PREPARATION OF TRANSPARENT SECTIONS OF ROCKS, ETC. La 


-care that you may use, you sometimes cannot avoid cracking them. 
In spite of your care you see a discontinuity between the rock and 
the glass. When this is the case you must warm the preparation 
cand allow the worked balsam to close up again; but that does not 
always succeed. Sometimes you must put Canada balsam over it 
and keep it hot for a considerable time. The balsam penetrates 
inside and the bubbles disappear. 
Sometimes I have been unfortunate enough to break the glass, 
or found that I could not very well get rid of certain of the bubbles 
in the way I have mentioned, and it was occasionally desirable to 
remove the thin section from the glass and put it on another. 
If you think it desirable thus to remove a thin slice of rock 
from one glass to another, you must clear away the Canada balsam 
all round up to the edge, cast plaster of Paris over it and allow it 
to harden, then make it hot and push the thin slice of rock 
completely off. A section can be moved completely off the glass, 
although it may be only the zo, of an inch in thickness. The 
plaster of Paris holds it firm, and you mount it as if it were 
bearing down a piece of the rock. You can get the plaster off and 
leave the section clean without any difficulty, and finally mount it 
in the usual way. 
I also devoted a considerable amount of time to preparing sections 
of shells and corals. You can make very good sections of shells 
perpendicular to the structure, not by attempting to cut a thin 
portion of the shell, but by taking the whole shell—supposing it 
were a uni-valve, one of the Gasteropods—and rubbing it down, 
so that the shell itself holds the portion firm that you want to deal 
with. It would otherwise be impossible to make a thin transverse 
section of a shell perhaps only = of an inch in thickness. With 
a thicker shell you deal as if it were a portion of tock. ‘The 
surface must be polished with putty powder and all sections made 
very thin. 
I must not conclude without saying a word or two with reference 
to the preparation of sections of minerals. 
In the case of some minerals, when wanted for the study of the 
fluid cavities, you deal with them just as though you were preparing 
a section of rock, modifying the process according to the character 
of the mineral ; but in some cases it is desirable to prepare much 
thicker sections, in order to study certain optical properties, 
especially to measure the index of refraction. It is very important 
that the two opposite surfaces that you polish shall be perfectly 
parallel with each other If you want to guide the sections in 
some very particular direction, as, for example, perpendicular to 
the axis, of course it requires a great amount of care, and you 
must make very careful observation of the angles of the crystal 
before you commence with it. You must do it by degrees, so as 
