The luster of coals as indicated by reflected light has been used in classification of coal by other workers. The present writers have measured the index of refraction of various vitrain samples by determining the angle of maximum polarization in reflected light. Measurements were made on a Fuess goniometer. The order of accuracy was not greater than 0.015 for the indices of refraction. Using this order of accuracy, it was found that the index of refraction of the vitrain in banded coals indicates the rank of the coal, and its approximate British thermal unit (B. t. u.) value on a “moist mineral-matter-free” basis. Other methods are proposed, and the experiments are to be continued.

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