MAGNETISM. 



of effect, resulting from the action of 

 a great number of particles, that the 

 controlling power of such a substance 

 becomes sensible. To this cause is ap- 

 parently owing the diminution of the 

 action of a revolving magnet on a disc 

 of copper, when the latter is intersected 

 by radiating grooves : since a portion of 

 the substance, every part of which is 

 concerned in the full development of 

 induced magnetism, is thereby removed. 

 In confirmation of this reasoning, Mr. 

 Harris found that the number of oscil- 

 lations made in vacuo in a given arc, 

 by a delicately suspended bar, sur- 

 rounded by several concentric copper 

 rings, did not materially differ from the 

 number performed, in similar circum- 

 stances, by the same bar, surrounded 

 by a solid mass of copper. 



(370.) The effects of various metals 

 in diminishing the oscillations of a mag- 

 netic needle, have been determined with 

 considerable accuracy by Professor See- 

 beck. An account of his experiments 

 is given in the Annales de Physique for 

 1826*. 



(371.) With regard to the theory of 

 magnetism, Poisson has remarked that 

 there is no necessity for supposing that 

 the phenomena of magnetism are pro- 

 duced in all bodies by a fluid, or fluids, 

 possessing everywhere the same inten- 

 sity of attractive or repulsive action, 

 and therefore requiring to be considered 

 as the same fluid in different substances. 

 No doubt can exist as to the identity of 

 the electric fluid; because we see it 

 passing from one conducting body into 

 another, and at the same time preserv- 

 ing all its properties, and exercising, in 

 like circumstances, the same attractions 

 and repulsions. But we have no evidence 

 of this kind in the case of the magnetic 

 fluids, because they are always confined 



* See Quarterly Journal of Science for January, 

 1828, p. 456. 



to the same particles ; and we cannot, 

 by mere reasoning, decide whether the 

 magnetism of two different bodies, such 

 as pure iron and pure nickel, should be 

 considered as the same imponderable 

 substance. It would assist us in the 

 determination of this question, were it 

 ascertained that similar and equal nee- 

 dles of iron and of nickel, when sub- 

 mitted to the magnetic influence of the 

 earth, or of any other magnet, would 

 make an equal number of oscillations. 

 An experiment was made by M. Gay-Lus- 

 sac with this view, from which it would 

 appear that the mutual action of the 

 magnetic fluids contained in steel and in 

 soft iron is decidedly greater than the 

 mutual action of the fluids belonging to 

 steel and to nickel. This experiment, 

 however, is not decisive of the question, 

 because we may still consider the mag- 

 netic elements of a body as not actually 

 in contact, but as constituting assem- 

 blages of particles, in which the two 

 fluids reside, and are separated by inter- 

 vals not greater than the dimensions of 

 those elements; and, as we formerly 

 observed ( 165), the ratio between the 

 sum of the volumes of all these elements, 

 and the total volume of the body, may 

 be different in different substances, and 

 at different temperatures, in the same 

 substance. This diversity may explain 

 the result of Gay-Lussac's experiment, 

 without the necessity of the supposition 

 of a difference in the intensity of the 

 magnetic power in substances differently 

 susceptible of magnetism*. But the 

 interest which attached to speculations 

 of this kind, are now, in some measure, 

 superseded by the new theory of magnet- 

 ism, proposed by Ampere: for an ac- 

 count of which, we must refer to the 

 forthcoming Treatise on Electro- Mag- 

 netism. 



* Memoires de 1'Acadcoiie Royale des Sciences, 

 torn. v. pp. 252, 254. 



ERRATA. 

 Page 64, line 35, for sin, read cos. 



37, .. to the to half the. 



41,.. sin sin. 



