MOUNTING IN BALSAM AND BENZOL. 299 



over it and hold the whole over the spirit-lamp until a sea 

 of the bubbles is seen underneath. Remove, and with a 

 gentle pressure press down the cover. The bubbles will 

 all disappear and the balsam become hard. To secure the 

 diatoms all in the same plane, turn the cover-side down- 

 wards, and leave in a warm place. This is best effected 

 in the whalebone clip shown by Fig. 243, but care should 

 be taken that the balsam does not project beyond the 



FIG. 243. 



cover, or it will stick to the whalebone, and cause consi- 

 derable annoyance. 



The old plan of mounting objects in pure Canada 

 balsam has been almost abandoned ; it is now usual to 

 put up objects in balsam and benzol whenever balsam- 

 mounted objects are required. 



To illustrate how this should be done, let us proceed to 

 mount one of the forelegs of the great water-beetle (Dytiscus 

 marginalis) shown in Fig. 244. 



After detaching the leg of the insect, the first operation 

 is to soak it in potash solution for a day or two, then take 

 it out and wash it in water, allow it to soak in dilute spirit 

 (i of spirit to 3 of water) for 24 hours, and then transfer 

 it to methylated spirit. After remaining here until all the 

 moisture has been extracted by the spirit it must be taken 

 out, drained on blotting paper, and placed in oil of tur- 

 pentine. Here it must remain until the colour of the 

 chitinous skeleton has become sufficiently reduced, when it 



