116 PREPARATION AND MOUNTING 
as warm as the hand can comfortably bear, will stop the 
expansion, when the portion which we wish to mount should 
be cut off from the mass of salt by simply scratching the 
film around, and pure Canada balsam with the thin glass 
used. Breathing upon the film, or allowing the slide to 
become cold and attract the moisture from the atmosphere, 
will cause the crystallization to extend, and sometimes 
greatly rob the effect; so it is necessary to mount quickly 
when the desired forms are obtained. As the crystals are 
very uncertain as to the place of their formation, I may 
here mention that they may be got in any part of the slide 
by piercing the film with a needle-point ; but in some degree 
this necessarily interferes with the centre. Into the cause 
of this we have no need to enter here, and as it has been 
elsewhere discussed, I can only give the above directions, and 
say that there is a great field in this branch of study which 
the microscope alone has opened. 
It would be useless to enter into particulars respecting the 
various salts and treatment they require, as a great differ- 
ence is effected even by the strength of the solution. There 
are some crystals, also, which are called forth in insulated 
portions, showing no formation upon the ground; but even 
when mounted in any preserving fluid, and unchanged for a 
year, a new action seems to arise, and a groundwork is pro- 
duced which bears little resemblance to the original crystal. 
Sometimes this new formation adds to the beauty of the 
slide; in other cases the reverse is the result, the slide being 
rendered almost worthless. This action, I believe, frequently 
arises from some liquid being contained in the balsam or 
other mounting medium used ; and this is rendered the more 
probable by the crystallization being called forth in an hour 
after the balsam diluted with chloroform is employed, 
whereas no change would have taken place for months (if at 
all) had pure balsam been used. 
Sections of some of the salts are very interesting objects ; 
but the method of procuring these and their nature will be 
described in Chapter VI. 
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