Metallic Vapours in an Exhausted Space. 109 
across the lamp is, however, the same in both cases. It is 
not possible to account for the existence of this lower limit 
by the low temperature of the arc at the low current, since 
we saw that cooling the arc increases the stability. 
The explanation is rather to be looked for in the properties 
of the cathode. The supply of ions coming from the cathode, 
one must assume that the ionization process to be stable 
requires at each voltage a certain current, and dies out when 
the current is lowered below a certain limit. The behaviour 
of the tube can be considered as being in favour of that 
explanation. When the current in the arc is reduced below 
that critical value, the lamp suddenly goes out after a certain 
interval of time, no gradual change in the potential drop 
across the lamp being observed, This phenomenon points to 
the existence of a cause which ceases to act in a discontinuous 
way. Still, this explanation is probably only a partial one, 
since it does not explain the influence of the magnitude ot 
the diameter of the tube on this lower Jimit. This influence, 
although not very pronounced, certainly exists, the critical 
value of current being lower the smaller the diameter of the 
tube. 
The phenomena described above, referring to the con- 
ductivity of the arc, being in accordance with the ionic 
conception, the question remains to be answered whether the 
flow of ions in the arc takes place in both directions or only 
in one; and if the latter be true, what that direction is. The 
total difference in appearance of the two electrodes speaks 
against the existence of a symmetrical process in the are 
The are around the anode is quiet and steady, while on the. 
surface of the cathode there is a small bright spot which is 
constantly wandering about upon that surface. The funda- 
mental importance of the cathode in the process of starting 
of the are, described in section 1, leads one naturally to the 
conclusion that the production of ions takes place at the cathode 
surface. This is further strengthened by the following facts. 
The properties of the arc are independent of the nature of 
the anode, whether this be made of iron, graphite, silicon, or 
of mercury itself. In the case of iron and graphite a slight 
disintegration of the anode takes place; but this is exceedingly 
small, so that it takes days to notice any black deposit on the 
glass; and is obviously a secondary phenomenon, due to vola- 
tilization of the material of the anode in vacuum in conse- 
quence of the high temperature of the anode. Replacing 
these substances by a hard substance, such as titanium carbide, 
does away with the disintegration altogether. The material 
© 
of the anode is therefore irrelevant, and its choice is deter- 
