12 



MAGNETISM. 



the magnetic polarities, and favours the 

 induction of magnetism. Striking on 

 the bar with a hammer is found to pro- 

 duce this effect in a remarkable degree ; 

 and the more so if it occasion a ring- 

 ing sound in the steel, which is an in- 

 dication that its particles are very gene- 

 rally thrown into vibratory motion. But 

 any other cause producing agitation 

 among the particles assists in the in- 

 duction of magnetism. 



(45.) The transmission of an electric 

 discharge through a steel bar under the 

 influence of a magnet, is sufficient to 

 produce permanent magnetism. That 

 the electricity acts here only by its me- 

 chanical operation, is proved by the 

 effect being the same, whatever be the 

 direction in which it is transmitted ; 

 that is, whether the positive stream of 

 electricity be made to pass from right 

 to left, or from left to right, along any 

 part of the steel bar ; or whether it be 

 passed longitudinally or transversely 

 through it. This mechanical operation 

 of electricity is, however, to be carefully 

 distinguished from an influence of a to- 

 tally different description, which it is 

 capable of exerting in producing mag- 

 netism, and the operation of which will 

 be the subject of a distinct treatise here- 

 after to be published. 



(46.) Heat also appears to act by re- 

 moving the obstructions to the trans- 

 mission of magnetism which exist in 

 steel, in its ordinary state, and by thus 

 reducing it nearly to the condition of 

 soft iron. Accordingly, if a steel bar 

 be heated, and placed in circumstances 

 favourable to magnetic induction, if it 

 be placed, for example, in the immedi- 

 ate vicinity of a magnet, and then sud- 

 denly cooled, it will be found, on its 

 removal from the magnet, to have be- 

 come strongly and permanently mag- 

 netic. The greatest degree of magnetism 

 is produced by heating the steel to red- 

 ness, and, while it is under the influence 

 of a strong magnet, quenching it sud- 

 denly with cold water. 



(47.) It will readily be understood 

 that since the magnetism of a steel bar 

 remains permanent, only because the 

 peculiar texture of the steel presents 

 an insuperable obstacle to its resuming 

 its natural state of uniform distribution, 

 all the causes which diminish this ob- 

 structing force, will give occasion to the 

 escape of portions of this imprisoned 

 magnetism, and will make the bar ap- 

 proach nearer to a neutral condition. 

 In other words, its magnetism will be 



impaired and weakened, by the very 

 same causes which favoured its acqui- 

 sition when under the inductive process. 

 It is accordingly found that any mecha- 

 nical concussion, or any rough usage, 

 has a tendency to destroy the power of 

 a steel magnet. Dr. Gilbert, who was 

 one of the earliest discoverers in this 

 science, found that a magnet which he 

 had impregnated very strongly was very 

 much impaired by a single fall on the 

 floor: and it has been observed since 

 his time, that a magnet is more injured 

 by falling on a stone pavement, or re- 

 ceiving blows which cause it to sound 

 or ring, than by being struck with any 

 soft or yielding substance. 



(48.) In like manner the application 

 of heat to a magnet is invariably at- 

 tended by a dissipation of its magnetic 

 power. It is even sensibly affected by 

 the heat of boiling water ; and a red heat 

 totally destroys its magnetism. It has 

 been observed by Mr. Canton, that if 

 the temperature of the magnet has been 

 raised only to that of boiling water, al- 

 though it loses much of its power during 

 the operation, yet that a great part of it 

 is again recovered on its becoming cool. 

 But after it has been heated to redness, 

 no part of its magnetism is recovered on 

 cooling. 



(49.) The precise nature of the in- 

 fluence which heat has upon magnetism 

 is far from being clearly understood; 

 and there appears to be much discord- 

 ance in the accounts given by different 

 authors on this subject. This appears 

 to have arisen in a great measure from 

 a want of attention to the circumstance 

 that the operation of heat is of two 

 kinds : for while, on the one hand, it 

 facilitates the induction of magnetism, 

 on the other it weakens magnetic action. 

 In those cases where the effects depend 

 upon the readiness with which a piece 

 of iron receives magnetism by induction, 

 heat will favour this process ; thus, soft 

 iron is more disposed to be attracted by 

 a magnet when hot than when cold, 

 provided the heat be not excessive. But 

 in as far as relates to permanent mag- 

 netism, the action of heat is to impair 

 or destroy it ; so that steel, when heated, 

 is less capable of retaining its power 

 than it is when cold. This happens 

 in consequence of its being brought 

 nearer to the condition of soft iron, by 

 the separation of its particles. By rais- 

 ing the temperature sufficiently high, to 

 a red heat for instance, the whole of its 

 permanent magnetism is at once de- 



