450 
POPLLAR SCIE^’CE REVIEW. 
part, D, of the camera screws when required into it. A section of the top 
of this tripod is shown^ with the manner of cramping the hall. C repre- 
sents a sensitised glass, which can he retained in place hv a piece of flat 
spring wire partly surrounding it. 
PHYSICS. 
A^i Experiment in Diamagnetism. — Mr. W. E. Kernan has given an accoimt 
of a new experiment which suggested itself to him some time since, and 
whose object is to show the diamagnetic phenomenon to a large number of 
people so as to he mauifest to all at once. For this pm’pose a disc of 
copper is made to revolve between the poles of the magnet by a pulley and 
band from a steady source of motion — for instance, an electro-motive engine. 
The diamagnetic effect maybe made manifest in three ways : — 1st. The band 
is let to run somewhat loosely. As long as the cmTent is not turned into 
the magnet coils, the disc moves Tvdth great velocity. The tuming-in of 
the current instantly stops the disc, forcing the band to slip. Thus motion 
and stoppage of the disc can be exhibited alternating, until the effect is fully 
appreciated. 2nd. The band is drawn tight. The turning-in of the current 
does not now stop the disc, but the great diminution of velocity in the 
prime mover shows the effort of the magnet to stop the disc. Here, too, 
alternation may be used as before. 3rd. The axis of the disc carries a 
small wheel with many teeth ; a card held against the teeth of the revolving 
wheel gives a flne clear high note (Savart’s wheel). As long as the magnet 
is not in action the prime mover keeps up a flxed velocity, and consequently 
the wheel gives the same note. On turning-in the current, if the band slips 
the sound stops ] if it does not slip the note is changed, from the decreased 
velocity of the prime mover. The alternations of sound and silence (or 
change of note) are very striking. 
Cause of the igniting Dower of Spongy Platinum. — This'^peculiar property 
of spongy platinum is supposed to be due solely to the influence of the 
metal on the occluded hydrogen. Professor Graham, however, offers the 
following representation of the phenomenon, with an apology for the pecu- 
liarly speculative character of the explanation.” The gaseous molecule of 
hydrogen being assumed to be an association of two atoms, a hydride of 
hydrogen, it would follow that it is the attraction of platinum for the 
negative or chlorylous ” atom of the hydrogen molecule which attaches 
the latter to the metal. The tendency, imperfectly satisfied, is to the for- 
mation of a hydride of platinum. The hydrogen molecule is accordingly 
polarised, oi'iente, with its positive or basylous ” side turned outwards, and 
having its affinity for oxygen greatly enlivened. It is true that the two 
atoms of a molecule of hydrogen are considered to be inseparable, but this 
may not be inconsistent with the replacement of such hydrogen atoms as 
are withdrawn, on combining with oxygen, by other hydrogen atoms from 
the adjoining molecules. It is only necessary to suppose that a pair of con- 
tiguous hydrogen molecules act together in a single molecule of the external 
