200 PRACTICAL MICROSCOPY. 



the chip has remained long enough in the solution it must 

 be dried in a warm place. Sedimentary rocks, as a rule, 

 do not require to be cut as thin as igneous rocks, so that 

 they may be left on the grinding slide, as it will not be 

 scratched. 



One thing is absolutely necessary in either method, that 

 is to get rid of all air bubbles in the balsam attachment, 

 as if any are left, the section is sure to wear into holes and 

 break. 



Sections of Echinus spine may be cut in the same way as 

 above, or with a fine saw improvised from a thin clock 

 spring, and the slice ground down by rubbing on a fine 

 level Turkey hone, and when thin enough should be 

 cleansed with water and a soft camel-hair brush, dried by 

 immersion in alcohol, passed through benzol, and finely 

 mounted in balsam and benzol, or dry, if required, when it 

 is ready for observation. 



Hard rocks, as a rule, are easier to prepare than soft 

 ones. The latter should be soaked in turpentine, and then 

 in balsam arid benzol, afterwards being heated till quite 

 hard. 



Coal may be cut and ground into sections in the follow- 

 ing manner : After careful selection of a piece free from 

 cracks, a prism is to be cut J inch or f of an inch square. 

 Slices of this, as thin as possible, may now be cut with a 

 fine saw, and one face rubbed flat upon a slab of pumice- 

 stone kept well wetted with water, and finally rubbed 

 smooth upon a Water-of-Ayr stone, water being continually 

 applied to it. The polished surface is now cemented to a 

 3x1 glass slide with marine glue, taking special care not 

 to include air bubbles, and the slide put by to set for some 

 days. When this has taken place, the slice may be rubbed 

 down on a flat piece of coarse gritstone with plenty of 

 water until almost thin enough to show the structure, and 



