572 



PEINCIPLES OF CHEMISTKY 



lines are observed in the spectra of two different compounds of one and 

 the same element obtained in the above -described manner, and also in 

 the spectrum of the metal, but they all have their peculiarities. The 

 independent spectra of the compounds of copper are easily observed 

 (fig. 75). Thus certain compounds which exist in a state of vapour, and 

 are luminous at a high temperature, give their independent spectra. 

 In the majority of cases the spectra of compounds are composed of 

 indistinct luminous lines and complete bright bands, whilst metallic 

 elements generally give a few clearly-defined spectral lines. 36 There is 



M Spectroscopic observations are still further complicated by the fact that one and 

 the same substance gives different spectra at different temperatures. This is especially 

 the case with gases whose spectra are obtained by an electric discharge in tubes. 

 Pliicker, Wiillner, Schuster, and others showed that at low temperatures and pres- 

 sures the spectra of iodine, sulphur, nitrogen, oxygen, &c. are quite different from the 

 spectra of the same elements at high temperatures and pressures. This may either 

 depend on the fact that the elements change their molecular structure with a change oi 

 temperature, just as ozone is converted into oxygen (for instance, from N 2 molecules are 

 obtained containing only one atom of nitrogen), or else it may be because at low tempera- 

 ture certain rays have a greater relative intensity than those which appear at higher tem- 

 peratures. If we suppose that the molecules of a gas are in continual motion, with a 

 velocity dependent on the temperature, then it must be admitted that they often strike 

 against each other and rebound, and thus communicate peculiar motions to each other 

 and the supposed ether, which express themselves in luminiferous phenomena. A rise 

 of the temperature or an increase in the density of a gas must have an influence on the 

 collision of its molecules and luminiferous motions thus produced, 

 and this may.be the cause of the difference of the spectra under 

 these circumstances. It has been shown by direct experiment 

 that gases compressed by pressure, when the collision of the 

 molecules must be frequent and varied, exhibit a more complex 

 spectrum on the passage of an electric spark than rarefied gases, 

 and that even a continuous spectrum appears. In order to show 

 the variability of the spectrum according to the circumstances 

 under which it proceeds, it may be mentioned that potassium 

 sulphate fused on a platinum wire gives, on the passage of a series 

 of sparks, a distinct system of lines, 583-578, whilst when a series 

 of sparks is passed through a solution of this salt this system of 

 lines is faint, and when Roscoe and Schuster observed the absorp- 

 tion spectrum of the vapour of metallic potassium (which is green) 

 they remarked a number of lines of the same intensity as the above 

 system in the red, orange, and yellow portions. 



The spectra of solutions are best observed by means of Lecoq 

 de Boisbaudran's arrangement, shown in fig. 76. A bent capillary 

 tube, D F, inside which a platinum wire, A a (from 0'3 to 0'5 mm. 

 in diameter) is fused, is immersed in a narrow cylinder, C (in 

 which it is firmly held by a cork). The projecting end, a, of 

 the wire is covered by a fine capillary tube, d, which extends 

 1-2 mm. beyond the wire. Another straight capillary tube, E, 

 with a platinum wire, B 6, about 1 mm. in diameter (a finer wire soon becomes hot), 

 is held (by a cork or in a stand) above the end of the tube, D. If the wire A be 

 now connected with the positive, and the wire B with the negative terminal of a 

 Ruhmkorff s coil (if the wires be connected in the opposite order, the spectrum of air is 

 obtained), a series of sparks rapidly following each other appear between a and b. and. 



FIG. 76. Method of 

 showing the spec- 

 trum of substances 

 in solution. 



