Ib89.] TJie Recalescence of Iron. 455 



is turned upon this rod, there is at first a sudden deflection of twenty 

 magnetometer-scale-divisions to the left, the spot afterwards moving 

 slowly and steadily towards the right. When the light is shut off 

 there occurs at once a jump of twenty divisions further towards the 

 right before the spot begins to move back in the zero direction. 



Some attempts have been made to repeat the experiments with light 

 polarised by means of a Nicol's prism ; but, either because the largest 

 prism at my disposal was too small (its aperture being barely 2 cm.), 

 or because too much of the radiant energy was absorbed by the spar, 

 I failed to get any magnetic effects whatever with the prism in either 

 position. 



[Professor Silvanus Thompson has quite recently been kind enough 

 to lend me a very large and excellent Nicol's prism. From a few 

 experiments already made with this instrument it appears that the 

 action of the light is quite independent of the plane of polarisation. 

 March 16.] 



There can be no doubt whatever of the reality of the effects here 

 described : they are perfectly distinct, and are at any time reproducible 

 with certainty. The only question is how much of them is primarily 

 caused by the action of light, and how much by mere incidental 

 change of temperature. But taking all the circumstances into con- 

 sideration, I think the evidence is in favour of the conclusion that 

 the instantaneous magnetic change, which occurs when a prepared iron 

 bar is illuminated, is purely and directly an effect of radiation. 



IV. Recalescence of Iron." By J. HOPKINSON, F.R.S. 

 Received March 7, 1889. 



Professor Barrett has observed that if an iron wire be heated to a 

 bright redness and then be allowed to cool, that this cooling does not 

 go on continuously, but that after the wire has sunk to a very dull red 

 it suddenly becomes brighter, and then continues to cool dowii. He 

 surmised that the temperature at which this occurs is the temperature 

 at which the iron ceases to be magnetisable. In repeating Professor 

 Barrett's experiments, I found no difficulty in obtaining the pheno- 

 menon with hard steel wire, but I failed to observe it in the case of 

 soft iron wire, or in the case of manganese steel wire. It appeared 

 to be of interest to determine the actual temperature at which the 

 phenomenon occurred, and also the amount of heat which was 

 liberated. Although other explanations of the phenomenon have 

 been offered, there can never, I think, have been much doubt that it 

 was due to the liberation of heat owing to some change in the 

 material, and not due to any change in the conductivity or emissive 

 power. My method of experiment was exceedingly simple. I took a 



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