EEV. T. E. EOBINSON OX SPECTEA OE ELECTEIC LIGHT. 
985 
trie discharge so iramensely superior to all others 1 and are there no means of estimating 
its real amount 1 Two of the facts which I have noted respecting these discharges may 
at least dhect attention to this subject. In general the spectra of the simple spark of 
an induction machine are much fainter than when a jar, however small, is connected 
with it; and those with a small jar than those with a large. Thus, with silver elec- 
trodes in air, the spark gave twenty- three lines, of which § and d were “ very bright a 
jar of 0’5 foot coating gave also twenty-three, but differently placed, more at the red, 
fewer at the 'violet, but seven of them ^^s; the normal one of 1‘25 foot gave thirty-one, 
with nine ^^s; and two large of 8-5 feet gave thirty-six, with ten *s and many others 
“ very bright.” In these cases, were one to judge from first appearances, the spark heats 
air much more than the jar-discharge, for it has much greater power to burn anything 
which it encounters ; but its section is larger, because much of it is conducted by the 
air which surrounds it ; and besides, from the diminished resistance, the amount of heat 
produced may be less, as well as less concentrated. We cannot say it must be ; we know 
too little of the nature of induction discharge to estimate the effect of changing resist- 
ance, for if it be increased it is possible that part of the electricity may be discharged 
through the coil itself. It must also be kept in mind, that while the jar-discharge is 
almost instantaneous, the other, at least in part, has a sensible duration. It could, how- 
ever, be easily decided by experiment whether more heat is evolved in the C.P. or R. 
discharge. 
Secondly. My induction machine, as I have already stated, can be used collaterally ; 
in this case the quantity is double, and should have a fourfold heating power. In fact, 
its discharge (or rather the air which that discharge heats) fuses a piece of platinum 
•wire, which the consecutive arrangement only reddens. Now, if the lines were produced 
by mere heat, the spectrum of the former discharge should be far the brightest : it is 
not so in the red and green ; in the violet there is a difference, but I think an unprac- 
tised observer w'ould scarcely notice it. I however saw the lines beyond H more easily 
and further. Unless the temperature is sufficiently raised by a weaker discharge than 
either of these to bring out all the lines (which seems inconsistent with the effects 
obtained by enlarging the jar), we might expect here, from the heat theory, a greater 
change. It is, I think, worth pushing the trial further, and I intend to repeat the 
experiment -with an induction machine of much greater quantity, and at the same time 
to ascertain if intensity also have any influence. 
These observations, on the whole, incline me to refer the origin of the lines to some 
yet undiscovered relation between matter in general and the transfer of electric action. 
According to the special properties of the molecules which are present, the brightness 
of these lines will be modified through a range from great intensity down to a faintness 
which may elude our most powerful means of observation. If several sorts of molecules 
be simultaneously present, there may be expected interferences which -will produce alter- 
nations of brilliancy or obscurity to any extent ; and if any of these be chemically united, 
analogy leads us to expect that such compound molecules will act with an influence of 
