980 
EEV. T. E. EOBINSON ON SPECTEA OE ELECTEIC LIGHT. 
and O. As to metalSj the lowest number, 12, occurs in No. 85 ; but the average is far 
above this. 
These facts may be explained in two ways. We may suppose that the action of the 
electric discharge on the molecules which transmit it has in itself intermittences which 
tend to produce maxima of light recurring at intervals, and which are effective in doing 
so when the forces inherent in these molecules are in accordance with them. If that 
accord be perfect, the development of light will be intense ; but if imperfect or wanting, 
the corresponding light will be feeble or will vanish. Or it may be supposed that our 
metals and gases are compounds of unknown elements which are separated by the dis- 
charge, and exhibit their appropriate lines. This hypothesis is tempting, for if true it 
would at once lead to analysis of many of our supposed simple substances ; and the facts 
which have been stated respecting the band No. 49 give some countenance to it. I how- 
ever think the first view of the matter more probable, for reasons which I shall soon 
state. According to it, the existence of a luminous line merely indicates the presence 
of matter in the circuit ; but its intensity depends on the nature of that matter, which 
may either make it extremely bright or obscure it, even to invisibility. It is generally 
supposed that the presence of a metal gives brilliant lines, and that those due to a gas 
or vapour are less bright ; but as the two are always simultaneously present it is not 
easy to separate their influences. A promising plan of effecting this has been proposed 
by Pluckek. Two balls connected by a capillary tube were filled with any gas and then 
exhausted ; the balls were coated with tinfoil, and when they were connected with the 
terminals of an induction machine recij^rocating discharges took place, which, though 
very faint in the globes, were bright enough when condensed in the capillary part to 
give a spectrum. Here nothing but glass is in contact with the rarefied gas, and there- 
fore he expected to obtain only gas lines. I repeated this experiment, adding a glass 
stopcock to one of the balls, that I might use the same glass with different gases. The 
spectra observed were the E. of N, O, and H ; the first had 13, the second 23, and the 
last 13, besides several in each too faint to be taken. Of these were — 
Common to three. 
In N and 0. 
In 0 and H. 
In 0, 
In N. 
In H. 
Nos. 18 
Nos. 27 
Nos. 49 
Nos. 16 
Nos. 14 
Nos., 5 
39 
68 
75 
24 
76 
13 
46 
33 
84 
82 
57 
36 
61 
41 
65 
42 
78 
44 
48 
52 
54 
72 
