NON-EXPANSION OF GASEOUS BODIES BY MAGNETISM. 
11 
Fig. 1. 
and to the third was attached a gauge (2732.) 
for the purpose of indicating and measuring 
any change of volume which might occur. 
The edges of the central copper plate and 
the heads of the countersunk screws, were 
touched with white hard varnish, and the 
chamber thus rendered perfectly tight, under 
every condition to which it had to be sub- 
jected (fig. 1). 
2732. The gauges were formed of small capillary tubes from 1*5 to 3 inches in 
length, the diameter in the middle of their length being less than one-half of that at 
either termination. These were 
fixed at one endinto a small socket, 
which screwed on to the third, 
or gauge- cock mentioned above 
(2731.). A minute portion of spirit, coloured by cochineal, being put into the external 
end of this gauge, from a slip of wood or glass, immediately advanced to the middle 
or narrowest part, forming, as it always should do, a single portion of fluid. By 
shutting the cock, this little cylinder could be easily retained in its place undisturbed 
during the filling of the air-chamber with gas, and the adjustment of its pressure to 
equality with that of the atmosphere. On shutting the other cocks and opening the 
gauge-cock, the gauge was then ready to show any change of volume which the 
supervention of the magnetic force might cause ; but to give it the highest degree of 
sensibility, it was necessary previously to make the liquid cylinder travel right and 
left of its place of rest, that the tube might be moistened on each side of the indi- 
cating fluid ; an effect easily obtained by inclining the chamber to and fro, the gravity 
of the fluid making it pass one way or the other. But this and many other necessary 
precautions as to position, temperature, &c., can only be learned from experience. 
2733. When this box was in its place, it stood between the poles of the great elec- 
tro-magnet, with the plane of the gas-chamber in the equatorial position ; then square 
blocks of soft iron, resting on the magnet poles, were made to abut and bear against 
the sides of the box, so that in fact the inner faces of the air-chamber were the vir- 
tual magnetic poles, and being 3 inches square were only ^th of an inch apart. 
Hence, whatever air or gas was within the chamber, would be subjected to a very 
powerful magnetic action, and could have very small changes in its bulk measured ; 
but it is perhaps necessary to observe, that it would be contained in a field having 
everywhere lines of equal magnetic power (2463. 2465.). 
2734. Air was introduced into the. box, and when all was properly arranged, the 
place of the indicating fluid was observed by a microscope. Then the magnet was 
rendered powerfully active, and there appeared a very slight motion of the fluid, as if 
the air were a little expanded ; on taking off the magnetic force the fluid returned to 
c 2 
