Page:Outlines of Physical Chemistry - 1899.djvu/151

 The methods adopted in the case of substances of high melting point are very different (V. Meyer, Le Chatelier, Holborn and Wien, and Heycock and Neville).

The comparison of the melting points of organic sub- stances has led to the discovery of certain regularities, similar to those which have been mentioned for the boiling points. But the laws to which allusion is made are as yet too limited in their application and have only been verified for a small number of groups.

The melting point as a qualitative test is of the greatest importance, and may even give quantitative indications, inasmuch as by it we may control the degree of purity of a substance. It has been found that the presence of a very small amount of impurity often lowers the melting point of a substance very considerably. This is very general for organic substances, and more especially for those belonging to the aromatic series. For instance, pure benzoic acid melts at 120°, but the presence of a trace of chlorobenzoic acid or nitrobenzoic acid lowers the melting point by several degrees. Para-oxybenzoic acid fuses at 210°, and the meta- isomer at 290° ; but a mixture of equal parts of the two melts at 148-152° (L. Lieber- mann).

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