ACTION OF HEAT ON MAGNECRYSTALS. 
175 
when reheated its direction was equatorial ; and this change could be repeated many 
times, without the crystal appearing to be at all altered, either by the production of 
caustic lime or of free oxide of iron : — its state and peculiar qualities were preserved. 
It was magnecrystallic at high and at low temperatures ; but at the upper it was like 
pure calcareous spar and at the lower like carbonate of iron. At the lower tempe- 
rature it was, as a whole, paramagnetic in air and therefore in carbonic acid gas. 
Whether at a given intermediate temperature it would cease altogether to be magne- 
crystallic, and would in carbonic acid gas be, as a whole, neither paramagnetic nor 
diamagnetic, and therefore in part, and for a given temperature, answer the inquiry 
before made (3393.), I cannot jsay. 
3417. In the carbonate of lime and carbonate of iron, the short axis of the rhom- 
boid is, emphatically, a line of direction, and points either equatorially or axially 
according to the nature of the crystal. A plane at right angles to it does not present 
sensible differences in particular parts, the force appears to be equal in all directions. 
So the whole change in the ferrocarbonate of lime appears to depend upon the 
circumstance, that, in the direction of the short axis, the aptness for magnetic in- 
duction decreases by heat more rapidly than in i\\% plane at right angles to it; so as 
not merely to overtake the latter but to pass hy it, and that in cooling it again 
returns towards, by, and beyond it: — the force in the equatorial plane or direction is 
probably varied, but nevertheless its whole range appears to be intermediate to that 
which the axial direction supplies. 
3418. It would seem that such crystals as these could not have been formed at a 
high temperature and common pressures, inasmuch as they cannot sustain such a 
temperature now. They may even be considered, physically, as different substances 
at high and low temperatures ; for a body which cannot expand and keep its inte- 
grity must have a very different arrangement of its molecular forces, when they are 
just about to burst the mass into particles, to that which exists when they are 
employed in giving permanency to the state into which they have brought them. 
The variations in magnetic relations are very striking for the two cases ; perhaps 
some of the optical characters may be found to be affected also. The crystals are, 
I think, harder than those of calcareous spar, and always more fissured. Such 
calcareous spar as contains veins of minute crystals of pyrites is almost sure to prove 
of this peculiar nature. 
3419. It would appear that magnecrystals (with the exception of the ferrocarbo- 
nate of iron), whether paramagnetic or diamagnetic, are all generally alike in their 
affections by heat; the differences of force in two given directions diminishing as the 
temperature is raised, increasing as temperature is lowered, and being constant for a 
given temperature. Such alterations might take place in various ways ; a diminution 
by heat of the force of the stronger axis would account for it, so also would an 
increase of the force of the weaker axis ; such doubtful points might be settled by 
combining with results like those I have given, others upon the whole paramagnetic or 
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