HANDLING SMALL AMOUNTS OF MATERIAL: SUBLIMATION 291 



of a hot stage such as that described on page 222 or by the method 

 recommended by A. W. Blyth. 1 A small porcelain crucible is 

 nearly filled with mercury, into which dips the bulb of a ther- 

 mometer. A thin cover glass, bearing at its center the material 

 to be tested, moistened and dried as usual, is 

 floated on the surface of the mercury. Upon 

 the cover glass is placed a low glass cell whose 

 upper and lower rims are accurately ground. 

 A second cover glass is placed above to receive 

 the film — see diagram, Fig. 151. A number 

 of clean covers should be placed near at hand. 

 The crucible is heated over the low flame of 

 a Bunsen burner. As the temperature rises, 

 the covers are changed, by means of a pair of 

 forceps, every five or ten degrees. The cover 

 glasses are examined under the microscope, 

 and a decision made as to the temperature of FlG - J 5i- Crucible 



i v a j j ii-* j Method of Micro- 



sublimation. A second and even a third ex- ... . 



sublimation. 



periment should always be made. If the 



material fails to sublime at a temperature below that at which 



the mercury itself is volatilized, a bath of a suitable low-melting 



alloy must be used. For accurate measurements it is essential 



to protect the crucible and cell from the cooling effects of air 



currents. 



Subliming upon a glass object slide as shown in Fig. 150 is 

 impracticable when only a minute quantity of the material is 

 available since the losses through incomplete condensation are 

 considerable. In such an event it is safer to employ the device 

 shown in Fig. 153, page 293, primarily intended for distillation 

 but yielding good results with solids as well as with liquids. 

 When, however, only an excessively small amount of material is 

 to be tested as in toxicological analysis, it is better to drop the 

 substance into a thin-walled glass tube of not over 1 millimeter 

 in diameter, sealed at one end. Tap the tube gently so as to col- 

 lect all of the material at the sealed end. With a very fine blast- 

 lamp flame draw out the open end to a hair-like capillary tube, 

 1 Poisons: Their Effects and Detection, 259, 4th Edition, London, 1906. 



