MAGNETISM. 



41 



importance, as they allow of a strict 

 comparison of the results of experiment 

 with the deductions from theory ; and 

 the accurate accordance which has been 

 obtained between them, in every instance 

 in which such a comparison has been 

 instituted, affords the strongest evidence 

 of the correctness of the views on which 

 that theory is founded. 



(165.) It has already been observed 

 that we have no data for determining 

 the question as to the size of the mag- 

 netic elements, compared with that of 

 the constituent molecules : we know not 

 whether they are coincident with these 

 molecules, or whether they occupy only 

 the interstices between the molecules : 

 neither can we determine whether they 

 do not actually comprehend certain de- 

 finite aggregates of molecules, or whe- 

 ther they are constituted in the intervals 

 of these aggregates. All that we can 

 be certain of is, that the sum of all the 

 magpetic elements, added to the sum of 

 all the unmagnetic elements (that is, the 

 spaces, whether occupied by matter or 

 not, which are devoid of magnetic fluid), 

 must together make up the total appa- 

 rent volume of the body under conside- 

 ration. Now the ratio between these 

 two sums may vary, not only in different 

 kinds of bodies, but also in the same 

 body, in different circumstances. It 

 may, for instance, be very materially af- 

 fected by changes of temperature : and 

 this consideration will probably furnish 

 a key to the explanation of many of the 

 anomalous appearances we have already 

 had occasion to notice in $ 49. 



(166.) The hypothesis of two mag- 

 netic fluids was first propounded by 

 Wilke and Brugmann ; but the first real 

 foundations of the theory were laid by 

 Coulomb, who, by the exercise of sin- 

 gular perseverance and sagacity, pre- 

 pared and established all the physical 

 principles upon which it rests. It has 

 recently occupied the attention of Pois- 

 son, and appears to have received its 

 last finish from his masterly hand ; for 

 by applying to it the refinements of mo- 

 dern analysis, this distinguished mathe- 

 matician has succeeded in discovering 

 formulae which represent, numerically, 

 all the principal phenomena of the sci- 

 ence, even in their minutest details, and 

 which furnish us with a ready and con- 

 sistent explanation of the physical mode 

 by which they are produced.* 



* A popular view of this theory is given by Biot, 

 in a note to his French translation of Fischer's 

 Physique Mechanique, 4th edition, page 342. 



(167.) Professor Prevost, of Geneva*, 

 has laboured to frame a theory of mag- 

 netism which shall dispense with all at- 

 tractive or repulsive agencies, and in 

 which all the phenomena shall be re- 

 solvable into the effects of impulsion. 

 For this purpose he admits two magnetic 

 fluids, each giving its respective polarity 

 to the two ends of a magnet, and neu- 

 tralizing each other by combination ; but 

 adopting the hypothesis of Le Sage, as 

 to the existence of another infinitely 

 more subtile fluid, pervading all space, 

 and giving rise by its inconceivably 

 rapid movements to all the phenomena 

 of gravitation, cohesion, and chemical 

 attraction, he supposes the magnetic 

 fluids themselves to be set in motion by 

 this primary and universal agent. But 

 it would be impossible in this place to 

 engage in the development of so abstruse 

 and complicated a system as this. 



CHAPTER V. 



Methods of making Artificial Magnets. 

 $ 1. General Principles. 



(168.) THE art of communicating mag- 

 netic power to bodies capable of retaining 

 it, is founded on the proper application of 

 the principles already explained ; and 

 the practical results of experience in this 

 art have, as might be expected, furnished 

 some of the most interesting illustra- 

 tions of the theory of magnetism. We 

 have seen that acquired magnetism of 

 every kind, whether temporary or per- 

 manent, of which the origin can be 

 traced, has been derived, by induction, 

 from a similar power already existing in 

 some other body. In this respect, then, 

 it differs from electricity, which may be 

 elicited from bodies all of which were 

 previously in a neutral state, by a variety 

 of processes either of a mechanical or 

 chemical nature. But the body which 

 is the cause of magnetism in another 

 body must itself be in an active state of 

 magnetism, and may be either a magnet, 

 whether natural or artificial, or else it 

 must be the globe of the earth itself: it 

 is therefore highly probable, that the 

 magnetism of the earth is the original 

 source of all other magnetism. This 

 view of the subject excludes, of course, 

 all consideration of electro-magnetic in- 

 fluence,which belongs to another division 

 of the science, hereafter to be treated of. 

 It will then be shewn that electricity in 



* Surl'Origine des Forces Magnetiques. 8vo. 

 Geneve, 1788. 



