PEOFESSOE STOKES ON THE LONG SPECTEUM OF ELECTEIC LIGHT. 617 
of the envelope and blue negative light, both which are drawn out, indicating a very 
appreciable duration. On making the electrodes approach somewhat nearer, the spark 
diminishes, and the envelope is formed in perfection, especially with broad electrodes. 
The air-lines now begin to show themselves well, but are brightest on the side of the 
spectrum answering to the blue negative light. 
It might be supposed at first sight that the permanence of the yellowish and of the 
blue light only indicated a glow of appreciable duration left by a sensibly instantaneous 
discharge ; but several circumstances indicate that the discharge itself lasts, and that it 
is under its action that the glow takes place*. The action, I am persuaded, is this: a 
spark first passes ; and this enables a continuous discharge to pass, which is due, in part 
at least, to the inductive action of the still falling magnetism, just as a voltaic arc may 
be started in a powerful battery by passing an electric spark between the slightly sepa- 
rated electrodes ; and the glowing of the air under the action of this discharge produces 
the yellowish envelope and blue negative light. Thus, when the electrodes are nearly 
at the greatest distance at which this sort of discharge takes place, the blue negative 
light is seen pretty sharply terminated in a moving mirror. Were it a dying glow, it 
ought to fade away ; but if produced imder a discharge, it ought to cease almost abruptly, 
inasmuch as at this distance of the electrodes a continuous discharge is unable to pass 
when the tension has sunk much below that under which it was first produced. 
The same conclusion may be drawn from an effect which I once obtained, the exact 
conditions for the production of which it is not easy to hit off. With a jar in connexion, 
each discharge due to a single breach of contact appeared in a moving mirror as a bright 
spark joined to a spark less bright by the blue negative light, and also by the yellowish 
or reddish light, brightest close to the positive electrode. Were the blue light due to a 
glow, it ought to be reinforced instead of being put out by the second spark, whereas 
the explanation of the result is easy on the supposition of a continuous discharge. The 
first spark started a continuous discharge, which emptied the jar less fast than it was 
filled by the secondary coil ; so that presently another discharge took place, which 
emptied the jar so that a continuous discharge could no longer pass. 
On viewing the broad discharge formed without a jar when the electrodes are at a 
moderate distance, through a revolving disk of black paper with a single hole near the 
circumference, while the envelope was being blown aside, so as to get a succession of 
momentary views of the discharge, the envelope was seen extravagantly lent, as a fiexible 
conductor might have been — not torn across, as a column might have been which was 
heated by a previous spark. The central spark, of course, was usually missing, as it is 
sensibly instantaneous. 
I have spoken of the arc, and especially the blue negative light, as exhibiting air-lines. 
The arc, however, is liable to be coloured not only by casual dust (as when it passes 
partly through the flame of a spirit-lamp with a salted wick, when it is coloured yellow 
* AltFougli this view may be considered already established (see the work by the Vicomte Du Moncel 
jnst q^uoted), the observations here mentioned wOl not, I hope, be altogether useless. 
4 P 2 
