314! On the anomalous maiinctic Action 



Mac. Pow. Ma". Pow. 



Malleable iron ... 100 



Cast iron 48 



Blistered steel soft . 67 

 hard 53 



Shear steel soft ... 66 



hard ... 53 



Cast steel soft .... 74? 

 hard .... 49 



As it was obvious from these experiments, that the softer 

 the iron the greater was its power, and the contrary, I was de- 

 sirous of determining how nearly these different kinds of me- 

 tal would approximate towards each other in their magnetic 

 action, when rendered perfectly soft by being heated in a fur- 

 nace. With this view, bars of equal size of cast-iron, malle- 

 able iron, shear steel, &c. were rendered white hot, and being 

 placed in the direction of the dip, as before, their powers, as 

 was anticipated, agreed nearly with each other ; but still the 

 cast iron, which was weakest while the meta) was cold, ex- 

 ceeded a little in power all the others when hot, and the mal- 

 leable iron which had the greatest power cold, had the least 

 when hot ; but the difference was not very great, and might 

 probably arise from some accidental circumstance. While 

 carrying on these experiments, it had been observed, both by 

 Mr. Bonnvcastle and myself, that between the white heat of 

 the metal, when all magnetic action was lost, and the blood- 

 red heat, at which it was the strongest, there was an interme- 

 diate state in which the iron attracted the needle the contrary 

 way to what it did when it was cold, viz. if the bar and com- 

 pass were so situated that the north end of the needle was 

 drawTi towards it when cold, the south end was atttracted du- 

 ring the interval above alluded to, or while the iron was pass- 

 ing through the shades of colour denoted by the workman the 

 bright red and red heat. 



As this anomalous action had never before beeii noticed, I 

 was desirous of examining it a little more particularly; and 

 with the assistance of Mr. Bonnjcastle, the following series 

 of experiments were performed, wholly directed to this in- 

 quiry. Before entering upon the detail, however, it may not 

 be amiss to notice those results which have hitherto been ob- 

 tained relative to the magnetic action of heated iron ; and to 

 show how the contradictory statements, that we find on the 

 subject, may be reconciled with each other. For example: 

 we find it stated in Newton's Optics, that red hot iron has 

 no magnetic property ; while Father Kircher asseits that the 

 magnet will attract red hot iron as well as cold, ("de Mag- 

 nete," lib. 1). Again, in Vol. xviii. No. 214. Phil.Trans. it is 

 stated, that hot iron not only has an attraction for the magnet, 

 but that its power is increased by the heat ; and these asser- 

 tions 



