Of all the spectra that have been determined, those of sodium and iron are the most important and interesting. In that of sodium, the only light is of the purest yellow condensed into a double line of intense brilliancy on a dark ground. The iron spectrum on the contrary is crossed by bright lines of all intensities and colours in such multitudes, that their number has not been ascertained. The calcium spectrum has one very bright green band in the orange, a red line in the yellow, and a well-defined yellow line in the indigo. As already mentioned, the red and orange parts of the strontium are crossed by many red lines separated by dark intervals; there is a bright blue line between the orange and yellow, and an orange line in the blue. One intense crimson band in the orange characterises the lithium spectrum. Seven broad green bands stripe the yellow and a part of the green, in the barium spectrum, and that of magnesium has many green bands and lines. All of these were determined by the heat of white coal gas flame, which amounts to 2350° Cent., and at the time MM. Bunsen and Kirchhoff were not aware that by an increase of temperature new bright lines were added to some of the spectra. That discovery was made by Professor Tyndall, while examining the spectrum of chloride of lithium, which with the low temperature has only one crimson band in the orange, but with the hotter flame of hydrogen gas, amounting to 3259° Cent., an orange line appeared in the yellow, and when Mr. Tyndall employed the electric lamp,[18] the spectrum acquired a broad brilliant blue band between the orange and yellow, while the crimson band remained unchanged. Professors Roscoe and Clifton confirmed Tyndall’s discovery, and upon comparing the spectra of strontium and lithium, they found where only one prism was employed that the blue line of lithium appeared to coincide with the blue line, delta, of strontium; but with an apparatus having several prisms like that of Kirchhoff, they saw that the two blue lines differed by one division of the measuring scale, the lithium line being the most refrangible. A great change was produced on the strontium spectrum by increased electric temperature: three of the red bands vanished, and new bright lines appeared, that were not coincident with those they replaced; the blue line was not affected, but four new violet lines were added. With the intense heat of the electric spark, the broad green band of the calcium spectrum is replaced by five green lines of less refrangibility, the well-defined yellow line vanishes, and instead of the red band three red or orange lines appear, of greater refrangibility than those that have vanished. Six of the bright green bands in the spectrum of barium entirely vanish, and bright new non-coincident lines appear. Thus, not only new lines appear at very high temperature, but the broad bands, characteristic of the metal or metallic compound at a low temperature of