MOUNTING 131 



seldom used, partly because most of the suitable substances are 

 inconveniently volatile, partly because it is so easy to prepare 

 hydrophobe media that will stick firmly to glass. Methyl sali- 

 cylate, however, has certain virtues. It is colourless and miscible 



OH OCH3 



Methyl salicylate 



in all proportions with absolute ethanol. It gives excellent trans- 

 parency to most preparations, because its refractive index (1-537) 

 is very close to that of proteins fixed in the usual ways (see p. 124). 

 Since it is a single substance, there is no question of change of 

 refractive index by diff"erential evaporation of components. Basic, 

 acid, and mordanted dyes seem to maintain their colours well in it. 



DPX.^^^ It has already been remarked that adhesive hydro- 

 phobe mounting media are more commonly used than any others. 

 They are valuable for making permanent preparations of dyed 

 objects. Dyes maintain their colours well in DPX, which possesses 

 the further advantage of being itself perfectly colourless. The 

 letters DPX are the initials of Distrene-Plasticizer-Xylene. 



The most familiar of all media in this group is Canada balsam, 

 which was described in earlier editions of this book. Although it 

 has great virtues, it is omitted from the present edition, partly 

 because it has a tendency to bleach basic dyes, partly because it is 

 very complex in composition (there are at least 7 constituents in 

 addition to the solvent). ^'^ 



In adhesive hydrophobe media the essential constituent is a 

 'resin', in the wide sense of that term: that is to say, a solid, 

 amorphous organic compound, insoluble in water, having no 

 definite melting-point (because composed of molecules of varying 

 lengths). The natural resins that exude from the bark of certain 

 trees are oxidation-products of terpenes, but plastics having 

 similar physical characters nowadays receive the same name. The 

 particular plastic that gives high refractive index and adhesiveness 

 to DPX is polystyrene, a polymer having the same general char- 

 acter as acrylic acid (p. 78), but diff"ering in the substitution of a 



