Basalt. 
269 
2. Finish the surface produced by rubbing on a 
water-of-ayr honestone. 
6. Some Canada balsam is now spread out in a 
thin layer on a glass plate, and heated or 
“cooked” until it is of such a consistency 
that when cold it is sufficiently hard to 
retain an indentation made by the finger- 
nail, but yet not so hard as to be brittle. 
The exact degree can be learned only by 
experience. When a wooden match is 
dipped into the balsam and brought away 
it will carry a thread of balsam after it. 
If this thread, on cooling, becomes brittle 
the operation has proceeded far enough. 
A slip of ordinary double thick window 
glass (about 2 x 2 inches) is now taken 
and heated on an iron plate. This iron 
plate must be supported in table fashion, 
so that a small lamp may be placed under- 
neath. 
4. Take some of the cooked balsam on the glass 
slip, and, having heated the rock suffi- 
ciently to expel all moisture, it is then 
pressed firmly into the balsam, in such a 
way as to exclude all air bubbles. The 
glass slip, with the rock cemented, smooth 
side down, is now removed and allowed 
to cool. Grinding operations are now 
resumed, on the other side of the rock, 
the square of glass serving as a handle, 
and, at the same- time, acting as a sup- 
port for the stone when ground down to a 
film. As the grinding proceeds, the stone 
will become more transparent as it becomes 
thinner and thinner, until at last, when 
pressed on a printed sheet, the print may 
be read through the stone. Towards the 
