525 



iron, red ferroprussiate of potassa, and also compressed bismuth, sur- 

 rounded in succession by the following media, phosphorus, alcohol, 

 oil, camphine, water, air, and saturated solution of protosulphate of 

 iron, he found the result to be the same as before. The mode of 

 estimating the set was as follows : The selected crystal being sus- 

 pended in the magnetic field by a torsion- wire, right-handed force 

 was then slowly applied by the revolutions of the torsion-head above, 

 until the crystal being gradually carried round, attained that position 

 at which any additional torsion-force would cause it to advance sud- 

 denly and considerably ; this position was called the upsetting point ; 

 then left-handed torsion was put on until the like point was attained 

 'n the opposite side: the amount of the revolution of the torsion-index 

 from one upsetting point to the other, minus the angle between the 

 upsetting points, was considered as the measure of the set of the 

 crystal under the constant magnetic force employed. 



As the setting force of a crystal remained constant for any sur- 

 rounding medium, it was evidently possible to select a crystal and a 

 medium such, that in one position the crystal would be attracted, 

 and in another, at right angles to the first, be repelled in the 

 same medium. This case was realized with the paramagnetic red 

 ferroprussiate of potassa and a solution of sulphate of iron, and also 

 with the diamagnetic crystal carbonate of lime and diluted alcohol. 

 A crystal was sought for amongst the ferrocarbonates of lime having 

 this relation to the assumed natural zero presented by a vacuum or 

 carbonic acid ; but this case was not realized. 



39. Action of heat on magnecrystals. When magnecrystals, 

 subjected to the same constant magnetic force, were raised or lowered 

 to different temperatures, it was found that the setting force was 

 affected ; and at all temperatures from F. upwards the force 

 diminished as the temperature became higher. Thus the torsion- 

 force of a crystal of bismuth at 92 being 175, was at 279 dimi- 

 nished to 82 ; that of a tourmaline, by passing from the tem- 

 perature of 79 to 289, was so far diminished ; that the power at 

 the lower temperature was nearly double that at the higher. A 

 like result occurred with carbonate of iron, and also with com- 

 pressed bismuth. In all these cases the bodies resumed their first 

 full power on returning to lower temperatures, nor was there any 

 appearance of magnetic charge in any part of the range of observa- 



3A2 



