OF FERRUGINOUS BODIES. 209 
11. From the time of Canton’s discoveries of 
the action of heat on magnetic bodies till the year 
1819, philosophers had been led to view the 
influence of any elevation of temperature as detri- 
mental to the full display of the magnetic powers 
of ferruginous bodies; and, although the idea 
amounts to a demonstrable fact, as far as previously 
magnetised steel and loadstone are concerned, 
Professor Barlow showed, by some highly inter- 
esting experiments conducted with great care, 
about the period last named, that ferruginous 
bodies generally, whether in the character of steel 
or iron, when under the influence of terrestrial 
magnetic forces only, acquire a greater action on 
the compass needle, when at a blood-red heat, 
than at any other temperature; but that at a 
full white heat the magnetic action entirely dis- 
appears.* 
12. The discovery of electro-magnetism, by 
professor Cirsted, in 1819, gave M. Ampére an 
opportunity of supposing that terrestrial mag- 
netism might possibly be due to electric currents 
circumflowing the body of the globe ; but it was 
not till the discovery of thermo-electricity, by 
Dr. Seebeck, in 1822, that any idea could be 
* Barlow’s Magnetic Attractions ; and Phil. Trans. 
