ELECTRO-MAGNETISM. 



magnetism " was not permanently re- 

 tained. In subsequent experiments, 

 by an exposure of needles under the 

 same circumstances, for six hours, a 

 very sensible degree of magnetism was 

 acquired, and remained permanent. 

 The rays transmitted through the blue 

 glass employed in this experiment black- 

 ened muriate of silver as powerfully as 

 those transmitted through uncoloured 

 glass ; thus proving that it was freely 

 permeable to the chemical rays of the 

 solar spectrum. Green glass was also 

 tried ; and the rays which had pene- 

 trated it were likewise found to commu- 

 nicate magnetism. The white light of 

 the sun produced no magnetic effect 

 whatever on needles exposed to its in- 

 fluence. 



(331.) Although the experiments, of 

 which we have just stated the results, 

 are minutely detailed in the paper above 

 referred to in the Philosophical Trans- 

 actions, yet, in many trials made by other 

 experimentalists, no success has been 

 obtained. The experiments on the os- 

 cillations of the needle were repeated by 

 Messrs. P. Riess and L. Moser without 

 any satisfactory result*. We may, 

 therefore, consider the subject as still 

 open to inquiry, and as requiring a more 

 minute and scrupulous investigation. 



CHAPTER XV. 

 Origin of Terrestrial Magnetism. 



(332.) SEVERAL causes have been as- 

 signed for the magnetic influence which 

 the globe of the earth is found to exercise, 

 not only over the magnetic needle, but 

 also, as we have seen, over currents of 

 voltaic electricity transmitted through 

 conductors. (See Chapter IX. 128, 

 et seq.~) Among the various substances 

 which occupy the interior of the globe 

 it is extremely probable that chemical 

 actions of different kinds are incessantly 

 occurring. These actions will, for the 

 most part, however, be very slow, and 

 will continue with a certain degree of 

 uniformity for very extended periods of 

 time. They will occur more especially 

 in the superficial strata of the earth, 

 where the combined agencies of water, 

 of atmospheric air, and of heat are in 

 constant operation. The influence of 

 the solar rays on a surface of such vast 

 extent must be very considerable : and 

 excepting in the vicinity of the poles, 

 every portion of that surface is exposed 

 in succession to their action, and acquires 

 * Aanales de Cliiuiie, xlii. 3U4, 



during that exposure a certain degree of 

 heat ; which heat' is again lost by noc- 

 turnal radiation. Although the effect 

 of these alternate changes of tempera- 

 ture may extend only to a small depth 

 below the surface, yet considering their 

 immense superficial extent, they may be 

 sufficient to give rise to thermo-electric 

 currents of considerable power. It has 

 been conjectured, also, that these effects 

 may be combined with an influence of 

 another kind, more directly derived from 

 the rotation of the earth on its axis, on 

 the principle that all bodies have been 

 found to exhibit magnetic polarity by 

 rotation. t 



(333.) That electric currents do really 

 circulate in different parts of the solid 

 strata of the earth, is not merely matter 

 of conjecture : the existence of such 

 currents has been lately proved, in the 

 most satisfactory manner, by Mr. Robert 

 Fox, in a paper " On the Electro-mag- 

 netic properties of metalliferous veins," 

 which has been recently published in the 

 Philosophical Transactions*. Having 

 been led from theory to entertain the 

 belief that a connexion exists between 

 electric action in the interior of the earth, 

 and the arrangement of metalliferous 

 veins, he was anxious to verify this 

 opinion by experiment. The first trials 

 he made with this view were unsuc- 

 cessful : but by persevering in his at- 

 tempts, he soon obtained decisive evi- 

 dence of considerable electrical action 

 in the mine of Huel Jewel, in Cornwall. 

 His apparatus consisted of small plates 

 of sheet copper, which were fixed in 

 contact with ore in the viens by copper 

 nails, or else wedged closely against them 

 by wooden props stretched across the 

 galleries of the mine. Between two of 

 these plates, at different stations, a 

 communication was made by means of 

 copper wire, one twentieth of an inch in 

 diameter, which included a galvanometer 

 in its circuit. In some instances three 

 hundred fathoms of copper wire were 

 employed. 



(334.) The intensity of the electric 

 currents was found to differ consider ably 

 in different places. It was generally 

 greater in proportion to the greater 

 abundance of copper ore in the veins, 

 and in some degree also to the depth of 

 the station. This curious fact may 

 possibly afford the miner some useful 

 indications as to the relative quantities of 

 ore which the vein contains, and also as 



* Cumming's Manual, &c. p. 231, 

 i For 1830, page 3D3. 



