14 



MAGNETISM* 



sition, liable however to some excep- 

 tions, that the power of retaining mag- 

 netism in any specimens of iron or steel, 

 is in proportion to its hardness. 



(55.) But some of the combinations 

 of iron with other substances affect its 

 capacity for magnetism, independently of 

 the hardness of the compound. A slight 

 degree of oxidation pervading the mass 

 of iron appears to increase its power of 

 retaining magnetism ; but a greater de- 

 gree renders it totally insusceptible of 

 being affected by the magnet, or of 

 possessing any magnetic properties 

 whatsoever. Combinations with phos- 

 phorus, with arsenic, or with tin, were 

 found by Mr. Gay Lussac to produce 

 compounds somewhat resembling those 

 of carburet of iron or steel in their ca- 

 pability of retaining magnetism. Every 

 thing depends, however, upon the pro- 

 portions in which these several sub- 

 stances are united with the iron ; for if 

 they exceed a certain quantity, they 

 totally incapacitate the compound from 

 acquiring any magnetic properties. 



(56.) It is only the finest and purest 

 soft iron, free from all knots and veins, 

 that returns to the state of perfect neu- 

 trality after it is removed from all extra- 

 neous magnetic influence. Iron is sel- 

 dom found in this perfectly pure state ; 

 but even the purest iron may be rendered 

 capable of permanently retaining mag- 

 netism, if it has been twisted or ham- 

 mered violently. The slight superficial 

 oxidation it undergoes by the action of 

 the atmosphere, will also make it suscep- 

 tible of some degree of fixed magnetism. 

 But in its common state, iron may, on 

 the whole, be regarded as incapable of 

 any long retention of the magnetism 

 which it may have received by induc- 

 tion. 



(57.) It would appear from the ex- 

 periments of Mr. Scoresby*, that the tex- 

 ture of all iron, even the most malleable, 

 presents a certain degree of resistance to 

 the transmission ot magnetic power; 

 for if a bar of iron be placed in circum- 

 stances favourable to its acquiring mag- 

 netism by induction, it does not acquire 

 it in the degree of intensity of which it 

 would be capable, were there no such 

 internal obstructions to the transmission. 

 If, under these circumstances, it be sub- 

 jected to percussion, which, as we have 

 seen, favours the transfer of magnetism 

 in obedience to the attractive and re- 



* Transactions of the Royal Society of Edin- 

 burgh, vol. ix. p. 252. 



pulsive forces that act upon it, it is 

 found to acquire a much higher inten- 

 sity of magnetic power than it would 

 have received without such percussion. 

 Nor is the whole of this power lost on 

 the removal of the inducting cause ; a 

 part is retained by the iron, the inter- 

 nal structure of which appears to have 

 undergone some alteration by the per- 

 cussion. 



As connected with this subject, we 

 may notice the following curious obser- 

 vation of the same experimentalist*. 

 Bars which had been strongly magne- 

 tized, and had their magnetisms de- 

 stroyed or neutralized, either by ham- 

 mering, heating, or by the simultaneous 

 contact of the two poles of another mag- 

 net placed transversely, were always 

 found by him to have a much greater 

 facility for receiving polarity in the same 

 direction as before, than in the contrary 

 direction. Hence, it generally happened 

 in his experiments, that one blow with 

 the original north end downward, pro- 

 duced as much effect as two or three 

 blows did with the original south end 

 downwards. He also observed, that the 

 polarity of pokers, generally supposed to 

 be permanent, and considerable in inten- 

 sity, was rather transient and weak ; for 

 in no instance did he meet with a poker 

 the magnetism of which he could not 

 destroy by a blow or two with a hammer 

 on the point ; and in general, two blows, 

 even when the poker was held in the 

 hand, and not rested upon any thing, 

 were sufficient to invert the poles. 



(58.) Soft steel is not much more reten- 

 tive of magnetism than iron in its ordinary 

 state. It is only when hardened that its 

 magnetic powers become in any degree 

 sensible. Dr.Robison states, that when 

 steel is tempered to that degree which 

 fits it for watch springs, it may acquire 

 a strong magnetism, which it exhibits 

 immediately on the removal of the mag- 

 net. But it dissipates very rapidly ; and 

 in a very few minutes it is reduced to 

 less than one half of the intensity it mani- 

 fested while in contact with the magnet, 

 and to less than two-thirds of what it was 

 immediately on removal from it. It con- 

 tinues to dissipate for some days, though 

 the bar be kept with care ; but the dissi- 

 pation diminishes fast, and it retains at 

 least one-third of its greatest power for 

 any length of time, unless carelessly kept 

 or injudiciously treated. 



(59.) Steel tempered for cutting-tools 



* Philosophical Transactions for 1822, p. 251. 



