NON-EXPANSION OF GASEOUS BODIES BY xMAGNETISM. 
9 
Forty-six feet from it was placed the great horseshoe magnet (2247.), ready to be ex- 
cited by twenty pairs of Grove’s plates ; the poles were in a line, so that the ray from 
the lamp passed for 4 inches close to the surface of the first pole, then through 
6 inches of air, and then, for 4 inches, close to the surface of the second pole. A very 
fine refracting telescope, belonging to Sir James South, having an aperture of 3 
inches and 46 inches focal length, received the ray. The telescope was furnished 
with a perfect micrometer, so that the smallest change in the place of the luminous 
image could be observed on the threads. The axis of the telescope was just above 
the level of the magnetic poles. Not the smallest change in either the character or 
place of the luminous image could be observed, either on the making or the breaking 
of the contact between the voltaic battery and the magnetic wire. 
2724. As the chief part of the light which came to the telescope consisted of rays 
which passed at some distance above the magnetic poles, these were cut olf by a screen, 
which rising only one-eighth of an inch above the level of the poles, allowed no ray 
to pass that was not within that distance. The intensity of the light was of course 
diminished, and the image was distorted by inflection ; still its place was well marked 
by the micrometer. Not the slightest change in that or any other character occurred 
in the supervention or the withdrawal of the magnetic force. 
2725. The terminals of the magnetic poles were then varied, so that the ray some- 
times passed parallel and close to a long right-angled edge, or parallel to and between 
two right-angled edges, a little above or below them, or over the line joining two 
hemispherical poles, placed close together (and also in many other ways), but in no 
case did the magnetic action produce any effect upon the course of the ray. 
2726. In another form of the experiment the telescope was dismissed, and a simple 
card, with a pin-hole of an inch in diameter, employed in its place. 
The image of the star of light could be seen through the pin-hole in the dark room, 
and yet every ray tending to its formation passed within 5^th of an inch of the sur- 
face of the magnetic pole ; still no effect due to the magnetic force could be ob- 
served. 
2727 . By another arrangement of the polar terminations, analogous to one I had 
formerly employed when experimenting on the diamagnetic relations of the gases*, I 
was able to surround them with other gaseous substances than air, and subject the ray 
for 2 inches of its course to these gases whilst under the influence of the magnet. 
Though the glass of the enclosing vessel disturbed the image of the object, /. e. the 
point of light, yet it was easy to perceive that no additional effect occurred when the 
magnetism was superinduced. 
2728. Oxygen, nitrogen, hythogen and coal-gas were thus employed; but whether 
any one of these, or whether air itself was submitted to examination, when in contact 
with the active pole of a very powerful magnet, it did not appear to be either expanded 
or condensed to such a degree as to cause any sensible change in its refractive force. 
* Philosophical Magazine, 1847, vol.xxxi. pp. 414, 415. 
MDCCCLI. 
C 
