170 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



forms of cements had to be used, and were a great disadvantage. 

 They are quite unnecessary except for sections of the central 

 nervous system ; if these are being dealt with the slide must be 

 coated with a very thin layer of a solution of egg-albumin in water 

 before the section is laid upon it. The process is then exactly 

 the same as before. 



STAINING AND MOUNTING PARAFFIN SECTIONS 



We will suppose that the sections have been cut, flattened out 

 on hot water, and caused to adhere to slides, and shall describe 

 in general terms the steps through which they must be taken 

 before they are ready for examination. In the first 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 turpentine 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 



