METHODS FOR HANDLING SMALL AMOUNTS OF MATERIAL 241 



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 



, . . , . , FIG. 127. Sublimation of Material 



water has for its object the from One Object Slide to Another. 



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. 127). 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 



