618 PEOT'ESSOE STOKES ON THE LONG- SPECTEITM OE ELECTETC LIGHT. 
by sodium), but also by matter torn from the positive electrode. This is well seen with 
electrodes of aluminium, when the arc or a portion of it is frequently coloured green. 
This green light has a very sensible duration, and a distinctive prismatic composition, 
and is brighter towards the positive than towards the negative electrode, but is not con- 
fined to the immediate neighbourhood of that electrode (extending indeed sometimes 
over almost the whole length of the arc), in which respect, and in its duration, it differs 
from the light of the spark proper*. With aluminium opposed to another metal, as 
copper or iron, the green light is seen only when the aluminium is positive. Even with 
aluminium this light may generally be got rid of by making the electrodes approach ; 
and it is the arc in what may thus be deemed its normal state that was observed for the 
construction of the last line of fig. 1, though I have not at present noticed variations in 
the invisible corresponding with those in the visible spectrum of the arc discharge. 
On the Cause of the Advantage of Broad Electrodes ; and on the Heating of the 
Negative Electrode. 
Although the spark appears instantaneous when viewed in a moving min’or, it must 
yet occupy a certain time ; so that we have in fact a brief electric current, to which we 
may apply Ohm’s laws. The electromotive force is here the difference of tensions of 
the coatings of the jar. As to the resistance, the short metallic part of the cu'cuit may 
be neglected, and we need only attend to the place of the discharge. The resistance 
here may be divided into that due to the air and that due to the parts of the electrodes 
close to the points of discharge. That the latter is by no means insignificant, may be 
inferred from the enormous temperature to which minute portions of the electrodes are 
raised, as indicated by the excessively high refrangibility of the rays emitted by the 
metals, in the state doubtless of vapour. By the use of flat electrodes the stiiking- 
distance is materially diminished, without any change in the difference of tension of the 
coatings of the jar. Hence the electricity which it contains passes at a higher velocity, 
and therefore produces a more powerful effect on the metals. 
The injurious effect of the introduction of a small resistance was very strikingly 
shown with broad, slightly curved copper electrodes, three inches in diameter, by leading 
wires from a coating of the jar into a tumbler of water, and from thence to the corre- 
sponding electrode, when the spark became quite insignificant in comparison to what it 
had been. 
With one sharp and one flat electrode placed near together, bright sparks passed 
when the connexion was metallic, and the invisible spectrum then showed the copper 
lines, with one or two air-lines not conspicuous ; but when water was interposed the 
spark was greatly reduced, and the invisible spectrum showed the air-lines. In both 
cases the spark was followed by an arc discharge, as might be seen in a moving muTor ; 
and in the latter case the arc discharge was increased in consequence of the diminution 
* The outer part of the jar-spark between aluminium electrodes has the same green colour and prismatic 
composition, though in this case the green light is sensibly instantaneous. — July 1862. 
