C.—GEOLOGY. 65 
Of the components CaO, lime, is a solid fusible in the electric are 
at a temperature about 2570° C, as measured by the optical pyrometer. 
There are reasons for believing that lime exists in two forms; one of 
_ these is nearly isotropic and perhaps amorphous, and is obtained by the 
dissociation of CaCO, at low temperatures; the other is cubic with good 
cleavage and generally occurs in rounded crystals ; at high temperatures 
this is probably the only form met with. 
CaCO, as a mineral and as a chemical compound has been so exten- 
sively studied that the literature would fill a considerable library. At 
least eight forms of it have been described. Four of these, ktypeite, 
- eonchite, lublinite, and vaterite are rare and doubtful, and are by no 
means satisfactorily known. Recently a form known as p»CaCQ, has 
_ been described, but it is not believed to be of importance as a mineral. 
- Two others are the well-known minerals calcite and aragonite. Calcite 
is the stable form under ordinary conditions. Aragonite is transformed 
into calcite slowly in presence of moisture and carbonic acid, and rapidly 
if heated to a temperature from 400° to 500°C., but is practically stable 
in dry air at ordinary temperatures and pressures. There is some 
reason for believing that aragonite would be the stable form in tempera- 
tures 100° C. or so below the freezing-point. It has a higher specific 
gravity than calcite. In all experiments in which well-formed crystals 
of aragonite have been heated they changed to granular crystalline 
aggregates of calcite before dissociation or melting began. ‘This 
transition so far as is known is irreversible. 
At temperatures between 450° and 970°C. calcite is the result of 
heating every known form of CaCO,, but above that point it is believed 
to change to another mineral, aCaCO,, not very different in crystallo- 
_ graphic and optical characters. The transition is reversible, and as 
the temperature falls calcite is again formed. The existence of this 
transition is indicated by the heating and cooling curves of calcite, which 
show a discharge of heat delaying the fall of temperature about 970°C. 
The change is very small. Optical studies have been made with the 
help of an electric furnace closed with transparent quartz-glass plates, 
but no measurements were obtained of the optical constants of the 
mineral, and of its crystallographic form nothing is known except that it 
is probably trigonal. The change in fact is very similar to that by which 
quartz passes into a-quartz at a temperature of 575° C., and it has been 
proposed to use calcite like quartz as a geological thermometer. 
Carbonate of lime when heated in a closed vessel melts at a tem- 
_ perature of 1289°. A pressure of not less than 110 atmospheres of 
carbonic acid gas is necessary to prevent dissociation at the melting tem- 
perature. It forms quite a liquid melt which will readily flow through 
~ eracks in the platinum vessel that contains it. | On cooling the melt 
crystallisation takes place readily, and the resulting mass when cold is 
finely granular and completely crystalline. 
The dissociation pressure of CaCO, when heated has been studied 
by several investigators and the results are somewhat discordant. 
The most reliable results obtained by actual experiment show that at 
587° dissociation has only begun, the pressure of CO, being only one 
‘millimetre of mercury. At 700° it is 25mm., at 800° about 160mm., 
