194 THE AMERICAN MONTHLY [July 
fuses, withdraw it from the heat before it chars, and al- 
low it to cool gradually. If successful, small circular 
plates or rosettes will appear on the film, and these may 
be mounted in the usual way in cold xylol balsam. A 
good slide of this description, viewed with dark ground 
illumination, or by polarised light, will fully repay any 
trouble involved in preparing it. This method is useful 
in enabling one to prepare totally different physical forms 
from the same salt. With salicine, for instance, an aque- 
ous solution deposits needle-shaped crystals, quite dis- 
tinct from the circular form obtained by fusion. A point 
to be observed in using the process is to avoid having too 
-much of the salt on the slip, as’on cooling, the film, if too 
thick, will probably star and crack. If the film should be 
too thick for viewing asa transparent object, it will often 
make a good opaque one by pasting a circle of black pa- 
per on the under side of the slide. 
Another class of objects, prepared in a similar way, 
are crystals of fatty substances, spermaceti, hard paraf- 
fin, etc. It is only necessary to place a small piece on a 
slip and warm it. When melted press down on it a cov- 
er-glass, the crystal forming as the mass cools. These 
slides cannot compare from an artistic point of view with 
those obtained from salts, but are interesting from the 
fact that the actual formation of the crystals can be 
watched under the microscope any number of times by 
simply warming the slide before viewing it. 
The third principal method is still more limited in ap- 
plication, being confined to those substances which are 
easily volatilized and crystalize on cooling. Preparation 
of slides by sublimation may be carried out as follows 
Take a dry narrow test-tube and place init any suitable 
chemical—benzoic acid, for instance. Hold the tube over 
the flame until the acid volatilizes, now invert the tube 
and stand it on a cold 3x1 in. slip. The characteristic 
crystals will form on the part of the slip covered by the 
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