MOUNTING 133 



The refractive index of DPX, while it still contains the original 

 amount of xylene, is 1 -532. Since the index of xylene is only about 

 1-496, that of the medium must increase as drying proceeds. The 

 final index is rather higher than that of the fixed proteins of 

 ordinary microscopical preparations. If all the xylene is deliber- 

 ately driven off" from DPX, the resultant substance (distrene 

 + plasticizer) is a solid, melting above 100° C.^°^ Although the 

 plasticizer is a solvent for distrene, not nearly enough of it is 

 present to make a solution. 



In whole mounts there is some tendency to retraction under the 

 coverslip, despite the presence of the plasticizer. 



Permanent preparations can be made of objects mounted in non- 

 adhesive media, by sticking the periphery of the upper surface of 

 the coverslip to the slide (fig. 7). The gelatine gel used for embed- 

 ding (p. 70) is a good adhesive for this purpose.-^ It melts readily 

 in an incubator at 37° C. The slide is dried with a cloth up to the 

 edge of the coverslip, and the melted gel applied with a paint 

 brush. It hardens at first by becoming a gel again, and subse- 

 quently by evaporation of the water. In a day or two it becomes so 

 hard that it can scarcely be marked with the finger-nail. Both 

 hydrophil and hydrophobe mounting media can be sealed with 

 gelatine. 



As an extra precaution against evaporation of the mounting 

 medium, it is wise to varnish the hardened gelatine and to extend 

 the varnish beyond the gelatine on to the upper surfaces of the 

 coverslip and slide (fig. 7). The varnish chosen for this purpose 

 should be one that makes perfect contact with glass surfaces and 

 is not readily softened or dissolved by the various fluids used as 

 immersion-oils. Gold size is convenient for the purpose. This is 

 a substance used by gilders in the preparation of surfaces to 

 receive gold foil. It consists essentially of an oleo-resin dissolved 

 with turpentine and linseed oil.^^° The oleo-resin (or resin dissolved 

 in an essential oil) is often wrongly called 'gum' animi. It exudes 

 from the bark of a leguminous tree, Hymenaea coiirbaril, a native 

 of the West Indies. ^^^ Two separate processes are involved in the 

 drying of this varnish: first, the evaporation of the essential oils; 



