188 J. B. JORDAN ON ROCK SECTIONS. 



of charging the edge of the disc with diamond powder, requires 

 some little care. Having reduced the diamond to the requisite 

 degree of fineness in a hardened steel mortar (so fine that no 

 sparkling is perceptible on exposure to light), a few grains are 

 placed in a watch-glass, made into a paste with a drop of sweet 

 oil, and applied to the edge of the disc with a quill ; while the 

 disc is being slowly revolved by hand, it must be gently pressed in 

 with a small roller of glass, or hard steel, until the particles of 

 diamond powder are securely bedded in the edge of the metal, care 

 being taken to avoid getting any of it on the sides of the sheer. In 

 cutting, it is necessary to steady the work with the hand, and not 

 to trust for pressure entirely to the suspended weight. The work 

 must be constantly lubricated with brick oil, but spirits of turpen- 

 tine will, I believe, answer the same purpose. I may mention that 

 when we can obtain a sufficiently thin piece of the specimen by a 

 dexterous chip with the hammer, or other means, the operation of 

 slitting may, of course, be dispensed with. 



Having prepared suitable slices of about -jIg- or |^ of an inch in 

 thickness, it is necessary to reduce them still further by grinding 

 with fine emery and water on a lead " lap," which is made to revolve 

 on the spindle (X)). This lap is eight inches diameter, and about 

 ■| of an inch thick in the centre, cast with rounded edges, and 

 slightly convex sides. I find that this form facilitates the grinding 

 of a uniform thinness, there being always a tendency on a flat sur- 

 face (which soon wears hollow) for the edges of the section to grind 

 away before it is sufficiently thin. One side of the section can 

 easily be ground and finished by holding in the hand, and this being 

 done, it must be cemented with hard Canada balsam to a small 

 square of plate glass, in order to grind the other side, which opera- 

 tion must be carefully carried on until the structure appears distinct 

 and well defined. Mr. Sorby tells me that he grinds his sections 

 on the glass slides on which they are intended to remain, thereby 

 obviating a second mounting — the corners of the glass, in this case, 

 must be protected with small pieces of zinc during the operation of 

 grinding. This plan requires very careful manipulation. When 

 sections thus prepared are to be mounted in balsam, under a glass 

 cover, I have never found it necessary to polish them ; the only 

 finish requisite is best given by careful rubbing on a flat surface of 

 Water-of-Ayr stone (without polishing powder of any kind) 



