CHAPTER XIX. 225 



Resinous Media. 



442. Resins and Balsams. Resins and balsams consist of a 

 vitreous or amorphous substance held in solution by an essential 

 oil. By distillation or drying in the air they lose the essential oil 

 and pass into the solid state. It is these solidified resins that should 

 be employed for microscopical purposes ; for the raw resins always 

 contain a certain proportion of water, which makes it difficult to 

 obtain a clear solution with the usual menstrua, is injurious to the 

 optical properties of the medium and to the preservation of stains. 

 All solutions should therefore be made by heating gently the balsam 

 or resin in a stove until it becomes brittle when cold, and then 

 dissolving in an appropriate menstruum. 



Solutions made with volatile menstrua, such as xylol and chloro- 

 form, set rapidly, but become rapidly brittle. Solutions made with 

 non-volatile media, such as turpentine, set much less rapidly, and 

 pass much less rapidly into the brittle state. 



Turpentine media preserve the index of visibility of the prepara- 

 tions much longer than do media made with more volatile menstrua. 

 Preparations made with these often become so transparent in course 

 of time that much fine detail is often lost. (Such mounts may, 

 however, be revivified without removing the cover by putting them 

 for a day or two into a tube of benzol ; the benzol penetrates the 

 balsam, and brings it down to a lower refractive index.) 



For a permanent mounting medium of somewhat low index I 

 unhesitatingly recommend Euparal. For cases in which a still 

 lower index is desired, Gilson's camsal balsam. Turpentine colo- 

 phonium is a safe and excellent medium, but is injurious to alum- 

 haematein stains. For these, and in general where a strongly 

 clearing medium is desired, xylol balsam is about the most recom- 

 mendable, though it is not perfectly safe, the mounts sometimes 

 developing granules. Seiler's alcohol balsam is a fine medium, and 

 perfectly stable. Oil of cedar is sometimes useful, it keeps perfectly, 

 and with time it thickens sufficiently to hold the cover in place ; or 

 if desired, preparations may be luted with BELL'S cement. 



443. Canada Balsam. Prepare with the solid balsam as described 

 last . The usual menstrua are xylol, benzol, chloroform, and 

 turpentine. Turpentine has the advantages pointed out last , but 

 the defect that it does not always give a homogeneous solution with 

 Canada balsam, as it does with colophonium. For most purposes 

 the xylol solution is the best. If time be an object, a benzol solution 

 should be preferred, as it sets much quicker than the xylol solution. 



M. 15 



