96 Dr. E. Weintraub on the Ae in 
A certain advance is represented by the patent of Rapieff *, 
who used a closed vessel with two mercury electrodes, and 
started the are by bringing the electrodes into contact and 
separating them. He used a cooling chamber to condense 
the vaporized mercury, and speaks of the advisability of 
exhausting the tube in which the arc is to play. 
The study of the mercury arc in an evacuated space 
begins, however, with Arons, who published his observations 
in Annalen der Physik, vol. lvii. p. 73 (1896), also in Verh, 
Phys. Ges. Berlin, p. 55 (1892), and Zeitschr. fiir Beleuchtungs- 
wesen, Aug. 15, 1895. From these articles we may date 
the second period in the development of the mercury arc. 
Arons described the general properties of the mercury arc, 
of the electrodes, and even constructed a small mercury- 
lamp which can be used as a source of light, especially for 
laboratory experiments. Arons also investigated, to a certain 
extent, the behaviour of amalgams in the tube. The work 
of Arons will be mentioned many times in the course of 
this article. 
Gumlich published a few observations on the use of cad-— 
mium amalgam as an electrode in Wied. Ann. vol. Ix1. p. 401, 
(1897). Further work on the mercury-lamp, especially the 
method of starting the lamp by an inductive high-voltage 
shock, has been done by Peter Cooper Hewitt. 
This publication contains some results of the work carried 
ont in the Research Laboratory of the General Electric 
Company. Only those results are mentioned which are of 
interest from a theoretical point of view, while the practical 
side of the question is not considered. 
Si 
Starting of an Arc in Metallic Vapours. Properties of 
the Cathode. 
Metallic vapours, such as vapours of mercury, alkali metals, 
and some of the heavy metals, which have been investigated, 
have, even when overheated to a considerable degree, only a 
very slight conductivity, which in the case of mercury, accord- 
ing to J. J. Thomson (Phil. Mag. [5] vol. xxix. pp. 358 & 441, 
1890; see also Strutt, Phil. Mag. [6] vol. iv. p. 596, 1902), is 
of the same magnitude as that of air. The starting of an arc 
between two mercury electrodes in a well-exhausted tube, by 
means of moderate voltage, presents, therefore, difficulties 
which have been overcome by previous investigators in two 
different ways, both of which were used by Arons. 
# Eng. Pat. 211 (1879). 
Vat 
