88 
POPULAR SCIENCE REVIEW. 
to the direction of the current and that of the previous longitudinal 
magnetization. 
The Photophone has "been reproduced in an exceedingly simple form by Mr.* 
Shelf ord Bidwell. The transmitter is a disc of thin microscopic glass silvered 
on its anterior surface, and placed in front of a tube by which the voice is 
conveyed to it so as to excite vibration. The lime, or electric light, is reflected 
from this mirror through a convex lens, so as to render the rays parallel ; 
these being received on a second lens at some distance, and again concentrated 
on a selenium receiver. This is the most important part of the apparatus. 
It consists of a slip of mica, 2j inches long and f inch broad, round which is 
wound No. 40 copper wire in the form of a flat screw, with a pitch of X V of 
an inch. The ends are fixed through holes drilled in the mica. A second 
wire is then wound beside, but not touching, the first. A few grains of 
vitreous selenium are melted and dropped on the surface of the mica, being 
afterwards evenly spread by means of another slip of mica. The temperature 
should be just above the fusing-point of selenium. It is then allowed to cool. 
It is next annealed for several hours and allowed to cool very slowly. The 
terminals of this cell are joined up with a battery of eleven Leclanche ele- 
ments and a pair of Bell telephones wound with finer wire than usual, in 
larger quantity than that required for ordinary telephonic communication. 
The voice is very fairly conveyed across a space of ten feet and into a neigh- 
bouring room by this simple form of apparatus. 
The Thermic and Optical behaviour of Gases, under the influence of the 
electric discharge, has been further investigated by Wiedemann, and the 
results have been given at some length in the Annalen. He had before 
shown that when a mixture of two gases, of which one is a metallic vapour 
and the other nitrogen or hydrogen, is exposed to the electric discharge, the 
lines of the metallic vapour are seen in the spectrum, while those of the 
other gas remain invisible ; so that the propagation of electricity is entirely 
due to the metallic vapour, — the discharge being entirely discontinuous. He 
had also shown that the temperature of the gas illuminating a Geissler’s tube 
may be below 100°. It is thus proved that the usual theory referring the 
emission of light by gases to an increase of temperature up to the point of 
incandescence is no longer tenable, and requires fresh experimental examina- 
tion. In this communication, he has thoroughly examined the thermic 
relations of the discharge of the induction machine under different conditions, 
and has discovered a peculiar behaviour of positive and negative electricity. 
Experiments follow to determine numerically the conditions under which the 
transformation of the band-spectrum into the line-spectrum occurs in 
hydrogen. The apparatus employed is described at length. It appears 
among other results that as pressure diminishes, the quantities of heat evolved 
diminish to a minimum and then rise again. With hydrogen the heating 
is generally less than with air. The number of discharges increases as the 
pressure diminishes, and then decreases again. The heating at the positive 
electrode diminishes continually and rapidly as the pressure decreases; 
that at the negative electrode at first decreases, and then increases 
rapidly. 
Electric Conductivity of Carbon. — Dr. Werner Siemens has lately de- 
scribed to the Berlin Academy a new series of experiments on the electric 
