Metallic Vapours in an Exhausted Space. 117 
4 amperes the light of potassium becomes visible to the 
naked eye. ‘This influence of current-density has already 
been noticed by Arons. The 20 per cent. amalgam has the 
peculiarity that it remains solid while the are is running, so 
that a mercury are takes place between two solid electrodes, 
and the tube can be given any position whatever. The more 
alkali metal is present in the amalgam the lower is the 
current limit at which the light of the alkali metal appears, 
until, beginning with the amalgam containing about 40 per 
cent. potassium or sodium, the behaviour is nearly the same 
as that of the pure alkali metals. 
Magnesium and lithium amalgams have also been investi- 
gated. Even relatively dilute amalgams are solid, and the 
tension of mercury vapour, and correspondingly (see § 3) 
the amount of light given off, are greatly reduced by the 
presence of small quantities of those two metals. The 
tension of mercury vapour being greatly diminished by 
addition of those and similar metals, these amalgams may be 
useful in connexion with the investigation of the amount of 
matter carrying the current in the arc. 
o 3. 
Alternating Current Phenomena in the Mercury Are. 
From experiments of Zuchristan (1893), Wirz (1893), 
Sahulka (1894), Arons (1896), and others, it is known that 
the establishment of an alternating current arc of any con- 
siderable length between two metallic electrodes presents 
difficulties. This is at least true for moderate voltages, and 
Arons has shown that this general rule holds for an arc 
between two mercury surfaces in a vacuum. Jamin and 
Manceuvrier found (188%, Comptes Rendus, vol. xciv. p. 1615) 
that in using two electrodes, one of carbon and the other of 
metal, the alternating current passes in form of an arc 
between the two electrodes much more readily than between 
two metal electrodes, and that at the same time partial recti- 
fication of the alternating current takes place, the current in 
the direction carbon-metal predominating over that in the 
opposite direction. Jamin and Manceuvrier used as a 
metallic electrode lead, iron, or mercury, and found the 
rectification best in the case of the last-named material. 
Blondel, Comptes Rendus, vol. exxvili. p. 727 (1899), in- 
vestigated the arc between one carbon and one metal elec- 
trode by means of his oscillograph, and showed that under 
certain conditions one half-wave of the alternating current 
is entirely suppressed. Duddell and Marchant performed 
similar experiments. 
