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 Hquid of tiny particles is thus 
remarkably clear since they vanish instantly on melting. The 
hot stage should in such cases be provided with glass wirdows. 
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.^ 
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, Wetzlar, Germany, n^nufactures 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, Ill. 
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. 
