226 ELEMENTARY CHEMICAL MICROSCOPY 



to be determined. A lens attached to the body-tube or held in 

 a separate stand serves to magnify the thermometer scale. It 

 is thus possible to look into the tube of the instrument and to 

 watch both the material and the thermometer. This arrange- 

 ment and its applications will be readily understood by refer- 

 ence to the illustration. 



With platinum wire coils a temperature somewhat higher than 

 700 C. may be obtained in the apparatus. 



The material to be tested may be either crystallized upon or 

 supported on a small thin cover glass held by the wire fingers C 

 or may be placed in a short piece, 5 millimeters long, of tiny thin- 

 walled capillary tube fastened to the thermometer by a wire 

 band. For ordinary materials these tubes are best held horizon- 

 tally but for fats, waxes, etc., better results are obtained by 

 slightly inclining the capillary and taking as the melting point 

 the thermometer reading at the instant the fat slides out of 

 focus. 



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 



