out more brightly under certain circumstances than others, and thus giving a different character to the spectrum. That the change, in a gaseous spectrum, may go so far as to produce a continuous spectrum is also certain. The flames of many gases, such as this blue one of carbonic oxide, burning in the air to form gaseous products of combustion, give continuous spectra ; indeed, we may see the beginning of such a continuous. spectrum in every soda flame; and Dr. Frankland has lately observed that when oxygen and hydrogen gases are inflamed under great pressure, they emit white light and show an unbroken spectrum. From these facts there is no doubt that, when intensely heated and under certain circumstances, gaseous bodies can be made to yield continuous spectra.2 This, however, in no way interferes with the fixity of position of the bright lines, nor can it influence the deductions derived from this fact. Several interesting observations have been made with respect to the changes produced in the spectra of some 1 See Appendix C to this lecture. of the metals by increase of temperature. Let me, in the first place, show you that new lines may make their appearance in the spectra of certain elements when the temperature is raised. Thus, for instance, if we heat lithium, either the metal or its salts, in the electric arc, we obtain a splendid blue band having the wave-length of 4605 ten-millionths of a millimetre (see Fig. 39), in addition to the red and orange rays a and B seen in the flame spectrum, showing that the undulations in this particular set of vibrations have become more intense. The same phenomenon is observed in the case of the strontium spectrum, where no less than four new lines (e, 7, x, and λ, Fig. 39) make their appearance on increasing the temperature of the incandescent vapour of the metal. Also in the case of sodium four sets of new lines appear when the temperature is increased, in addition to the wellknown "D" lines; and singularly enough, each of these four lines is double. Dr. Watts has shown that the first of these higher sodium lines (wave-length 5889 and 5687) become visible at a temperature of 2000° C., whilst the next set (wave-length 5155 and 5152) appear when the temperature rises to about 3000° C. The analogy between the production of these more highly refrangible rays and that of the overtones or harmonics of a vibrating string will occur to all. On the other hand, by reducing the temperature, and therefore the intensity of the spark, only the most prominent lines of a metallic spectrum may be seen. Thus Lockyer and Frankland have shown that the magnesium (b) lines vary in length and intensity when the electrodes are separated, so that in a certain position one of the four well-known magnesium lines disappears. We shall see the application of this observation in a subsequent lecture. The second set of facts with regard to the effect of increased heat has reference to the changes which the spectra of compound bodies undergo when the temperature is increased. These changes are clearly seen in the following experiments. Let us first put a bead of fused chloride of calcium, a common lime salt, into the colourless gas flame: we observe a peculiar spectrum, which is represented roughly on this diagram, in which the red is supposed to be on the right and the blue on the left hand (Fig. 40, No. 1, and Frontispiece, No. 9). If, however, we now pass an electric spark over a bead of chloride. of calcium, and then look at the coloured spark, we find that the spectrum thus obtained is not the same as that observed in the flame. FIG. 40. Here you notice the difference between these two spectra: the lower drawing gives you the spark spectrum, and the upper one what we may call the flame spectrum. This variation can be readily explained. It is a well-known fact that certain chemical compounds, when they are heated up above a given temperature, decompose into their constituent elements, whilst, below that temperature, these compounds are capable of existing in a permanent state. When we once get the spark spectrum, we find that no alteration in the intensity of the spark can then alter the position of those lines. The position of the red lithium line never varies, although the blue line comes out. It naturally strikes every observer that these bands seen in the flame spectrum are produced by a compound of calcium (say the oxide or chloride), which remains undecomposed at the temperature of the flame. When we increase the temperature, as in the spark spectrum, a dissociation of the compound occurs, and we get the true spectrum of the metal. The position of these true metallic lines never alters at all, although, when the intensity of the electric spark is increased, new lines may sometimes make their appearance. Hence we can fully rely upon the spectrum test as a proof of the presence of the particular metal. No such change in the character of the spectra is noticed in the case of those metals whose compounds undergo dissociation at low temperatures: thus we do not see any such phenomenon in the alkaline metals, although it is observed in the case of barium, strontium, and calcium; for if I take two beads, one of sodium nitrate and the other of sodium chloride, I obtain in the flame the same sodium spectrum with both salts, because each is decomposed, yielding incandescent sodium vapour. Another fact which bears out the truth of this explanation has been observed by Plücker, that, in the case of bodies whose spectra change from bands to lines on increase of temperature, a recombination of the elements occurs on cooling, and the band spectrum of the compound reappears. Many other observations crowd upon us to convince us that compound substances capable of existing in the state of glowing gas yield spectra different from those of their constituent elements. Thus the spectrum of terchloride of phosphorus exhibits lines differing from those of either phosphorus or chlorine, and the chloride and iodide of copper each yields a distinct. set of bands bearing no resemblance to the bright lines of the metal. It is here important to learn that a distinguished spectroscopist, Professor Angström, does not endorse the conclusions of Plücker and Wüllner respecting the existence of several spectra for one element, inasmuch as the spectra observed in the Geissler's tubes with low intensity are, according to Ångström, those of compound bodies, and it is only when the discharge becomes disruptive that the constant spectrum of the element appears. Ångström doubts the truth, therefore, of reputed dualism in the spectrum of one element; and he explains the observed changes by supposing that traces of some foreign body are present, and he shows that it is almost impossible to obtain any gas perfectly pure when in an extreme state of rarefaction. Thus, on one occasion, Ångström rarefied air in a Geissler's tube to the utmost extent attainable by a mercury pump, and on allowing a discharge from an induction coil to pass through, he obtained, (1) the ordinary air spectrum, (2) the fluted spectrum of nitrogen, (3) that of carbonic oxide, (4) the bright lines of chlorine and of sodium. Proceeding to criticise in detail Wüllner's experiments, Ångström concludes that two of the four spectra attributed to hydrogen are really the spectra of the hydrocarbon acetylene and sulphur; whilst of the other two, one is the true hydrogen spectrum, and the other the broad band hydrogen 1 See Appendix A, Lect. V. |