METHODS OF MOUNTING. 315 



pungent odor of the oil derived from the leaves. This essential 

 oil does not discharge anilin-colors, and is, therefore, useful when 

 those dyes have been employed. It clears slowly, but well, and 

 may be used after dehydration with 95 per cent, alcohol. 



Methods of Mounting-. 



Sections that have been treated by the foregoing methods of 

 preparation are fitted for mounting in a solution of some resin. 

 The most commonly employed are Canada balsam and dammar. 

 The best solvent for these resins is xylol, though chloroform and 

 benzol are sometimes used for this purpose. All traces of turpen- 

 tine should be removed from the balsam before its solution, to avoid 

 the discharge of stains with hematoxylin or anilin-dyes which tur- 

 pentine occasions. 



When sections are transferred from alcohol to a clearing-agent 

 they float upon the surface of the latter, and can then be flattened 

 and all folds removed. As the alcohol is extracted the sections 

 sink in the clearing -agent. In order to transfer them from the 

 clearing-agent to a slide, the first step in mounting, a good method 

 is to slip a strip of cigarette-paper under the section, withdraw it 

 along with the section (using a platinum needle as aid, if necessary), 

 drain oft' the superfluous fluid, and then lay the cigarette-paper on 

 the slide, section side down. Light pressure will now squeeze out 

 -considerable of the clearing-agent, when the paper can be stripped 

 from the section and slide, leaving the section nearly dry and with- 

 out folds or wrinkles. With a little care, this method of transferring 

 the section to the slide rarely fails. When such is the case the 

 manipulations must be repeated. 



A drop of the mounting-medium is now placed upon the section 

 and a cover-glass laid on and gently pressed down until it comes in 

 contact with the section and the excess of balsam or dammar is 

 expelled from beneath the cover. If the sections tend to raise the 

 <;over, the latter may be weighted with a bullet placed in its centre. 

 Freshly mounted specimens are not so favorable for examination 

 with high powers as those that have been mounted for a few hours 

 or days. This is because the refractive indices of the clearing-agent 

 and mounting-medium are not identical. When these have become 

 thoroughlv mixed, or the former has evaporated, the specimen is 

 impregnated with and surrounded by a homogeneous medium that 

 does not scatter the light passing through it. 



