Metallic Vapours in az Exhausted Space. 107 
lamp, the current, &.; and in what follows it will always be 
supposed that the vacuum is perfect, the starting. instan- 
taneous, and the appearance of the arc from the very beginning 
uniform and deprived of any visible striations. Under these 
conditions the resistance of the arc is lowest at the beginning, 
and increases rapidly during the first half minute, “when it 
reaches its normal value, the are havi ing fully developed. If, 
the tube being perfectly cold, the are is started and the instru- 
ments carefully watched, the voltage is seen to begin witha 
low value and to increase, while the ammeter shows first a 
high value which rapidly decreases. In one case, for instance, 
the indications at the beginning were 46 volts, 7 amperes 
(resistance 64 ohms); and after a few seconds, when the 
normal condition had been established, 62 volts, 5°5 amperes 
(resistance 11°3 ohms). The resistance, therefore, has nearly 
doubled ; and we have the, at first sight, paradoxical result 
that the conductivity of the mercury are is the larger the less 
vapour there is in it, the conductivity being highest in the 
first moment when the pressure of mercury is but ‘001 mm. 
The following experiment leads to the same conclusion. 
Experiment 10.—The lamp is provided with a jacket through 
which cold water can be kept in constant circulation. The 
temperature of the are is hereby lowered, the amount of 
mercury vapour in the tube diminished correspondingly , and 
this is accompanied by a diminution of the resistance of the 
arc. Ina lamp which consumed, when surreunded by air, 
42 volts, the potential drop across the arc went down to 34, 
when surrounded by constantly renewed water, the current 
being kept up to the same value (44 amperes) by i increasing 
the resistance in series with the lamp. The explanation of 
this phenomenon on the basis of the ionic theory is simple. 
In the are-stream we must distinguish between two different 
kinds of mercury vapour, the ionized conductive one and the 
ordinary mercury vapour produced by superfluous vapori- 
zation of mercury. This latter part of the mercury vapour 
hinders the motion of the ionized particles, and, by doing this, 
increases the resistance of the arc-path. 
The amount of light sent out by the arc, under the con- 
ditions of experiment 10, is perfectly insignificant ; whereby 
it is shown that the light-emission i is not intimately connected 
with the electric conductivity, but is a function of the tem- 
perature of the are. 
Thus the conductivity and the luminosity of the are do not 
go parallel, the maximum conductivity nearly coinciding with 
the minmum light-emission. With the increase of pressure 
of the mercury vapour the conductivity steadily diminishes, 
while the amount of light emitted increases up to a certain 
