EXAMINATION OF STRUCTURE OF FOSSIL PLANTS. 13 



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 

 will 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 i:>ossess 

 a slight convexity. This gives a certain amount of rigidity to 

 the edge, which is useful in slitting by the hand ; 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. Then, by 

 holding the agate with the diamond powder with a moderate 

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

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

 with invisible teeth. In pounding the diamond, some care is 

 necessary, as also a fitting mortar. The mortar should be 

 made of an old steel die, if accessible ; if not, a mass of steel, 

 slightly conical, the base of which ought to be 2 inches in dia- 

 meter, and the upper part 1 J inch. A cylindrical hole is now 

 to be turned out in the centre, of fths of an inch diameter, 

 and about 1 inch deep. This, when hardened, is the mortar ; 



