. When the section is wet with xylol it 

 will be quite transparent; this is because the refractive index 

 of the xylol is almost the same as that of glass, and the rays 

 of light which come through the section are not bent. But 

 when the alcohol is added the section will suddenly become 

 opaque, and for the opposite reason. 



If there is a milkiness on the section or slide when the 

 water is poured on, it is a sign that the xylol has not been 

 completely removed; xylol will not mix with water, and 

 forms an emulsion with it. If this should happen, you must 

 give the section another dose of absolute alcohol, and 

 rewash in water. 



It is an advantage to wipe the surface of the slide (of 

 course avoiding the section) before going from one fluid to 

 another. 



A cardinal rule in dealing with paraffin sections is never to 

 let the section get dry from the moment the first dose of 

 xylol is added until the final mounting" in balsam. 



The methods of staining which are in use are legion, and 

 it would be far beyond the scope of this book to describe 

 even a few of those which are used in histological work, and 

 to give indications for their use. It will be sufficient to 

 describe (i) a method suitable for the diagnosis of tumours, 

 etc., and for ordinary histological purposes; (2) a method of 

 staining to demonstrate bacteria which stain by Gram's 

 method; (3) a method for bacteria which do not stain by 

 Gram's method; and (4) the process for demonstrating 

 tubercle bacilli in the tissues. 



I. Staining sections for histological purposes : 



i. Xylol, two lots (to remove paraffin), 



2. -Absolute alcohol, two lots (to remove xylol). 



