
106 THE NORTHERN MICROSCOPIST. 

will just break to powder between your fingers it is satisfactory. 
If it is not hard enough you will perhaps find subsequently that 
portions of this solution will break away; whilst if it is too hard 
and brittle portions break off from the opposite fault. If you 
maintain the heat sufficiently long to make the balsam so hard that 
when a portion is taken up with a pin you give it a bit of a squeeze 
with your fingers and it just breaks to powder, it is, I take it, in 
the state of a happy medium between the two objectionable 
extremes. Then you have your portions of rock hot; you put it 
on the tripod stand with the finished side upwards, and keep it 
sufficiently hot so as to be ready to mount it. But you must not 
make it too hot; if so you might sometimes expel the fluid from 
the fluid cavities that occur in some minerals, which would spoil 
the whole object as far as certain questions are concerned. ‘Then, 
having kept the rock moderately hot, and having got your Canada 
balsam sufficiently hard, the next object is to spread over the 
surface of the rock a portion of Aard Canada balsam kept for the 
purpose. You then put this on and spread it so as to get the 
balsam to penetrate to a certain extent. Do not put the rock 
straight on the Canada balsam, but get it coated over, and then 
take off all you can,—stroke it off with a flat wire till you have 
got the surface just wetted with balsam,—the object being to 
prevent the formation of too many bubbles when you come to put 
the portion of rock on. Having done this, press it down with a 
pencil carefully till you squeeze out all the excess of balsam. It 
should not be pressed into absolute contact. A thin portion of 
Canada balsam should be left between the glass and the object, 
and then you may turn it upside down to see that you have no 
bubbles underneath it. You have now your portion of rock 
fastened down, but probably a considerable amount of balsam 
more than is necessary, which you take off with a large pin. ‘This 
is the balsam that you keep for the purpose just mentioned, ze., 
to put on the surface of the rock when you are going to mount it 
down. 
The next thing is to reduce the thickness of the portion of the 
rock thus fastened down. 
The manner I adopted was to place them in the hands of a very 
intelligent glass cutter, and he used to grind them down till he 
left them about the thickness of a good stout card,—not thinner, 
for fear of portions being torn up and damaged. 
If you can avoid it, it is as well not to scratch the corners of 
the glass, though it is of no very great importance. I found that 
the most satisfactory method was to cement at each corner a 
portion of thin sheet zinc, fastening it down with Canada balsam. 
(Zo be continued. ) 

