EXAMINATION AND PRESERVATION MEDIA 225 



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 em[)loyed for microscopical ])urposes ; for the raw 

 resins always contain a certain proportion of water, which makes it 

 difficult to obtain a clear solution with the usual menstrua, and is 

 injurious to the optical properties of tlie 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 men- 

 struum. 



Solutions made with volatile menstrua, such as xylol and chloroform, 

 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 visability of the preparations 

 much longer than do media made with more volatile menstrua. Pre- 

 parations made with these often become so transparent in course of 

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

 be rejuvenated without removing the cov^r 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 

 Lee unhesitatingly recommended Euparal. For cases in which a 

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

 colophonium is a safe and excellent medium, but is injurious to 

 alum-hsematein 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. Seller'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. 



483. Canada Balsam. Prepare with the solid balsam as 

 described last §. The usual menstrua are xylol, benzol, chloro- 

 form, and turpentine. Turpentine has the advantages pointed 

 out last §, but the defect that it does not always give a homo- 

 geneous 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. The chloroform (and clove oil) 

 solutions become very brown with age, and are injurious to stains 

 made ivith tar dyes. Benzol is good when chemically pure and 

 free from water. 



Sahli {Zeit. wiss. Mik., ii, 1885, p. 5) dissolves in cedar oil. 



Apathy {Fauna Flora Golf. Neapel, xxii, 1909, p. 18) takes 

 balsam 2 parts, cedar oil (immersion) 1, and chloroform 1. 



VAUE-UECUM. 8 



