FOSSIL SECTIONS. 787 



On preparing fossils for microscopic examination, Mr. Alexander 

 Bryson remarks : — * 



The usual mode of proceeding in making a section of fossil wood is 

 simple, though tedious. The first process is to flatten the specimen to 

 he operated on by grinding it on a flat lap made of lead charged with 

 emery or corundum powder. It must now be rendered perfectly flat 

 by hand on a plate of metal or glass, using much finer emery than in 

 the first operation of grinding. The next operation is to cement the 

 object to the glass plate. Both the plate of glass and the fossil to be 

 cemented must be heated to a temperature rather inconvenient for the 

 fingers to bear. By this means moisture and adherent air are driven 

 ofi, especially from the object to be operated on. Canada balsam is 

 now to be equally spread over both plate and object, and exposed again 

 to heat, until the redundant turpentine in the balsam has been driven 

 off by evaporation. The two surfaces are now to be connected while 

 hot, and a slow circular motion, with pressure, given either to the 

 plate or object, for the purpose of throwing out the superabundant 

 balsam and globules of included air. The object should be below and 

 the glass plate above, as we then can see when all the air is removed, 

 by the pressure and motion indicated. It is proper to mention that 

 too much balsam is more favourable for the expulsion of the air-bubbles 

 than too little. When cold, the Canada balsam wiU be found hard and 

 adhering, and the specimen fit for slitting. This process has hitherto 

 been performed by using a disc of thin sheet-iron, so much employed by 

 the tinsmith, technically called sheet-tin. The tin coating ought to be 

 partially removed by heating the plate, and when hot rubbing off much 

 of the extraneous tin by a piece of cloth. The plate has now to be 

 planished on the polished stake of the tinsmith, until quite flat. If 

 the plate is to be used in the lathe, and by the usual method, it ought 

 to be planished so as to possess a slight convexity. This gives a 

 certain amount of rigidity to the edge, which is useful in slitting by 

 the hand j while by the method of mechanical slitting, about to be 

 described, this convexity is inadmissible. The tin plate, when mounted 

 on an appropriate chuck in the lathe, must be turned quite true, with 

 its edge slightly rounded and made perfectly smooth by a fine-cut file. 

 The edge of the disc is now to be charged with diamond powder. This 

 is done by mingling the diamond powder with oil, and placing it on a 

 piece of the hardest agate, and then turning the disc slowly round ; 

 and holding the agate with the diamond powder under ;a moderate 

 pressure against the edge of the disc, it becomes thoroughly charged 

 ■«rith a host of diamond points, becoming, as it were, a saw with 

 invisible teeth. In pounding the diamond some care is necessary, as 



* On an improved Method of preparing Siliceous' and other Fossils ^for Microscopic 

 Investigation, witli a description of a new Pneumatic Chuck. By Alex. Biyson^ in EclAn. 

 N. Phil. Jmrnal, N. 8., iii. 297. 



