STAINING AND MOUNTING. 199 



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 abso- 

 lute alcohol. 



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 histo- 

 logical work and to give indications for their use. It 

 will be sufficient to describe (i) a method suitable for 

 the diagnosis of tumours, &c., and for ordinary histo- 

 logical purposes, (2) a method of staining to demon- 

 strate 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 : 



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



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



3. Water (to remove the alcohol). 



4. Stain with haematin (or hsematoxylin) for ten 

 minutes or more according to the nature of the speci- 

 men and the condition of the stain. The exact length 

 of time can only be learnt by trial, but ten minutes will 

 be about right. Rinse in distilled water. 



