94 Investigations on the Nature of Spectra. 



enclosed in one of the tubes of average width, with points as 

 electrodes ; the pressure of the nitrogen amounted to about 10 

 millims. Even when it was heated to 220°, the nitrogen- 

 spectrum was still distinctly visible together with that of 

 iodine — though, it is true, a series of dark absorption-lines 

 proceeding from iodine vapour were drawn through it. As 

 the iodine vapour filled the whole of the tube, the light (which 

 was emitted from the incandescent gas occupying only the 

 middle of the tube) must have passed through a considerable 

 thickness of it. At all events the tension of iodine at 220° is 

 much greater than that of mercury at the same temperature, 

 since the former boils at 180°, but the tensions of the latter are 

 uncommonly small (at 100°, according to Regnault, 0*146 

 millim., at 140° 3*059, and at 200° 19*19). Nevertheless in 

 the mercury-tubes the nitrogen-lines disappeared, even with 

 100 millims. pressure of nitrogen, at 140° at the positive elec- 

 trode*. 



Fourthly, that the phenomenon cannot be conditioned by 

 mercury precipitated on the electrodes being disruptively car- 

 ried away we learn, first, from the experiment described on 

 p. 92 ; and then, again, it was indicated by observations 

 which were made with glass tubes without internal electrodes, 

 in which the ends were only coated with tinfoil, and yet, when 

 the coverings were connected with the sources of electricity, 

 the nitrogen-lines vanished nevertheless. 



To mercury, therefore, must be attributed a behaviour 

 towards electricity which differs from that of hydrogen and 

 nitrogen. We might assume that the mercury molecules 

 alone take part in the discharges, or that they take a prepon- 

 derating measure of the electrical charge. 



In the mercury vapour mixed with hydrogen the discharge 

 perhaps takes place in this way: — There is an accumulation of 

 electricity at the electrodes ; and this distributes itself to the 

 molecules of the surrounding gas. But the mercury mole- 

 cules will be more charged than those of hydrogen ; and by 

 them will the discharge be chiefly brought about, because on 

 their encountering other mercury and nitrogen molecules they 

 will in preference give up their electricity to the former. But 

 the passage of the electricity conditions the luminous pheno- 

 mena ; hence only mercury-lines appear. 



Since, further, the electrodes do not behave alike, but the 

 nitrogen-lines are much longer visible at the negative than at 

 the positive electrode, we may perhaps assume that mercury 

 charges itself more readily with positive than with negative 



* Even mixtures of hydrogen and nitrogen constantly show the spectra 

 of both gases. 



