EXAMINATION AND PRESERVATION MEDIA. 245 



Turpentine media preserve the index of visibility of the 

 preparations 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 ma} 7 , 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 Enparal. For cases in which a 

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

 tine colophonium is a safe and excellent medium, but is 

 injurious to alum-hasmatein stains. For these, and in general 

 where a strongly clearing medium is desired, xylol balsam 

 is about the most recommendable, though it is not perfectly 

 safe, the mounts sometimes developing granules. Seller's 

 alcohol balxam 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 BELI/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 xi/lol solution is the 

 best. If time be an object, a benzol solution should be pre- 

 ferred, as it sets much quicker than the xylol solution. The 

 chloroform solutions become very brown with age, and are 

 injurious to stains made irith tar dyes. Benzol is good when 

 chemically pure and free from u'ater. 



SAHLI (Zeit. u'iss. Mils., ii, 1885, p. 5) dissolves in cedar 

 oil. 



APATHY (Fauna Flora Golf. Neapcl, xxii, 1909, p. 18) 

 takes balsam 2 parts, cedar oil (immersion) 1, and chloro- 

 form J . 



Samples of balsam that are acid are frequently met with, and are 

 injurious to some stains. Griibler & Hollborn now prepare a neutnd 

 balsam, in which MAYER has found that very delicate preparations, that 

 lost colour immediately in any other sort of balsam, have kept perfectly 



