940 
EEY. T. E. EOBINSON ON SPECTEA OF ELECTEIC LIGHT. 
electrodes and medium would produce separately 1 Lastly, is it certain that electricity 
produces light merely by its heating power I and may not the same action which 
produces thermic vibrations, also (and independently) produce luminous 1 
Some of these questions I have endeavoured; to investigate and the attempt can 
scarcely fail to be of use, were it only to direct attention to the subject. 
My attention was originally turned to it by observing, some years ago, that the dis- 
charge in carbonic oxide, which is white at common pressures, becomes bright green 
when the gas is highly rarefied. The spectra in these cases difiered so much that I 
determined to examine them in various gases and metals ; and I procured the apparatus 
which seemed necessary. That of Steinheil had not been contrived then ; and my want 
of experience in such researches caused at first some mistakes, but by degrees I corrected 
them ; and though my instrument is of comparatively limited power, the results are, I 
hope, not without importance ; and I have given details sufficient to estimate the en-ors 
which may afiect them. 
The source of electricity in my experiments was an induction-machine. Till July 
1860 I used one by Hearder, having three miles of secondary wire ; after that, one which 
I have described*, with the substitution of copper wire for iron. It has 9-5 miles, of 
which three are not lapped, merely varnished ; and when excited by three Grove’s, each 
of which has 49 square inches of platinum acting, it gives abundantly sparks 6 ’8 inches 
long. By a simple device its two coils can be in an instant made to act collaterally 
instead of consecutively, thus reducing the intensity but doubling the quantity. Then 
the sparks are only 2-5 inches, but very dense and luminous, and possessing a far higher 
power of ignition. A Leyden jar, each coating 1‘25 foot, was normally connected with 
the terminals, as the spectrum of the simple spark is far more faint and unsteady; 1 
have given one as a specimen in Table III. With this jar there is a continuous stream 
of discharge at 0-6 inch. 
The discharges were made in the open air, or in tubes about 0'2 inch diameter and 
6 inches long. With rarefied gases they would be more luminous if the tubes were capil- 
lary ; but in that case it would be very difficult to clean them from the coatings of metal 
or oxide which are deposited, most densely near the negative electrode, but often on the 
whole interval. In some metals, of which the most notable are lead, cadmium, bismuth, 
antimony, arsenic, and above all tellurium, this deposit was so thick that at the close of 
an experiment it was difficult to see the fainter lines, and I found it necessary to employ 
for common density a tube 1 inch diameter. Even this was coated, but not so thickly 
as to give much trouble. The upper electrode (which was, in all cases but one, positive) 
was attached to platinum wire fused in the tube. It was sometimes soldered to it, at 
other times twisted with it ; and when I could not procure the metal as wire or foil, a 
globule of it was fused by a gas-blowpipe, a platinum wire inserted in this, and the heat 
withdrawn. For nickel and cobalt, the blowpipe was oxyhydrogen. As the electric 
light does not spread over the positive electrode, it was unnecessary to insulate the 
platinum wire ; but "with the negative it is otherwise ; and when I had not enough of 
* Philosophical Magazine, April 1859. 
