CUTTING ROCK SECTIONS. 199 



balsam boil. When the balsam is properly hard, warm the 

 chip gently over the lamp or on a hot metal plate, brush it 

 over with a little turpentine, and re-melt the balsam ; then 

 lower the chip slowly into the balsam until it is cemented 

 firmly and evenly by its flat surface to the glass slip. When 

 the balsam is quite cold, the chip is to be rubbed down on 

 the iron plate with coarse emery until it is too thin to bear 

 any further rough friction. With care, many rocks may be 

 brought to the requisite thinness on the iron plate alone, 

 and will require little finishing. The necessary degree of 

 thinness will vary according to the nature of the rock ; but, 

 as a general rule, most hard rocks must be cut thin enough 

 to read through when placed on the page of a book. When 

 the section will no longer bear the friction of the coarse 

 emery, remove it to the glass plate, and grind it thinner 

 with flour emery, and finally finish it off on the Water-of- 

 Ayr stone. The slide, at the finish, will be disfigured by 

 deep scratches from the emery, and the section must be 

 transferred to a clean slip. Warm the section enough to 

 melt the balsam, and push the section off with a needle 

 into a cup of turpentine, and wash carefully with a small 

 brush. Now pour a little balsam and benzol solution on 

 the clean slip, place the section upon it, add a little more 

 balsam, and cover. 



2. Preparation of soft rocks and sedimentary rocks gene- 

 rally : These are prepared and mounted in the same way 

 as hard rocks, but no emery is to be used ; they must be 

 ground down and finished on the three stones mentioned 

 above. Some very friable rocks will require a preliminary 

 hardening by immersion for some days in a solution of 

 (i) balsam in benzol. The balsam must be first baked in a 

 cool oven, until on cooling it becomes hard and brittle ; 

 then dissolve it in benzol ; or (2) in a solution of shellac in 

 alcohol. This is, perhaps, the better of the two. When 



