Sections of Pumice-stone, <&c. By H. J. Johnston-Lavis. 23 



of hard balsam* is rubbed over its surface, so as to thoroughly 

 fill in all the opened cavities and leave a superabundance on the 

 surface. It should be left quiet on the hot plate for another minute 

 or two, and more balsam added if the first should have much sunk 

 in; it is then removed and allowed to cool whilst in a hori- 

 zontal position. When cold it should be ground down on the side 

 of a smooth grindstone, or, still better, upon a slab of sandstone 

 slightly inclined, over which is flowing a constant stream of water.! 

 The grinding should be continued till all the broken septa are 

 brought flush with the surface. It should then be thoroughly 

 washed with a camel's-hair pencil and submitted to a powerful jet 

 of water from a tap or syringe, so as to clean the newly opened 

 cells, after which it is dried and replaced on the warm plate and 

 rnbbed with the balsam stick. When cooled the excess of balsam 

 may be removed by grinding it on the sandstone, after which it is 

 washed. 



The following solution, which is next required, should be kept 

 ready in a bottle : — Yellow soap, 1 part ; methylated spirit, 2 parts. 

 Dissolve. AVater, 3 parts. Prepare a hone (I use a Washita stone, 

 but probably any hard hone stone would answer) of about 8 in. x 

 2^ in. X li in. Fix it conveniently on a board slightly inclined, 

 with the narrowest edge uppermost, and drop on a few drops of the 

 soap solution. At its upper end a small quantity of water should 

 be constantly dripping, which by preference should also be slightly 

 soapy. Now grind and polish the specimen on the hone until the 

 surface is brilliant. Whenever the balsam begins to " rool " or cause 

 hitching of the specimen add a few drops of the soap solution.:]: 



The pohshing being complete, the specimen is thoroughly 

 cleansed and put aside in a warm and dust-free place to dry, after 

 which it is cemented by hard balsam to a clean slide. Since it 

 must never be removed from its new position, as is done in the case 

 of more durable rocks, more care is required in protecting the glass 

 from injury. We now grind down the opposite side to almost 

 transparency on a well-watered grindstone, and by practice in pre- 

 senting different parts of the specimen to the grinding surface we 

 may reduce the slice to a sufficient thinness for almost any micro- 

 scopic work. The specimen is then washed and ground perfectly 

 level and polished on the hone, with water and soap-solution. The 

 application of the latter requires much practice to regulate, since 

 if too much be used it softens and saponifies the balsam, making an 

 opaque preparation ; or if insufficient, the balsam catches to the 



* Prepared as usual for rock sections. 



f Should the sandstone clog with balsam, it may be washed with a little 

 strong soda lye. 



X I have tried alkalies, spirit and many other lubricants, and feeble solvents 

 of balsam, but the above answers best, as we want to dissolve the balsam at the 

 same rate as we grin 1 down the specimen. 



