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 film 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 



