564 PRINCIPLES OF CHEMISTRY 



independent spectra of the compounds of copper are easily observed 

 (fig. 75). Thus certain compounds which exist and are luminous at a 



i .in- it i inni 



3*4 S 6 7 8 



\;ilSI^f' ;i lliiriiai >! 1 1 1 



F 





311 



Fig. 75. Bright Spectra of Copper Compounds. 



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 no reason for thinking 



36 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 different 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 of 

 temperature, just as ozone is converted into oxygen, or else because at low temperature 

 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 movement, 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 movements 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 movements 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 is enough to say 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 absorption spectrum of the vapour of 

 metallic potassium (which is green) they remarked a number of lines of the same inten- 

 sity 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, 



