MAGNETISM. 



13 



stroyed, though it will still be suscep- 

 tible, while in that state, of receiving tem- 

 porary magnetism by induction, and 

 therefore of being attracted by another 

 magnet 



(50.) The direct tendency of heat to 

 diminish magnetic power must also be 

 taken into account in our estimate of 

 the preceding phenomena. It not only 

 promotes the destruction of permanent 

 magnetism, but diminishes likewise the 

 effects of that which is of a tempo- 

 rary nature. The degree in which heat 

 possesses this direct influence, can be 

 estimated only under circumstances in 

 which no permanent change has been 

 produced in the magnetism of a bar 

 subjected to the change of tempera- 

 ture ; that is, provided we find on the 

 return of the bar to its former tempera- 

 ture, that it has retained all the power it 

 had before the experiment. The limit 

 beyond which no proper distinction can 

 be accurately drawn between the effects 

 of this twofold operation of heat, ap- 

 pears, according to the experiments of 

 Mr. Christie, to be below 100 of Fah- 

 renheit. From this temperature down- 

 wards the power of a magnet increases 

 as it becomes colder ; and this augmen- 

 tation proceeds as far as the lowest 

 temperature that has been tried. 



(51.) The following are the results of 

 an extensive series of experiments upon 

 this subject made by Mr. Christie*. 

 Commencing with a temperature of 

 -3 of Fahrenheit, up to one of 127, 

 the intensity of magnetic power de- 

 creased as the temperature of the mag- 

 nets increased. From an experiment 

 he made at the Royal Institution, in 

 conjunction with Mr. Faraday, in which 

 a small magnet, enveloped in lint, well 

 moistened with sulphuret of carbon, 

 was placed on the edges of a basin con- 

 taining sulphuric acid, under the re- 

 ceiver of an air-pump, he found that the 

 intensity of the magnet increased to the 

 lowest point to which the temperature 

 could be reduced, and that the intensity 

 decreased on the admission of air into 

 the receiver, and consequent increase 

 of temperature. This, he observes, is 

 in direct contradiction to the notion 

 which has been entertained of intense 

 cold destroying the magnetism of the 

 needle. Captain Middleton had an- 

 nounced ! his having frequently observed 

 that a compass appeared to be deprived 



* Philosophical Transactions for 1825, p. 62. 

 t PhilosophicalTrans. for 1733, vol. xl. p. 310. 



of all magnetic power from cold, while 

 he was navigating among the 'ice in 

 Hudson's Bay ; but recovered its power 

 when brought into the cabin and warmed 

 by the fire : and that this repeatedly oc- 

 curred. There can be no doubt that 

 this must have been owing to some other 

 cause than the one he assigned. 



With a certain increment of tem- 

 perature, the decrement of intensity is 

 not constant at all temperatures, but 

 increases as the temperature increases. 

 From a temperature of about 80 the 

 intensity decreases very rapidly as the 

 temperature increases, and beyond the 

 temperature of 100, a portion of ihe 

 power of the magnet is permanently de^ 

 stroyed. 



(52.) The effects produced on unmag-- 

 netized iron, by changes of temperature* 

 were observed by Mr. Christie to be 

 directly the reverse of those produced on 

 a magnet ; an increase of temperature 

 causing an increase in the magnetic, 

 power of the iron, the limits between, 

 which, he observed to be 50 and 100. 

 This is in perfect conformity with the 

 views we have above explained, of the 

 nature of the operation of heat with re- 

 gard to magnetism. 



(53.) Although the direct tendency of 

 heat to diminish magnetic power may, in 

 a red hot bar, be not sufficient to prevent 

 its receiving induced magnetism, yet 

 when the temperature is still further 

 raised, even this capability is destroyed 

 and accordingly we find that, at a white 

 heat, iron appears to be totally insuscep- 

 tible of any magnetic action. There are 

 still, however, some curious anomalies 

 occurring in the magnetic action of iron 

 at these very high temperatures, of which 

 further investigation alone can furnish 

 the explanation. 



(54.) In .the account we have now 

 given of trie properties of iron and of 

 steel, with regard to their capabilities of 

 acquiring and of retaining magnetism, 

 we have all along referred to those of 

 pure metallic iron in its softest and most 

 ductile state, and to those of steel which 

 has been brought to its greatest degree 

 of hardness by immersion in cold water 

 after being heated ; for it is in these two> 

 states that they exhibit the strongest 

 contrast in these respects. We often, 

 however, meet with this metal in states 

 possessing intermediate degrees of the 

 above qualities ; that is, acquiring mag- 

 netism with less facility than soft iron, 

 and retaining less of it than hard steel. 

 It may be laid down as a general propo- 



