600 PEOrESSOE STOKES ON THE LONG SPECTEIJM OE ELECTEIC LIGHT. 
spectrum was obtained from the voltaic arc, and this was sufficiently bright to be exhi- 
bited to the audience, the arc passing between copper electrodes, and the pure spectrum 
formed by quartz apparatus being received on a piece of uranium glass cut for the pur- 
pose. The spectrum thus formed was found to consist entirely of bright lines *, whereas 
the spectrum of the discharge of a Leyden jar had appeared (perhaps from not having 
been truly in focus) to be continuous, or at least not wholly discontinuous. 
The mode of absorption of light by coloured solutions, as observed by the prism, affords 
in many cases most valuable characters of particular substances, which, strange to say, 
though so easily observed, have till very lately been almost wholly neglected by chemists. 
Having obtained the long spectrum above mentioned, I could not fail to be interested 
with the manner in which substances, especially pure but otherwise imperfectly known 
organic substances, might behave as to their absorption of the rays of high refirangibility. 
But the difficulties attending the habitual use of a nitric-acid battery of 30 or 40 cells 
deterred me from entering on this investigation, and I determined to confine myself to 
the solar spectrum. 
On account of some inconvenience attending the tarnishing of the speculum of my 
heliostat, I was induced to order a quartz plate, intended to be either silvered or coated 
with the usual amalgam of tin. On trying on a small scale the reflecting power of such 
plates with respect to the invisible rays, which may be done by means of fluorescence 
almost as easily as if those rays were visible f, I noticed a remarkable falling off in 
the reflecting power of the silvered plate for the most refrangible of the solar rays, which 
I readily found was due to a peculiarity of the metal silver. This metal is highly reflective 
for the invisible as it is for the visible rays up to about the fixed line S J, when its reflecting 
power falls off, with remarkable rapidity, and for the more refrangible rays of the solar 
spectrum is comparable with that of a vitreous substance rather than vrith that of a metal. 
Steel, gold, tin, &c. showed nothing of the kind, but copiously reflected the invisible rays. 
A few years ago, as Dr. Robinson was showing me some experiments with the 
induction coil, it seemed worth while to try whether the spark obtained when a Leyden 
jar has its coatings connected with the secondary terminals might not be sufficiently 
strong to exhibit by projection the long spectrum shown by electric light. On 
projecting a spectrum formed by a prism and lens of quartz on a piece of mnnium glass, 
the long spectrum was in fact exhibited. It was not, indeed, so bright as when formed 
by means of a powerful voltaic battery, but nevertheless was quite bright enough to 
work by. It was discontinuous, consisting of bright lines. On changing the metals 
between which the spark passed, we found that the lines were changed, which showed 
clearly that they were due to the particular metals. 
* Proceedings of the Eoyal Institution, vol. i. p. 264. f Philosophical Transactions for 1852, p. 537. 
X According to the notation employed in the Map published in the Philosophical Transactions for 1859, 
Plate XL VII. In this Plate the group S should have been represented as three lines, of which the middle 
(specially named S) divides the interval between the 1st and 3rd in the proportion of 3 to 2 neaily, the spaces 
between the lines being a little darkened by shading. 
