290 
ELEMENTARY CHEMICAL MICROSCOPY 
first deposit being obtained, the receiving slide is moved along a 
few millimeters and a second sublimation made; again the slides 
are partly removed from the source of heat, the receiving slide 
moved along a trifle, and again the temperature is raised until a 
third film has been condensed. The process is continued as 
long as the material holds out on the first slide or fails to yield 
any further sublimate. If the drops of water, used to keep the 
receiver cool, evaporate, replace them by others. When dealing 
with compounds which melt on heating, the supporting slide must 
be slightly inclined so as to keep the material at the corner of the 
slide. Or we may sublime from a watch glass upon an object 
slide, as shown in Fig. 152, page 293. 
It sometimes happens that a more crystalline and characteristic 
sublimation fihn is to be obtained when the receiving slide is 
slightly warm, in which event the water is omitted, or, if this is 
not sufficient, a little cylinder made of carbon, such as is used in 
arc lamps, is warmed over a burner and placed upon the slide. 
Such pieces of carbon remain warm for some time and will be 
found to give excellent results. 
With the beginner it is always best to obtain each fractional 
sublimate upon a separate slide, carefully laying them down film 
side up in the order in which they have been obtained. Other¬ 
wise the films first formed are apt to be driven off by the in¬ 
creasing heat required to vaporize the last portions or will be 
rubbed off by the fingers or by contact with the support. 
When a series of sublimation films are obtained upon a single 
slide always see that the films succeed each other in such a man¬ 
ner as to bring the first ones farther and farther from the source 
of heat as each film in turn is formed. 
When dealing with sublimations taking place only at tempera¬ 
tures so high that ordinary glass will soften, quartz slips may be 
employed or nickel or platinum foil or small nickel or platinum 
spatulas. The method of procedure will in any event be similar 
to that above described, intimate contact between substance and 
support being first accomplished when possible by moistening 
with water and careful drying. 
The temperatures of sublimation may be determined by means 
