140 MICROSCOPIC OBJECTS. 



chalk, and more rarely with animal matter. When the 

 shells are mixed with chalk, the latter must be separated 

 by dissolving in dilute hydrochloric acid. A mixture of 

 one part acid to from six to ten parts water should be 

 made in a test-tube, and a little of the powder put into it 

 and weU shaken. It should then be left until the following 

 day, when the dilute acid is to be poured off, and the 

 shells at the bottom examined. If the chalk is not 

 sufficiently dissolved, the process must be repeated until 

 the whole has been removed, when the sihceous sheUs 

 must be washed in pure water. They are then ready to 

 place upon a glass shde for mounting. When in search 

 of sihceous bodies among animal matter, as in guano, the 

 substance must be digested in nitric acid, and subse- 

 quently washed in clean water. 



To Prepare Crystals for Polarizing Microscope. — 

 Pour a few drops of a saturated solution of the salt to be 

 mounted on a glass slide ; then gently warm it over a 

 spirit-lamp, so as to evaporate the excess of fluid. In 

 this operation care must be taken not to apply too much 

 heat, as in that case the salt wiU lose its water of ci*ystal- 

 lization and become opaque. When the crystals are 

 formed, examine under the polarizing microscope, and if 

 perfect they are ready for mounting. 



On Mounting Ferns, to exhibit their Fructification. — 

 Tlie genera are mainly distinguished by the position and 

 arrangement of the organs of reproduction. These are 

 mostly on the under side or along the margin of the leaf 

 or frond. Tliey are best examined as opaque objects, 



