Answers to Queries. 73 
professional lapidaries do not use so much diamond-powder as 
they pretend, but like to talk of it, as it seems to enhance the 
value of their work. Suppose G. H. B. wishes to grind down 
(say) a piece of Mountain Limestone, the plan I recommend is as 
follows :—Get a copper plate (I have used the back of a lady’s 
card-plate, but it is not quite large enough) and a tin of “ Wel- 
lington Knife Polish,” which is in fact flour of emery, and proceed 
to rub down a flat surface, about half-an-inch square, on this plate 
with emery and water. Having obtained this surface, wash the 
specimen carefully, so as to get rid of every grain of emery, and 
then grind the same surface on a school-boy’s common slate, so as 
to get rid of the scratches the emery has made. You can then 
proceed further to polish the surface on a smooth deal board, 
rubbing against the grain, but this is not necessary. A piece of 
rock which seems covered with scratches when dry will show no 
scratches when it is mounted in balsam. Next, take a glass slip, 
or, better, a piece of thickish plate glass, 1} inches square. Place 
on it a little Canada balsam and some fragments of common 
shellac, in the proportion of one part of balsam to two of lac. 
Heat the glass over a spirit-lamp, and unite the balsam and lac by 
stirring with a knitting-pin as they melt. Continue to stir for a 
few seconds after removing the slide from the lamp to get rid of 
bubbles, and then press down the ground surface of the specimen 
into the hot mixture. Hold it pressed down for a minute, and 
then set the glass aside to cool. As soon as it is cold, you can 
begin to rub down the other side, using the glass as a handle. I 
generally reduce the mass on a common grindstone till I can see 
light through it, and then begin on the copper plate and slate 
again. In this way I have reduced and mounted a great many 
rock specimens with, generally speaking, fairly good results. 
When the rubbing is finished, put your slide (if you have used a 
slide) in methylated spirit for about an hour. You can then, 
with a camel-hair brush, remove the cement round the edge of the 
specimen, and having washed it with benzole, mount in balsam. 
If the cementing has been done properly, the thin film of cement 
beneath the specimen does not interfere with the transparency of 
the object. But if you have used a piece of thick glass, or if you 
have rubbed your slide and wish to use another for mounting, 
leave the whole thing in spirit for a few days in an upright posi- 
tion. The cement melts, and the specimen comes off and falls 
down. In rubbing turn the slide frequently. You are thus more 
likely to get an even surface, and not to rub away one side more 
than the other. I have used a piece of thick plate glass instead 
of the copper plate. When the glass gets scratched, the emery 
grains get sufficient hold to do their work. Coarser emery must 
be used if there is much work todo, G. H. B. should consult 
Rutley’s “Study of Rocks,” Be of 2s 
