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 by the general name of Mica, and he thinks he has decisive reasons to believe that several substances of natures so extremely different as to their composition and structure have been improperly comprised under that name. He has also made use of the phænomena of alternate polarization, to construct an instrument which he calls a colorigrade, which, producing in all cases the same series of colours in exactly the same order, merely by the nature of its construction, affords a mode of designation just as convenient for comparison as that furnished by the thermometer for temperatures.

Many other experiments have been made, and are daily making; many other properties have been discovered in polarized light; but the limits of this Work do not allow us to give any detailed account of them, so that we have been obliged to confine ourselves to the results, which are, perhaps not the most important part of the subject, but the easiest to explain; our aim in this rapid sketch being rather to stimulate than satisfy the desire of knowledge on this branch of science which presents so vast a field for research both in theory and experiment, and which, though so lately discovered, has already furnished some useful applications to Physics and Mineralogy.