356 Wolff- — Apparatus for the Geological Laboratory. 



charge a saw properly. By this method, which has been per- 

 fected in the laboratory by Mr. C. L. Whittle, the bort is 

 forced into the tin and held fast by the teeth, which owing to 

 the oblique cutting of the notches press tightly together when 

 hammered back to place. The teeth are cut obliquely on oppo- 

 site sides in adjoining quadrants in order to distribute the bort 

 equally on both sides of the saw. A refinement on the notch- 

 ing process described above consists in the use of a brass disk 

 with guide notches cut into its edge, by which the knife can 

 be guided and the notches made evenly. 



Saws thus made will do a surprising amount of work before 

 wearing out. In two cases, where a record was kept, the saws 

 cut respectively 300 and 400 square inches of rock, mainly of 

 crystalline varieties. 



Method of sawing sections thin. — Power is furnished for 

 the laboratory from a 5 H. P. electric motor which is run from 

 a street current (500 volts) but only 2 H. P. of current is used 

 and paid for as this suffices for the lathes and electric light. 

 For grinding rock surfaces the usual revolving zinc and iron 

 plates are used with three machines, for coarse, medium and 

 fine emery or corundum (Nos. 60, 80, and F.F.F.). The dia- 

 mond saw, placed in an ordinary lathe, revolves at a speed of 

 600 to 700 revolutions per minute in the narrow slit of a brass 

 platform on which an adjustable gauge slides in a groove at 

 right angles to the saw, with a pivot and clamp so as to allow 

 the adjustment to exact parallelism between its face and the 

 plane of the saw. The piece of rock, sawed and polished on 

 one side, is cemented to a square piece of thick glass but little 

 larger than the specimen. The cementing substance used is a 

 mixture of Venetian turpentine and shellac boiled down to 

 the proper consistency. White shellac is preferable to brown 

 on account of the increased transparency but the white stick 

 shellac is often impure and refuses to melt, hence it is best to 

 use liquid white shellac. The tenacity of this mixture is 

 greater than that of Canada balsam and when white shellac is 

 used may often be left under the section when finally mounted. 

 The glass, with the specimen attached, is put against the gauge 

 which is then moved close to the saw so that it almost touches 

 the glass at the base of the specimen and, is also adjusted to 

 parallelism. The saw is then started and gently pressed 

 against the glass while the longest side of the specimen is pre- 

 sented to the cutting edge; as the saw cuts in, the glass is 

 slowly turned around while pressed against the gauge, until a 

 cut is made completely around the specimen, when the central 

 part is cut through and the slide removed. Where many per- 

 sons use the same machine, it is impossible to keep saw and 

 gauge in permanent adjustment, but after adjusting them to 



