HANDLING SMALL AMOUNTS OF MATERIAL: SUBLIMATION 289 
the material from being blown off by air currents, and brings the 
substance in intimate contact with the glass slide — a matter of 
prime importance. If the material is already in solution, evapo¬ 
rate a tiny drop, but in this case it should not be spread out, as 
is commonly done with test drops. When the drop is dry, add 
another tiny drop on top of the residue left by the first; this in 
turn is dried, the process being repeated until, in the judgment 
of the operator, there is sufficient material for work. In all 
cases the residue to be treated should occupy but little space, 
yet should not be too thick, since, if fractional sublimation is 
to be practiced, a thick mass is apt to be heated unequally and 
fallacious results will be obtained. 
Everything being ready, the slide is held in the left hand and 
the heating begun over the micro-flame, not directly beneath 
the spot of material, but slightly 
nearer the center of the slide. 
This is done in order to avoid rais¬ 
ing the temperature too rapidly 
and too high. As soon as the sub¬ 
limation point is almost reached 
(which can easily be recognized 
by practice) a second clean slide, 
carrying a drop or two of water, 
is taken in the right hand and 
lowered over the first slip, with 
the drop of water on the upper 
side directly over the material 
to be sublimed. The drop of 
water has for its object the 
keeping of the upper slide cool, 
thus far more effectually condensing any vapors produced by the 
heating. The receiving slide is supported on an edge of the other 
and is brought to within 2 to 4 millimeters of the substance (see 
diagram. Fig. 150). The temperature is gradually raised by 
moving the spot of substance nearer the flame. As soon as there 
is evidence of the appearance of a sublimate, raise the two slides 
above the flame so as to prevent too rapid vaporization. The 
Fig. 150. Sublimation of Material 
from One Object Slide to Another. 
