Some time ago it was discovered that some limestone, which had been
submitted for eighteen months to a heat of nearly 1,000 degrees in
the smelting furnaces of Leroy-Descloges (France), had given rise to
perfectly crystallized anhydrous lime. Figure C shows three of these
crystals magnified 300 diameters. It will be noticed that they have a
striking analogy with grains of common salt. They are, in fact, cubes
(often imperfect), but do not polarize light, as a substance of the
first crystalline system should. However, it is rarely the case that the
crystals do not have _some_ action on light. Most usually, when the two
Nicol prisms are crossed so as to cause extinction, the crystals present
the appearance shown at D. That is to say, while the central portion
is totally inactive there are seen on the margins zones which greatly
brighten the light.
[Illustration]
A and B.--Calcareous Pisolites and Oolites produced artificially.
A.--External aspect and section of a Pisolite. B.--Details of internal
structure as seen by the microscope.
C and D.--Crystals of anhydrous Lime obtained artificially. C.--Crystals
seen under the microscope in the natural light. D.--Crystals seen under
the microscope in polarized light.
The phenomenon is explained by the slow carbonization of the anhydrous
lime under the influence of the air; the external layers passing to the
state of carbonate of lime or Iceland spar, which, as well known, has
great influence on polarized light. This transformation, which takes
place without disturbing the crystalline state, does not lead to any
general modification of the form of the crystals, and the final product
of carbonization is a cubic form known in mineralogical language as
_epigene_. As the molecule of spar is entirely different in form
from the molecule of lime, the form of the crystal is not absolutely
preserved, and there are observed on the edges of the epigene crystal
certain grooves which correspond with a loss of substance. These grooves
are quite visible, for example, on the crystal to the left in Fig. D.
Up to the present time anhydrous lime has been known only in an
amorphous state. The experiment which has produced it in the form noted
above would doubtless give rise to crystallized states of other earthy
oxides likewise, and even of alkalino-earthy oxides.