SCIENTIFIC SUMMARY. 
373 
PHYSICS. 
A Vacuum Tube of variable Resistance was exhibited before the Physical 
Society by Dr. Stone, at the last meeting for the season, on June 26th. 
It consisted of a barometer tube 32 inches long, terminating above in a 
short vacuum chamber arranged transversely, and closed at either end 
by adjustible india-rubber stoppers, through which platinum terminals 
passed. Above this the vertical tube continues to a glass stopcock, by means 
of which small quantities of air can be introduced. The foot of the tube is 
attached to a flexible india-rubber pipe, with a cistern similar to that of 
Frankland’s gas apparatus. The cistern full of mercury is counterbalanced, 
and can be raised or lowered through the whole 32 inches. A Torri- 
cellian vacuum can thus be made in the upper chamber, or one of less 
perfectness. On passing the induction spark between the terminals in the 
former case, all the discharge is carried off, none appearing at the dis- 
charger. By gradually raising and lowering the cistern, after admitting 
a little air by the stopcock, the resistance of the partial vacuum can be 
altered within wide limits. A point can be found where the spark of 
breaking contact is shimted through the vacuum tube, while the weaker 
discharge of making contact is stopped. The induction-current is thus 
obtained in a single direction, a matter of some importance in physiological 
experiments. 
Pneumatic Clocks have been successfully established in Paris, both for 
public and private purposes. The subscribers are supplied with dia’s on 
this system for the sum of a halfpenny per day. Air is compressed to five 
atmospheres in a reservoir at the central station. A distributing-clock 
places this in communication with distributing-pipes for twenty seconds 
every minute, the used air being again employed to wind automatically the 
original train. The distributing-tubes are of iron, 27 millim. in bore, 
carried underground. These, by leaden or indiarubber connexions, com- 
municate with the affiliated dials. The dial has a small caoutchouc bellows, 
similar to that of the pneumatic telegraph, acting on a lever, which takes, 
by means of a ratchet, into a wheel of 60 teeth, carrying the minute-hand. 
The hour-hand is moved by the usual motion-work. Striking-clocks are 
also fitted up on the same system for the small increase in price of a single 
centime, namely, six instead of five per diem. It appears that the whole 
expense is from fifteen shillings to a pound per annum. 
The Magneto-Optic properties of Gases are being investigated by M. 
Henri Becquerel. He has recently examined oxygen, hydrogen, car- 
bonic dioxyde, nitrous oxyde, and olefiant gas. Except in the case of 
oxygen, the magnetic rotation of the plane of polarization due to a field 
of given intensity varies inversely as the square of the wave-length of the 
ray, as is the case in solids and liquids. This implies that violet rays are 
more rotated than red, or that there is a positive dispersion. In the case of 
oxygen, it is found that the red rays are rotated more than the green. This 
is the more remarkable, as oxygen gives a positive rotation as if it were a 
diamagnetic body. He remarks that oxygen behaves as if it were a 
mixture of a magnetic and a diamagnetic b:>dy, the magnetic having small 
negative rotation and a great negative dispersion, the diamagnetic having 
great rotation and small positive dispersion. 
NEW SERIES, YOL. IY. NO. XVI. C C 
