174 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXX.) 
as the heat fell and losing it when temperature inereased, and that repeatedly. When 
the temperature was further raised and continued for a minute or more, the crystal 
ceased to be magnecrystallic, and lost nearly all magnetic power ; but when lowered 
beneath a certain temperature, it became intensely magnetic, and was found to have 
lost its carbonic acid and become converted into magnetic oxide of iron. 
3413. Carbonate of lime . — The sphere of calcareous spar was comparatively so 
weak in magnetic force, as to give no sufficient indication when a metallic torsion 
wire was employed with the Logeman magnet ; to employ a silk torsion thread 
would have been unsafe. A very high temperature, amounting to full ignition (being 
the highest that a spirit-lamp flame could communicate to a small rhomboid), did 
not take away the magnecrystallic condition of calcareous spar, or interfere with the 
pointing of the optic axis equatorially ; for though the heat was sufficient to convert 
the exterior of the crystal (to which the aqueous vapour from the flame had access) 
into quick lime, still the internal crystalline part pointed magnecrystallically, and 
carried the altered part with it. This permanency, coupled with the low magne- 
crystallic state possessed by the crystal at common temperatures, shows that the 
power would decrease at a very low rate and in a very small degree, whilst rising 
from 0° of temperature to 300°. 
3414. When a crystal of red ferroprussiate of potassa is heated, either in air or oil, 
it flies to pieces at a certain high temperature; beneath that degree, however, it 
retains its magnecrystallic character unaltered, except that the pointing is with less 
force at the higher temperatures than at the lower. 
3415. The f err o calcareous spar before described (3393.) suggested some very 
curious points of inquiry. It seemed probable that the iron within the crystal would 
retain its state of chemical combination under the action of heat, if the crystal as a 
whole should preserve its integrity at high temperatures: and if so, then, because of 
the slow alteration of calcareous spar by heat, and the much quicker alteration of 
carbonate of iron, as regarded magnetic force, it seemed further probable, that such 
a magnecrystal being heated sufficiently, would change its character ; and that the 
axis of magnetic power, which at low temperatures was the maximum, would at high 
temperatures become the minimum axis, or line of minimum force: which indeed 
upon investigation proved to be the case. 
3416. It was very difficult to raise these crystals above a certain temperature; 
near a given point, about 300°Fahr., they either broke up suddenly like a Rupert’s 
drop, or crumbled to pieces. No previous slow elevation of temperature appeared 
to prevent this disruption. Nevertheless some pieces were obtained, both in air and 
in oil, which, though much fissured, still adhered together, so as to represent the 
crystal. When these were properly suspended in the magnetic field, the short axis 
of the rhomboid (or optic axis) pointed axially at common temperatures, but when 
raised by a spirit-lamp to a point clearly below a red heat, the short axis pointed 
equatorially. When the crystal was allowed to cool, it again pointed axially, and 
