194 SELECTION AND USE 



washed away and everything combustible has been burnt either 

 with fire or nitric acid. So too the siliceous cuticles of plants 

 may be procured by destroying all the other parts by chemical 

 means. The best way is to heat them in nitric acid, and add 

 to the hot liquid a small quantity of powdered chlorate of pot- 

 ash. The quantities used must be very small, and great care 

 must be exercised. 



It is frequently necessary to separate a small quantity of 1 

 deposit from a large amount of liquid, filtering being inadmis- 

 sible. For this purpose use a conical glass or a large test tube, 

 allow plenty of time for the deposit to settle, and give occa- 

 sionally a slight stir, so as to detach the particles from the 

 sides of the vessel. Then pass a large dipping tube (one quar- 

 ter of an inch in diameter) to the bottom, the upper end of the 

 tube being closed with the finger. On withdrawing the finger 

 the liquid and deposit rush in. Have ready a small ball of 

 soft cement (resin and beeswax equal parts, softened with oil) 

 and with it close the upper end of the tube, which may now be 

 withdrawn, carrying the liquid with it. Place the tube in a 

 vertical position, with its lower end on a slide or in a watch- 

 glass, and support it either by means of the ring of a small 

 retort stand or by a simple wire- having a ring (horizontal) at 

 the upper end, and a small piece of board for a foot. Beale 

 directs us to cork the tube, but this is difficult unless the tube 

 is made specially for the purpose with a mouth like that of a 

 test tube. Tubes made in this way are, however, the most con- 

 venient, and a good velvet cork closes them perfectly. 



There is a class of insect preparations, which are quite inter- 

 esting, though they are not as instructive as inferior prepara- 

 tions made by the process of dissection. We refer to the whole 

 insects found in most collections. They are prepared by soak- 

 ing the insect in liquor potassse, which may be had from any 

 druggist; this renders the internal organs soluble and the outer 

 horny skeleton transparent. The viscera are then expelled by 

 pressure with a camel hair pencil, the insect well washed in pure 

 water, soaked first in alcohol, and then in turpentine, and 

 finally mounted in balsam. The points requiring attention are 

 these: Soaking just the right length of time in the potash, for if 

 the insect remains too long in this liquid it will be destroyed ; 



