MAGNETISM. 



91 



magnets, arranged in the same line, and 

 separated about a quarter of an inch 

 more than the length of the needle that 

 was to oscillate between them. What- 

 soever was the substance of which the 

 needles were formed, they always ranged 

 themselves accurately in the direction of 

 the magnets ; and if disturbed from this 

 position, returned to it with oscillations, 

 which were often as frequent as thirty 

 or more in a minute, and considerably 

 more frequent than when the magnets 

 were removed : thus indicating a very 

 decided force of attraction. Needles 

 made of tin, lead, copper, silver, and 

 gold, and cylinders of glass, chalk, bone, 

 and different sorts of wood, together 

 with a great variety of other organic 

 substances, both animal and vegetable, 

 were tried in succession, and with the 

 same result. 



These experiments were repeated in 

 England, by Dr. Young, at the Royal 

 Institution, but with less decided suc- 

 cess: the force of attraction indicated 

 was estimated at rather less than the 

 two- thousandth part of the weight of the 

 substance employed. 



(352.) There are but two ways of ex- 

 plaining these phenomena: they are 

 either owing to the presence of minute 

 quantities of iron entering into the com- 

 position of all the bodies which manifest 

 magnetic properties, or else they war- 

 rant the inference that all these bodies 

 possess a certain degree of inherent 

 magnetism. If the former mode of ex- 

 planation be the true one, we shall be 

 forced to admit that iron may exist in 

 bodies, in quantities so minute as to 

 elude detection by the severest chemical 

 examination, and yet have sufficient 

 power to be sensibly affected by a mag- 

 net. A set of very delicate experi- 

 ments was undertaken by Coulomb 

 with a view to determine this point : in 

 the course of which he satisfied himself, 

 that a smaller quantity of iron than can 

 be discovered by any chemical test yet 

 known, will, when added to a body, impart 

 to it a very decided magnetic suscepti- 

 bility. This is the case when a metal 

 contains only the 130,000th part of its 

 weight of iron. The magnetic powers of 

 different specimens of metals were found 

 to differ materially according to the 

 methods employed for their purification. 

 Hence he concluded that the greater 

 part, if not the whole, of the effect ob- 

 served, is to be ascribed to the presence 

 of iron. So confident was he of the truth 

 ot this theory, that he imagined the 



magnetic action of all substances might 

 safely be taken as a criterion of the pro- 

 portion of iron they contain. 



(353.) On the other hand, the indi- 

 cations of magnetic power given by 

 nickel and by cobalt are far too con- 

 siderable to be accounted for by the 

 agency of any ferruginous admixture. 

 Biot was in possession of a needle made 

 of nickel, which Thenard had exerted 

 all his chemical skill in rendering as 

 pure as possible : the directive force of 

 this needle, when magnetized, was not 

 less than one-third of a similar needle 

 made of steel *. Now the proportion of 

 iron which, if added to the nickel, would 

 be required to impart an equal degree of 

 magnetic power, is far beyond what can 

 ever reasonably be supposed to enter in- 

 to the composition of nickel so purified. It 

 is certainly just possible that nickel may 

 be a compound metal, containing iron 

 as one of its ingredients ; but we are not 

 justified in admitting an explanation so 

 extremely hypothetical, especially as 

 there are other facts besides those above 

 mentioned, which tend strongly to cor- 

 roborate the universality of magnetism. 

 Of this nature are the evidences of an 

 influence exerted by bodies not reputed 

 magnetic, in controlling the oscillations 

 of a magnetic needle placed in their im- 

 mediate vicinity; and also the pheno- 

 mena of the mutual influence exerted 

 between these bodies and magnets, when 

 the one is revolving rapidly, in exciting 

 sympathetic rotation in the other. These 

 very curious phenomena will be de- 

 scribed in the ensuing chapter. 



CHAPTER VIII. 

 On the Magnetism of Rotation. 



(354.) CONSIDERABLE .light has lately 

 been thrown upon the question of the 

 universality of magnetism, by the dis- 

 covery of the unexpected effects which 

 result from the reciprocal action of 

 magnets upon other bodies, when 

 the one or the other is maintained in a 

 state of rapid rotation. In the year 

 1 824 M. Arago showed that if a plate 

 of copper, or of any other substance, be 

 placed immediately under a magnetic 

 needle, it exerts sufficient influence upon 

 its movements to diminish sensibly the 

 extent of its oscillations, without, how- 

 ever, affecting their duration ; and the 

 needle is brought to rest in a shorter 

 time than happened when no such sub- 

 stance is placed under it The con- 



* Traite de Physique, torn, iii., p. 126. 



