406 Dr. Faraday on the Diamagnetic conditions 



It will be evident to you that I have considered flame only 

 as a particular case of a general law. It is a most important 

 and beautiful one, and it has given us the discovery of dia- 

 magnetism in gaseous bodies: but it is a complicated one, 

 as I shall now proceed to show, by analysing some of its 

 conditions and separating their effects. 



For the purpose of examining the effect of heat alone in 

 conducing to the diamagnetic condition of flame, a small helix 

 of fine platina wire was attached to two stronger wires of cop- 

 per, so that the helix could be placed in any given position as 

 regarded the magnetic poles, and at the same time be ignited 

 at pleasure by a voltaic battery. In this manner it was substi- 

 tuted for the burning taper, and gave a beautiful highly-heated 

 current of air, unchanged in its chemical condition. When 

 the helix was placed directly under the axial line, the hot air 

 rose up between the poles freely, being rendered evident above 

 by a thermometer, or by burning the finger, or even scorching 

 paper; but as soon as the magnet was rendered active, the 

 hot air divided into a double stream, and was found ascending 

 on the two sides of the axial line; but a descending current 

 was formed between the poles, flowing downwards towards the 

 helix and the hot air, which rose and passed off sideways 

 from it. 



It is therefore perfectly manifest that hot air is diamagnetic 

 in relation to, or more diamagnetic than, cold air ; and, from 

 this fact I concluded, that, by cooling the air below the natural 

 temperature, I should cause it to approach the magnetic axis, 

 or appear to be magnetic in relation to ordinary air. I had 

 a little apparatus made, in which a vertical tube delivering 

 air was passed through a vessel containing a frigorific mix- 

 ture; the latter being so clothed with flannel that the ex- 

 ternal air should not be cooled, and so invade the whole of 

 the magnetic field. The central current of cold air was di- 

 rected downwards a little on one side of the axial line, and 

 falling into a tube containing a delicate air-thermometer, there 

 showed its effect. On rendering the magnet active, this effect 

 however ceased, and the thermometer rose ; but on bringing 

 the latter under the axial line it again fell, showing that the 

 cold current of air had been drawn inwards or attracted to- 

 wards the axial line, i. e. had been rendered magnetic in rela- 

 tion to air at common temperatures, or less diamagnetic than 

 it. The lower temperature was 0° F. The effect was but 

 small; still it was distinct. 



The effect of heat upon air, in so greatly increasing its dia- 

 magnetic condition, is very remarkable. It is not, I think, at 

 all probable that the mere effect of expanding the air is the 

 cause of the change in its condition, because one would be led 



