MOUNTING 127 



benzene. It has the percentage composition shown here, whether 

 CHs CH3 CH3 



k^ k^CH3 



o-xylene, 23% m-xylene, 43% p-xylene, 19% ethyl benzene, 



15% 



The components of xylene 



it is derived from petroleum or coal.^"^ Xylene is chosen as a 

 solvent for hydrophobe mounting media because it evaporates 

 sufficiently slowly to allow the coverslip to be adjusted at leisure. 

 Hardening is usually hastened by putting the slide on a warm 

 plate after the coverslip has been arranged in position. 



Hydrophil media are used when low refractive indices are re- 

 quired, or when it is desired to avoid dehydration at all stages. 

 Most hydrophobe media have high or rather high indices, approxi- 

 mating to those of proteins that have been fixed by formaldehyde 

 or by coagulant fixatives. These hydrophobe media generally give 

 more transparent preparations than hydrophil ones. 



Non-adhesive media present the advantage that the final re- 

 fractive index is known, because there is (or should be) no loss of 

 a particular component (or components) by evaporation. 



One cannot state the effective refractive indices of those adhe- 

 sive media that rely for adhesion on the evaporation of a volatile 

 component, because the exact amount of this component (if any) 

 that remains in the preparation at any particular time cannot be 

 controlled. One can only state the extreme limits of refractive 

 index : that is to say, the index of the complete medium, ready for 

 use, and the index of the medium from which the volatile com- 

 ponent has been completely driven off. It is unlikely that this 

 component is quite eliminated from ordinary microscopical 

 preparations. 



Since high refractive index and firm adhesion of the coverslip 

 are desirable in most cases, adhesive hydrophobe media are more 

 often used than the others. 



When non-adhesive media are used, it is generally desirable to 



