


134 THE NORTHERN MICROSCOPIST. 


there, the section must be thicker. If I had to prepare my own 
again I should be disposed to make some of them thicker than 
they are. 
But I found it very convenient to examine these sections as I 
went on, so as to learn what was the general character of the rock, 
as a guide to finishing it off and leaving it the best thickness for 
the study of its characters. 
Having thus reduced the rock to the proper thickness, and 
being satisfied that all is right, the next point is of course to 
mount over the whole a portion of thin glass, and I found it very 
desirable not to do this at once, but to leave the preparation for 
some weeks so that the Canada balsam would get thoroughly hard, 
in order to prevent the section breaking up when you mount the 
_ thin glass over it with the balsam. ‘Taking glass $5 Or zoo Of an 
100 
inch thick (because when you have to mount a larger cover, if you 
have the finest glass it would all break to pieces) you melt a small 
portion of balsam on the cover glass, keeping it sufficiently soft to 
get rid of all the bubbles ; then I found it very desirable to wet 
the section with turpentine and wipe it off. The effect of this is 
that when you put the thin section of rock down on the Canada 
balsam, the balsam spreads all over it without difficulty ; if it were 
dry you would have a great number of bubbles formed. 
The next thing is to press the thin glass down on to the 
object so as to get it as nearly as may be in contact, keeping the 
section warm so that the balsam may be fluid, but not so that the 
balsam which is employed to fasten down the object will melt, 
otherwise it might break up. You will now see the reason why 
the balsam should be allowed to get thoroughly hard. With a 
little dexterity you may mount the thin glass on and have the 
whole as perfect as you may desire. 
I may make a few remarks on some of the difficulties you have 
to contend with in the case of some rocks. 
When you see certain kinds of mica-schist in the field you would 
think it impossible to prepare a thin section perpendicular to the 
foliation. I never dreamed of being able to do such a thing at 
first. It is almost like making a thin section of the leaves of a 
book at right angles to the plane surfaces. Mica-schist is so friable 
in one direction that a very little thing causes it to break asunder. 
To think of making a section perpendicular to the foliation at first 
sight appears a very unlikely thing, but ultimately I found that it 
could be done. The plan that I adopted was this: I broke off a_ 
portion of fair thickness,—you cannot break them off thin, they fall 
to pieces directly, you must have one fairly thick, then you deal with 
it in the way I have described,—but having reduced it toa certain 
thickness, say about 4”, you must contrive so as to get it hardened. 
Having got it into something like the proper shape, I wetted it 


