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. 
