NON-EXPANSION OF GASEOUS BODIES BY MAGNETISM. 
15 
coiled round it, and arranged (when I used it) in one continuous length. The 
moveable terminal pieces for the poles are massive in proportion to the magnet. 
Eighty pairs of Grove’s plates were used to excite this magnet, and as it was found, 
by preliminary trials, that these were most powerful when arranged as four twenties, 
with their similar ends connected, they were so used, constituting a battery of twenty 
pairs of plates, in which each platinum plate was 4x9 inches in the immersed part, 
and therefore presented 72 square inches of surface towards the active zinc. 
2748. On repeating the former experiments (2743.) the effect of pressure was again 
evident, and it was manifest that the magnet itself, though 5 inches in thickness, was 
a little bent by the mutual attraction of its poles. The effect was very small, because 
of the unity of the iron core passing through the centre of the experimental gas- 
chamber (2738.). It was the only effect indicated by the gauge, and was the same 
for all the gases ; and when allowance was made for it, nothing remained to indicate 
any change in volume of the gas itself. 
2749. Air, oxygen, nitrogen, carbonic acid and nitrous oxide were submitted, in 
varying order, to the effect of this very powerful magnet, but not the slightest trace 
of change of bulk in any of them appeared. 
2750. I think that the experiments are in every respect sufficient to decide that 
these gases, whether they are considered as magnetic or diamagnetic bodies, or whe- 
ther they include bodies of both classes (for oxygen is in striking contrast to the 
rest), are not affected in volume by the magnetic force, whether in fields of equal 
power (273705 or in places where the power is rapidly diminishing. I think this de- 
cision very important in relation to the true nature of the magnetic force, either as ex- 
isting in, or acting upon the particles of bodies ; and as in the magnetic field the force 
exhibits itself, not as a central but as an axial power, so the further distinction of the 
phenomena, into such as are related to the axial direction (2733.), and such as are 
related to or include the equatorial direction (2737.), is not unimportant, for they 
show that the particles do not tend to separate either parallel to the lines of magnetic 
power, or in a direction perpendicular to these lines. Without the experiments, the 
mind might have considered it very possible that one of these modes of expansion 
might have occurred and not the other. 
2751. No doubt it is true, that even yet changes in volume in these directions may 
occur, provided the change in one direction is expansion and in the other contraction, 
and that these are in amount equal to each other. It was partly in reference to such 
possible changes (which may be considered as molecular), that the experiments with 
the ray of light were made (2723. 2729.), and also that in these and other experiments 
instituted for the purpose, a polarized ray was employed as the examiner ; but the 
results were always negative, when by repetition and care sources of error w^ere 
removed. 
2752. The great differences in the degree of diamagnetic susceptibility and con- 
dition which the gases employed in the foregoing experiments possess or can assume, 
are such as to make one ready to suppose, that if they show no tendency in any case 
