Changes in Iron fyc. at different Temperatures. 171 



shown that a piece of soft iron at a bright red heat loses all ordi- 

 nary signs of magnetism ; and Faraday has shown* that wrought 

 iron retains only traces of its ordinary magnetic capacity at that 

 temperature. He has further shown f that nickel first loses its 

 distinctive magnetic power at about 635° F. ( = 335° C), and 

 that the temperature of boiling oil is sufficient to render large 

 masses of that metal insensible to the action of common mag- 

 nets — also that on raising the temperature of iron and nickel 

 from 0° F. to 300° F. (=-17°'7 C. to 149° C.) the magnetic 

 capacity of iron remains constant, whilst that of nickel dimi- 

 nishes gradually. According to MatteucciJ, the magnetism of 

 iron increases up to a certain temperature, then decreases rapidly; 

 it retains, however, even at a white heat, a very minute degree of 

 its magnetic capacity, calculated at only '000015 of its ordinary 

 amount ; for a globule melted in a lime spoon was still attracted 

 by a very powerful electromagnet. 



In a paper "On a Momentary Molecular Change in Iron 

 Wire," published in the Proceedings of the Royal Society, No. 

 108, p. 260, January 28, 1869, I described a singular pheno- 

 menon which I observed in the cooling of iron wire which had 

 been heated to full redness whilst under a suitable degree of lon- 

 gitudinal tension by means of a spring attached to one of its 

 ends; the iron during cooling, and whilst still red-hot, gradually 

 diminished in length, then suddenly elongated by diminution of 

 cohesion, and finally contracted gradually nearly to its original 

 length during the remainder of the cooling process : a corre- 

 sponding but reverse phenomenon did not occur during the pro- 

 cess of heating the wire. Various other metals were similarly 

 examined, but no such peculiar phenomenon was found. In an- 

 other paper, " On the Development of Electric Currents by Mag- 

 netism and Heat"§, I showed that, by cooling a brightly red-hot 

 iron wire (under the influence of a permanent magnet) within 

 the axis of a coil of thin insulated copper wire, an electric current 

 was induced in that wire; and during the first few seconds in the 

 process of cooling, and at apparently the same temperature at 

 which the aforesaid elongation and loss of cohesion occurs, an 

 irregular action took place in the induced current, which u was 

 probably connected with the momentary molecular change." In 

 the present paper I have employed a different method of exa- 

 mining these molecular movements and magnetic changes of iron 

 and nickel under the influence of heat, and have obtained some 



* Experimental Researches in Electricitv, 2344-2347. 

 t Phil. Trans, vol. cxlvi. (1856) pp. 1/8-180. 

 X Bibl. Univ. de Geneve, [Arch, des Sc. xxiii.] xxiv. 

 § Proc. Roy. Soc. Januarv 28, 1869, p. 265 [Phil, Mag. Julv 1869, 

 pp. 59, 64.). 



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