The researches of which an account is given in the present communication have been undertaken in continuation of those carried on by Dr. Frankland and myself at the Royal College of Chemistry, from which we arrived at the conclusion that the thickening of spectral lines was due to pressure, and not to temperature per se . In our joint communications we pointed out that this held good for hydrogen in Geissler’s tubes and for magnesium vapour when the spark was taken in air, by means of a method which enabled us to spectroscopically examine its various portions. The magnesium experiment was important not only so far as the decrease of thickness of lines with decrease of density was concerned, but because it showed that one of the well-known triple lines in the spectrum of magnesium absolutely vanished altogether from the spectrum at some distance from the source of the supply of the vapour—that is, the pole of metallic magnesium. This result we also obtained, as stated in our note, when we observed the spectrum of the spark between two magnesium poles enclosed in a Geissler’s tube in an atmosphere of hydrogen in which the pressure of the gas was gradually reduced.