Even when a body is not in a fluid condition, the smallest particles may
undergo certain relations in their various modes of arrangement, which are
manifested by the different action on light.*
[footnote] *On gypsum as a uniaxal crystal, and on the sulphate of
magnesia, and the oxyds of zinc and nickel, see Mitscherlich, in Poggend.,
'Annalen.' bd. xi., s. 328.
The phenomena presented by devitrification, and by the formation of steel by
cementation and casting -- the transition of the fibrous in the granular
tissue of the iron, from the action of heat* and probably, also, by regular
and long-continued concussions -- likewise throw a considerable degree of
light on the geological process of metamorphism.
[footnote] *Coste, 'Versuche am Creusot uber das bruchig werden des
Stabeisens.' Elie de Beaumont, 'Mem. Geol.', t. ii., p. 411.
Heat may even simultaneously induce opposite actions in crystalline bodies;
for the admirable experiments of Mitscherlich have established the fact*
that calcareous spar, without altering its condition of aggregation, expands
in the direction of one of its axes and contracts in the other.
[footnote] * Mitscherlich, 'Ueber die Ausdehnung der Krystallisirten Korper
durch die Warmelehre', in Poggend.
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