476 Royal Sociefi/. 



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 

 in 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 tliat the setting force was 

 affected ; and at all temperatures from 0° 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 tempei'atures, nor was there any 

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

 tions. Between 32° and 300° the force of bismuth appeared to alter 

 by regular equal degrees ; but with tourmaline and carbonate of iron 

 the change was greatest for an equal number of degrees at the lower 

 temperatures. At a full red heat, however, both tourmaline and 

 calcareous spar retained a portion of their magnecrystallic force or 

 condition, and so did carbonate of iron up to that temperature at 

 which it was decomposed. 



It is known that pure calcareous spar points with its optic axis 

 equatorially, but that calcareous spar containing a trace of iron 

 points with its optic axis axially. Calcareous spar retains its mag- 

 netic characters at very high temperatures, but carbonate of iron and 

 oxide of iron lose almost the whole of their m.agnetic force at a dull 

 red heat. It was therefore expected that a ferrocarbonate of lime 

 crystal might become absolutely reversed in condition by cliange of 

 temperature, and this was found to be the case : at low temperatures 

 the optic axis pointed axially, and at high temperatures equatorially ; 

 and that through any number of changes, as the temperature of the 

 crystal was alternately lovi'ered and raised. 



§ 40. Effect of heat upon the absolute magnetic force of bodies. — 



