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period of vibration is increased, and consequently that the length of the wave of sound sent out is lengthened. Now, suppose a molecule or atom, the spectrum of which I am acquainted with, enters into combination with another; and suppose that the vibrations of the second molecule are weak, or lie outside the visible range of the spectrum: then the most simple assumption which I could make would be that the addition of the new molecule is equivalent to an increase of the mass of the other. An increase of mass without alteration of the force of the molecule will, as in the case of the tuning-fork, lengthen the period of vibration, and increase the wave-length. If a case of that kind were actually to happen, I should observe the whole spectrum shifting toward the red; and this is what is observed in the few simple cases to which I have referred. The first observation to that effect is due to Professor Bunsen, of Heidelberg. Examining the absorption spectra of different didymium salts, he found such a close resemblance between them, that no difference could be detected with instruments of small powers; but with larger instruments it was found that the bands varied slightly in position, that in the chloride they were placed most toward the blue end of the spectrum, that when the sulphate was substituted for the chloride a slight shift toward the less refrangible end took place, and that a greater shift in the same direction occurred on examining the acetate. Prof. Bunsen remarks that the molecular weight of the acetate is larger than that of the sulphate, and that the molecule of the sulphate, again, is heavier than that of the chloride. He adds: "These differences in the absorption spectra of different didymium compounds can not, in our present complete state of ignorance of any general theory for the absorption of light in absorptive media, be connected with