DETERMINATION OF MELTING POINTS 225 



The melting point of anisotropic substances is sharply obtained 

 by making the observations with crossed nicols and a selenite 

 plate; the change from solid to liquid of tiny particles is thus 

 remarkably clear since they vanish instantly on melting. The 

 hot stage should in such cases be provided with glass windows . 



The upper window of the stage consists of a thin glass object 

 slip (or one of mica or of quartz) held in place by the guides B, B, 

 permitting sliding the cover. This is essential when dealing with 

 materials which sublime, for in these cases the upper window 

 becomes fogged with condensed material, and in such an event 

 the cover is simply pushed along until a clear section is 

 obtained. 



In determining melting points with any type of hot stage, it is 

 obvious that the usual procedure should be followed, namely: 

 make a preliminary observation and then start anew, raising the 

 temperature very gradually as the melting point first observed 

 is approached. 



Determinations of the subliming points of tiny particles may 

 also be made by means of the hot stage. 



Electrically heated stages of several forms and for different 

 ranges of temperature may now be had from several different 

 optical firms. 1 



The Determination of Subliming Points may be made in the 

 hot stage illustrated in Figs. 121 and 122, or by the crucible 

 method of Blyth described on page 291. 



1 For other types of hot stage see: Cram, J. Am. Chem. Soc., 34 (1912), 954. 

 Cottrell, J. Am. Chem. Soc. 34 (1912), 1328. Dox and Roark, J. Am. Chem. Soc. 

 39 (1917), 742. 



E. Leitz, \Vetzlar, Germany, manufactures several convenient hot stages of low 

 range. 



Electric incubators for use upon the stage of the microscope and available for 

 melting point determinations are made by the Chicago Surgical & Electrical Co. 

 Chicago, 111. 



For a microscopic method for the estimation of high melting points, as for example 

 those of metals and alloys, consult: Burgess, A Micropyrometer. Circ. 198, U. S. 

 Bureau of Standards. Applications of the Micropyrometer, J. Frank. Inst. 182, 

 (1916), 19. 



