S 11-S 21 



SOLVENTS AND OILS 



623 



dehydrating reagents renders it dangerous 

 to use. The only all-round dehydrating 

 agent which can be recommended is 

 calcium sulfate, commercially available in 

 suitable form as Drierite. 



It is not usual to include glycerol as a 

 dehydrating agent even though it will, in 

 point of fact, readily extract water from 

 tissues. The advantage of glycerol is that 



it permits the transfer of material from 

 water to absolute alcohol without the 

 danger of collapse. Objects may be placed 

 in dilute gljTerol, the glycerol concen- 

 trated by evaporation, and the most 

 delicate material then transferred to ab- 

 solute alcohol. This reverses the direction 

 of osmotic pressure so that the pressure 

 tends to keep the object expanded. 



S 11 PHYSICAL PROPERTIES OF DEHYDRATING AGENTS 



S 20 Clearing Agents 



The phrase clearing agent, as an alter- 

 native to the more correct de-alcoholiza- 

 tion agent, is used because the high refrac- 

 tive index usually renders the object more 

 or less transparent. Clearing agents are 

 required either before resinous mounting 

 media or before wax embedding media. 

 It is recommended in general that essen- 

 tial oils be used for preparing whole- 

 mounts and that the sjmthetic clearing 

 agents be used before embedding. 



S 21 ESSENTIAL OILS 



Essential oils are the natural oils col- 

 lected from the leaves, stems, flowers, and 

 fruits of plants. The table below presents 

 the important characteristics of those 

 commonly employed in microscopical 

 technique. The column showing the 



strength of alcohol from which specimens 

 may be transferred to the oil in question 

 supposes the oil to be anhj'drous. Most 

 essential oils, as they occur in commerce, 

 are water-saturated and this water must 

 be removed with some dehydrating agent 

 before they are used. It must be remem- 

 bered that "miscibihty with Canada bal- 

 sam," shown in the seventh column, does 

 not of necessity mean that these oils are 

 miscible with any other resin used for 

 mounting; and, particularly, when syn- 

 thetic mounting media are employed, pre- 

 liminary tests should always be made. The 

 column showing the "solubility of nitro- 

 cellulose in" each oil should be consulted 

 before clearing nitrocellulose-sections, to 

 make sure that the support will not be dis- 

 solved while the specimen is cleared. 



