320 
POPULAR SCIENCE REVIEW. 
teresting manner. He says : 1 A medal and plate formed of the new metal, 
palladium, will be interesting to scientific men. The discovery of this metal 
by Professor Graham a few years ago finally settled the long- disputed point 
as to whether or not the gas hydrogen was a metal. He proved that palla- 
dium was simply hydrogen condensed. This may be easily exemplified by 
placing a piece of the metal under the receiver ot an air-pump and exhaust- 
ing the air. The a solid metal at once flies off as a gas, and on readmitting 
the air it shrinks again into its former size. The little medal shown con- 
tains 100 times its volume of the gas.’ We will only add, in transferring 
this gem to our columns, that we hope it is not a fair sample of the teaching 
at the Leeds Mechanics’ Institute — the worthy object for whose benefit the 
Yorkshire Exhibition is being held.” 
A Revolving Polariscope was exhibited by Mr. Spottiswoode, F.R.S., at 
the Physical Society, on the evening of May 22, 1875. The following is 
an account of the apparatus : — A luminous beam passes from a small circular 
hole in a diaphragm through a polariscope, the analyser of which is a double- 
image prism, the size of the hole being so arranged that the two luminous 
discs shall be clear of each other. If the prism be made to revolve rapidly, 
one of the discs revolves round the other, and is merged into a ring of light 
which is interrupted at opposite sides by a dark shaded band, the position 
of which depends upon that of the original plane of polarisation. The 
discs may be coloured by inserting a selenite plate, and the rapid revolution 
of the analyser then gives alternating segments of complementary colours ; 
or, if a quartz plate be used, the rotating disc passes successively, twice in a 
revolution, through all the colours of the spectrum, and when the revolution 
is rapid merges into a prismatic ring. The effect of the interposition of a 
^-undulation plate, which converts plane into circularly polarised light, was 
then shown, and Mr. Spottiswoode also interposed a concave plate of quartz 
and exhibited the effect of rotation on the characteristic rings of quartz. 
The Effect of having Glass Rods Cleaned. — Mr. Tomlinson, F.R.S., whose 
views on this subject are generally known to our readers, has recently ( {l Phil. 
Mag.,” April, 1875), been opposing the opinions of M. Gernez. Mr. Tomlin- 
son maintains that the inactivity of a glass rod or other solid body intro- 
duced into a gaseous solution depends on its being chemically clean. A 
cage of fine wire gauze was submerged in soda-water, but there was no 
escape of gas so long as it was chemically clean. When taken out, rolled 
between the slightly greased hands, and again lowered into the soda-water, 
the gas escaped from its side in bubbles with an audible noise. Supposing a 
liquid, at or near its boiling point, to be constituted like soda-water, Mr. 
Tomlinson refuses to admit that a solid, such as a glass rod, introduced into 
a boiling liquid (water for example), becomes covered with bubbles of steam 
by virtue of the air carried down by the rod. If the rod be unclean (that 
is, contaminated with a greasy film), the steam-bubbles cover it precisely 
after the manner of gas-bubbles, because there is adhesion between the 
steam-bubbles and the film, and not between the water and the film, and 
hence there is a separation. A chemically clean glass rod has no such action, 
not because the act of cleaning it deprives it of its adhering air, but because 
there is perfect adhesion between a vaporous supersaturated solution and a 
chemically clean surface. 
