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132 than white, the spectrum of course gives only the primitive colours necessary to make up the given colour. If, for example, yellow is wanting, the spectrum will give a black band instead of a yellow one. If, on the other hand, the colour transmitted through the prism be an elementary colour, there will be only one band, as in the case of the yellow flame produced by sodium; all the other colours are wanting, and only one yellow band is given. The flame in which lithium is diffused, gives only two bands, yellow and red.

The value of the discovery of the researches of the Heidelberg professors lies in this, that these bands are proved to be perfectly characteristic of the substances which produce them; that is, each substance has its own distinctive bands, so that they cannot be confounded with the bands of any other. The distinctive marks are—their number, breadth, tone, and grouping in the spectrum. The characteristic lines are as invariable as the circles of longitude and latitude; and just as these circles fix with certainty the locality of any city, so the bands in the spectrum determine the nature of the substance emitting the colour thus decomposed.

The delicacy of the prism in detecting minute quantities, far surpasses the reagents of the laboratory. One hundred-millionth of a grain can thus be detected. A very minute quantity of sodium diffused in a room, can at once be revealed by its effect on the