ATMOSPHERIC MAGNETISM — NITROGEN. 
45 
able to take cogriizance of by our observing instruments, which issue from the earth, 
proceed, at least at all parts of the globe where there is a sensible dip; but, as it were, 
between the earth and this space, there is interposed the atmosphere, which, however 
considerable we may estimate it in height, is so small when compared to the size of 
the earth, or to the extent of space beyond it into which the lines of force pass, that 
the idea of its being a changeable, active something interposed between two systems 
far more extensive and steady in their nature and condition, will not lead to any 
serious error. It is at the bottom of this atmosphere that we live and make all our 
inquiries, whether by observation or experiment. 
2853. The atmosphere consists, as far as we are concerned at present, of four volumes 
of nitrogen and one volume of oxygen, or by weight, of three and a half parts of the 
former and one part of the latter. These substances are nearly uniformly mixed 
throughout, so that, as regards their manner of investing the earth, they act magneti- 
cally as a single medium; nor does there seem to be any tendency in the terrestrial 
magnetic forces to cause their separation*, though they dilfer very strikingly in their 
constitution as regards this power. 
2854. The nitrogen of the air does not appear to be either paramagnetic or dia- 
magnetic ; if removed from zero, in either of these respects, it is only to a very small 
extent (2783. 2784.). Whether dense or rare, it has apparently the same relation to 
and equality with space, as far as the present means of observation have proceeded. 
As respects the other element of change, namely, temperature, I concluded, from 
former imperfect experiments'!', that nitrogen became more diamagnetic when heated 
than before ; but as it was then mixed with the oxygen of the air, and the results were 
mingled together, I have, for the purposes of the present research, repeated the expe- 
riments far more carefully. 
2855. A small helix of platinum wire, fixed at the end of thicker copper wires, could 
be placed in any position beneath the poles of the great electro-magnet, and being 
ignited by a voltaic battery, served to raise the temperature of the gas around it. 
The magnetic poles were raised, were terminated by hemispheres of soft iron 076 of 
an inch in diameter and 0 2 of an inch apart, and were covered by a glass shade, 
resting upon a thick flat bed of vulcanized caoutchouc. A tube passed through the 
bed, rising up to the top of the shade, by which any required gas could be introduced. 
A very thin plate of mica, about 3 inches square, was covered with an attenuated 
coat of wax on the upper side, and fixed horizontally over the magnetic poles within 
the shade. The small platinum helix was so placed as to be beneath the space, be- 
tween the poles, and a little on one side of the axial line, so that a current of hot air 
rising upwards from it, could pass to the mica plate, and by melting the wax show 
where it came against the mica. 
2856. All acted exceedingly well, air being in the glass shade. When there was no 
magnetic power on, the hot air from the ignited helix rose perpendicularly, and 
* Philosophical Magazine, 1847, vol. xxxi. p. 416. t Ibid. p. 418. 
