Elementary chemical microscopy (1921) Elementary chemical microscopy elementarychemi00cham Year: 1921 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
Elementary chemical microscopy (1921) Elementary chemical microscopy elementarychemi00cham Year: 1921 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
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