a 
On 
44 SEVENTH REPORT—1837, 
q 
at the ordinary temperature of the air forms one continuous plane, j 
becomes distinctly two planes, making a very obtuse angle with each — 
other, and meeting in the line of junction of the two crystals. 
The alteration of the angle between the 
two edges that meet at C, amounts to Artif. Sec. 
7' 26" for a change of temperature of 100° 
centigrade, the angle being more obtuse 
when the crystal is hot than when it is 
cold. 
The fact of the unequal expansion of 
crystallized bodies in different directions, 
was first established by Mitscherlich, in 
the case of calcareous spar, by actually 
measuring the angle between the planes at 
low temperature, and when the crystal 
was heated. The change due to 100° 
centigrade was 8! 34, the angle be- 
tween the cleavage planes, which at ordi- 
nary temperatures is 105°, 5', becoming 
smaller when the crystal is heated. Prof. 
Miller’s way of showing it is as follows :-— 
Two rhombohedrons are clamped together, 
with their obtuse edges in contact—the two 
crystals are then held so that the flame of 
a candle may be seen, after two reflexions, 
~ one at each of the two surfaces of the cry- 
stals, which form a re-entering angle. By 
a well-known optical property, the angle 
between the candle and its reflected image, 
as seen by an eye close to the crystal, will 
be twice the angle between normals to 
the reflecting planes. Hence, if, when heated, the angle of each 
crystal undergoes a change e, the angle between the normal salters b2 e, 
and the angle between the candle and its image, as seen from the ery- 
stals, by 4 e. 
In his observations, the image of the candle is viewed through the te- 
lescope of a theodolite. The angle through which the telescope revolves, 
in order to keep the image of the candle always bisected by the cross 
wires, of course measures 4. A good method of heating the crystals 
is by a small crucible, quite full of mercury, heated by a lamp, into 
which the lower part of the crystals is immersed, as also the bulb of a 
thermometer. The clamp must not be strong, but a kind of weak 
spring, in order that the crystals may experience no mechanical obstruc- 
tion to their change of form. 
pe 
