ATMOSPHERIC MAGNETISM — OXYGEN. 
47 
indifferent body ; it does not appear to be either paramagnetic or diamagnetic ; neither 
does it present any difference in its relation, whether it be dense or rare, or at high 
or low temperatures. 1 formerly found that the diamagnetic metals, when heated, 
did not seem to change in their relation to the magnet (2397.)} and this now appears 
to be the case with such neutral or diamagnetic bodies as nitrogen and carbonic acid 
gases. 
2861 . The oxygen of the air differs in a most extraordinary degree from the nitro- 
gen. It is highly paramagnetic, being, bulk for bulk, equivalent to a solution of proto- 
sulphate of iron, containing, of the crystallized salt, seventeen times the weight of the 
oxygen ( 2794 .). It becomes less paramagnetic, volume for volume ( 2780 .), as it is 
rarefied, and apparently in the simple proportion of its rarefaction, the temperature 
remaining the same. When its temperature is raised, the expansion consequent 
thereon being permitted^, it loses very greatly of its paramagnetic force ; and there is 
sufficient reason, from a former result with air-f-, to conclude, that when its temperature 
is lowered its paramagnetic condition is exalted. How much its paramagnetic in- 
tensity might be increased by lowering it to the temperature of freezing mercury, as 
at the north or south poles of the earth, we cannot at present tell. Though a gas, it 
is apparently like the solid metals, iron, nickel or cobalt, when they are within the 
range of temperature which affects their magnetic forces ; and it may, perhaps, like 
them, rise by cooling to a very high state. 
2862 . These relations it preserves when mingled with nitrogen in the air, as long 
as its physical and chemical conditions remain unchanged ; but it is not irrele- 
vant to remark, that every operation by which this active part of the atmosphere 
changes in its nature and passes into combinations, takes away its paramagnetic 
powers, whether the result be solid, liquid or gaseous. 
2863 . Hence the atmosphere is, in common phrase, a highly magnetic medium. 
The air that stands upon every square foot of surface on the earth, is equivalent, in 
magnetic force, to 8160 lbs. of crystallized protosulphate of iron ( 2794 . 2861 .). This 
medium is, by every change in its density, whether of the kind indicated by the baro- 
meter, or caused by the presence or absence of the sun, changed in its magnetic rela- 
tions. Further, every variation of temperature produces apparently its own change of 
force, in addition to that caused by the mere expansion or contraction in volume, and 
none of these alterations can happen without affecting the magnetic force emanating 
from the earth, and causing variations, both in its intensity and direction, at the earth’s 
surface. Whether these changes are in the right direction and sufficient in quantity 
to supply a cause for the variations of the terrestrial magnetic power, is the point 
now to be considered, for the illustration of which I will endeavour to construct a 
type case, and then apply it, as well as I can, to the natural facts. 
2864 . Let us assume the existence of two globes of air distinct from the surround- 
ing atmosphere, by a difference of temperature or by a difference of density : the 
* Philosophical Magazine, 1847, vol. xxxi. p. 417. f Ibid. p. 406. 
