PREPARATION OF TRANSPARENT SECTIONS OF ROCKS, ETC. 105 

so that these grains of, perhaps, the 35>" OF zoo in diameter, 
shall be ground down and polished on both sides, and I could 
show specimens of slates in which sections of grains of sand not 
thicker than ;,;5 of an inch are cut and ground down so as to be 
shown as transparent objects. 
It is very important indeed in studying some kinds of rock to 
slice through the minerals in that manner, because otherwise you 
might be misled by false appearances. 
In some cases a single grain of sand has a complete history of 
its own—there are fluid cavities and enclosed crystals—and possibly 
in the case of some volcanic rocks you have minute glass cavities, 
all of which can be distinctly seen in a grain of sand go of an 
inch in diameter. Of course, to see these satisfactorily it is requisite 
that the preparation should be made in a satisfactory manner. 
When you come to rub the pieces of rock down on these 
smoother stones, you find that the emery has ground them down 
so as to be a little convex. When you rub on the smoother stones 
you do not tear up portions from the specimen, but wear it down 
by a perfectly legitimate grinding of the constituents. For example, 
in the case of a limestone the emery pulls out the little grains of 
sand ; it makes a flat surface, but it is not a mathematically true 
section of the rock, which, of course, is unsatisfactory. When 
you begin rubbing on the smoother stones you find that these 
grains of sand are worn down on a level, and the first result is 
that you begin to polish the surface of these grains in the centre 
of the specimen, and gradually by continuing the grinding it is 
ground flatter and flatter, until you get close up to the corners. 
The process must be continued till you feel persuaded that the 
harder portions are not only polished but that they are worn down 
to the level of the softer portions, and then made as smooth as 
they can be got. 
I have never used polishing material in preparing’ sections of 
rock, because it penetrates into the rock and makes you see things 
which are not naturally there, and so may deceive you. 
Having got your portion of rock dressed off as described, the 
next point is to fix it down on a piece of glass with Canada balsam. 
I have a little tripod stand, under which I can have a jet of gas, 
and on the top of this a piece of brass so that it will keep the 
glass in the proper place in the centre. You heat this sufficiently, 
and put a portion of Canada balsam on to it, and an important 
point is to get rid of all the bubbles,—they rise to the top, and 
with a little manceuvring you may get them to the centre and with 
a pin draw them all out. Then you carefully stir it, keeping it 
hot until you have got the balsam sufficiently hard. I have found 
the best indication of that was to take a portion out with a 
large pin, and when the balsam has got so hard that when cold it 




