DETERMINATION OF MELTING POINTS 219 



measurements are available, it follows that melting-point deter- 

 minations may be obtained of material actually invisible to the 

 naked eye. Furthermore, these determinations will, in most 

 cases, be as accurate as those made by the usual capillary-tube 

 sulphuric-acid method, 



Method A. (Approximate.) — Where a series of pure com- 

 pounds, readily crystallizable and each of known melting point 

 is at hand the melting point of an unknown substance may be 

 ascertained approximately by placing similar sized fragments 

 of the known and the unknown side by side at the corner of a 

 thin object slide. The rotating stage of the microscope is re- 

 moved and a piece of asbestos board, perforated at the center, 

 substituted as a stage. A bent glass or quartz tube drawn out 

 to a jet at one end serves as a tiny burner and may be fastened 

 temporarily to the substage ring. The tiny burner is so adjusted 

 that the flame falls nearly in the line of the optic axis of the 

 microscope. The slide carrying the material to be tested is 

 placed under the microscope and focused and the tiny flame is 

 very slowly brought nearer the preparation by means of the 

 screw which serves to raise or lower the substage. The behavior 

 of the material is watched very closely through the microscope, 

 to determine whether the known or the unknown substance 

 melts first. Other compounds of known melting point are tried 

 until a known compound is found with which the unknown 

 simultaneously melts or the unknown is found to melt between 

 the melting points of two knowns. This indirect method is 

 quick and convenient where mere approximations are needed. 

 The operator after one or two trials soon learns to judge the 

 temperatures given by the tiny burner according to the size of 

 its flame and the distance below the slide. When comparing 

 melting points in this manner first try the pure material with 

 which the unknown is believed to be identical. Place the two 

 substances so close together on the slide that when they melt, 

 the molten masses will flow together; if they melt simultaneously 

 and mix to form a homogeneous melt, the presumption is strong 

 that the two fragments are of the same composition. If so, when 

 the melt solidifies (freezes) a single component will result. 



