526 



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 magnetic force at a dull 

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

 crystal might become absolutely reversed in condition by change 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 lowered and raised. 



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

 Results were sought for, by which the magnetic force of bodies, 

 already examined in the condition of magnecrystals, might be com- 

 pared with the whole paramagnetic or diamagnetic force of the same 

 bodies taken in the granular or amorphous state ; but they were not 

 satisfactory. The carbonate of iron gave the most distinct results ; 

 and in its case the change of power by change of temperature was 

 not the same for the two conditions. An examination of the three 

 metals, iron, nickel, and cobalt, at temperatures between and 

 300 F., gave a very interesting result, which the author is not 

 aware has as yet been noticed. As the temperature rises, the force 

 of the nickel diminishes, the force of the iron remains constant, the 

 force of the cobalt increases ; these facts suggest that there is a 

 temperature at which the magnetic force is a maximum, and above 

 or below which it diminishes. The order with the three bodies 

 accords perfectly with that in which they lose the chief amount of 

 their magnetic power, for much loss occurs with nickel at the tem- 

 perature of boiling oil, with iron at a dull red heat, and with cobalt 

 at a temperature near that of melting copper. 



