WITH THE MICROSCOPE.. l8l 



done at a very gentle heat, so as not to cause- the section to break up 

 by melting the balsam which holds it fast to the glass pJate. 



Sections of very soft rocks, which would easily break up in water,, 

 may be prepared in the same manner by hardening them. with balsam. 

 They should be first soaked with turpentine, and then with soft 

 balsam, and kept hot until quite hard. 



We may modify the above plan, with advantage in preparing 

 sections of such hard minerals as quartz. If ground down with 

 emery and water, deep scratches are produced, and it takes a long 

 time to remove them by means- of the softer stones. This may be 

 avoided by using fine emery paper, held flat on a piece of plate glass. 

 After grinding down to nearly the proper thickness with emery and 

 water, common English flour-emery paper may be used, which soon 

 removes the deep scratches ; and afterwards the surface may be almost 

 polished by using the finest French 1 emery paper employed in pre- 

 paring steel plates for engraving ; a perfect polish can then be 

 easily given by means of rouge on parchment. Crystals of salts 

 soluble in water may also be ground, down and dressed smooth on 

 emery paper, and finally polished with rouge in the same manner; 

 but in many cases they may be examined without this preparation, 

 and may be fastened on glass with balsam. Some are decomposed 

 by contact with balsam, and must be kept dry in small covered 

 cells ; others may be mounted in a concentrated solution of the 

 same salt, when it is desirable to retain the liquid enclosed in the 

 fluid-cavities ; and when very small they may be mounted in Canada 

 balsam, or, if that be objectionable, in: castor oil. 



Sometimes the structure of a rock or other mineral substance 

 may be studied to great advantage by grinding it to a suitable 

 shape, moderately thick and flat, fixing one side to glass with 

 balsam, and acting on the other with a dilute acid. If one part 

 is soluble and the other not acted on, some valuable facts may 

 be learned. As an example I refer to the Eozoon Canadense, 

 which has lately attracted so much attention. One part consists 

 of carbonate of lime, and the other of siliceous minerals insoluble 

 in diluted acid ; and when the former is dissolved a most beautiful 

 and minute structure may be seen, which appears to be due to 

 minute tubes and other opert spaces filled with the insoluble 

 minerals. 



27O. Of the Jlicroscopieal Structure of Iron and Steel.- The 

 microscopical structure of iron and steel is best shown by polished 

 surfaces slightly acted on by very dilute nitric acid. The section 

 should be cut in the required direction by means of a saw, and 

 ground or filed down to a convenient thickness, and fixed to a 



