276 CHAPTER XX. 



Braunschweig, 1889). It is uncrjstallisable, and preserves well carmine 

 and coal-tar stains (haematoxylin stains fade somewhat in it). The index 

 of refraction is somewhat higher than that of glycerin. Objects may be 



brought into it out of water. 



Glycerin Media. 



423. Glycerin. Glycerin diluted witli water is frequently 

 employed as an examination and mounting medium. Dilution 

 with water is sometimes advisable on account of the increased 

 visibility that it gives to man}' structures by lowering the 

 index of refraction of the glycerin. But from the point of 

 view of efficacious preservation it is always advisable to use 

 undiluted glycerin, the strongest that can be procured. 



Long soaking of tissues in glycerin of gradually increased 

 strength is a necessary preliminary to mounting in all cases 

 in which it is desired to obtain the best possible preparations, 

 and to ensure that they shall keep well. If this soaking is 

 done on the slide (the cover being removed and the object 

 treated with fresh glycerin every one or two days), it is well 

 to take the precaution recommended by BEALE, of luting the 

 edges of the cover so as to make the preparation air-tight, as 

 glycerin is so highly hygroscopic that a drop of it exposed 

 to the air rapidly diminishes in strength to a very con- 

 siderable deo-ree. In order to facilitate the removal of the 



O 



cover in this process, the slide may be gently warmed by 

 passing it two or three times through the flame of a spirit 

 lamp. No preparation can be considered to be made 

 secunduin, artem until every part of the object has been 

 thoroughly impregnated with strong* pure glycerin. 



For closing glycerin mounts, the edges of the cover 

 should first (after having been cleansed as far as possible 

 from superfluous glycerin) be painted with a layer of glycerin 

 jelly ; as soon as this is set a coat of any of the usual 

 cements may be applied. See next Chapter. 



Glycerin dissolves carbonate of lime, and is therefore to 

 be rejected in the preparation of calcareous structures that 

 it is wished to preserve. 



424. Extra-refractive Glycerin. The already high index of refrac- 

 tion of glycerin (Pi-ice's glycerin, n = 1'46) may be raised to about that of 



