84 COSMOS. 



all magnetism to eL^ctrical currents wliicli lie in a plan-, a* 

 right angles to the axes of the magnet, advanced the in 

 genious hypothesis that terrestrial magnetism (the magnetic 

 charge of the Earth) was generated by electrical currents, 

 circulating round the planet from east to west ; and that the 

 horar}^ variations of the magnetic declination are on this ac- 

 count consequences of the fluctuations of heat, varying with 

 the position of the Sun, by whose action these currents are 

 excited. These views of Ampere have been confirmed by 

 Seebeck's thermo-magnetic experiments, in which differences 

 of temperature of the points of contact of a circle composed 

 of bismuth and copper, or other heterogeneous metals, affect 

 the magnetic needle. 



Another recent and brilliant discovery of Faraday's, the 

 notice of which has been of almost simultaneous occurrence 

 with the printing of these pages, throws an unexpected light 

 on the same important subject. While the earlier researches 

 of this great physicist showed that all gases are diamagnetic, 

 i. e., assume a direction from east to west, as bismuth and 

 phosphorus, but that this property is most feebly exhibited in 

 oxygen, it has been shown by his latest researches, which 

 were begun in 1847, that oxygen alone, of all gases, like iron, 

 assumes a position from north to south, and that oxygen gas 

 loses a portion of its paramagnetic force by expansion and by 

 elevation of the temperature. Since the diamagnetic activity 

 of the other constituents of the atmosphere, such as the nitro- 

 gen and carbonic acid, are not modified by expansion or by 

 an elevation of temperature, it only remains for us to consid- 

 er the oxygen, " which surrounds the whole Earth, as it were, 

 like a large sphere of sheet tin, and receives magnetism from 

 it." The half of this sphere which is turned toward the Sun 

 is less paramagnetic than the opposite half; and as the bound- 

 aries of these halves are constantly altered by their rotation 

 and revolution round the Sun, Faraday is inclined to refer a 

 portion of the variations of terrestrial magnetism on the 

 Earth's surface to these thermic relations. The assimilation 

 thus shown by experiment to exist between a single gas (oxy' 

 gen) and iron, is an important discovery of our ow,i age,=* 

 which derives additional value from the fact that oxygen 

 probably constitutes the half of all the ponderable mattera 



* Faraday upon atmospheric magnetism, in tlie Exper. Researchet 

 on Electricity, series xxv. and xxvi. (Philos. Transact, for 1851, part i.) 

 ^ 2774, 2780, 2881, 2892, 2968, and for the history of the iuvpstigationj 

 4 2847. 



