igO CLINICAL BACTERIOLOGY AND HEMATOLOGY 



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 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 hasmatin (or haematoxylin *) for ten minutes or 

 more, according to the nature of the specimen and the con- 

 dition 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. 



5. Wash thoroughly in tap-water, continuing the washing until 

 the sections have a decidedly blue tinge. The haematoxylin 

 compounds are very much like litmus, being red in presence of 

 acids and blue in presence of alkalies ; the sections are to be 

 coloured blue, and the necessary alkali is contained in the tap- 



* Delafield's hasmatoxylin is the best for ordinary work, and is best bought 

 ready made. 



