TOURMALINE &ec., WITHIN MICA AND OTHER MINERALS. 549 
tinctly seen in the mica, and cracks or luminous streaks often occur in the 
tourmaline plates themselves. I have observed, too, in portions of the mica 
where I cannot find any cavities or crystals, distinct luminous sectors of polar- 
ised light, which could only be produced by a force emanating from their centre. 
This force may have been that of gas discharged from some neighbouring cavity, 
and driven by change of temperature to some other part of the mica plate; and 
in the following remarkable phenomenon we may perhaps find some evidence in 
favour of this opinion. 
Plates of mica contain many beautiful systems of Newton’s rings, occupying a 
_ circular space where the laminze have been separated by some cause or other, 
and where, of course, there must be either air or some gaseous body. The colours 
of the first order are at the circumference of the circular space where the laminze 
are in optical contact, and the higher orders of colour extend towards, and often 
to the centre of the space. Now it is a curious fact, that wherever there is a 
| cavity which has projected its fluid and probably gaseous contents, it 7s sitwated 
in the circumference of one of these circular spaces. When two cavities have been 
near each other, the circular spaces unite and lose their form, and when the cavi- 
ties have been more numerous, the circular spaces unite into very irregular 
shapes. That these circular hollows or spaces between the laminz have been 
produced by something which has issued from the cavity to which they are so 
constantly related, cannot admit of a doubt. That it has not been a fluid is evi- 
dent, and therefore it must have been a gas, which is either there still, or has 
escaped through some minute openings between the lamine, where optical contact 
has been restored.* 
There are some specimens of mica in which the crystals of tourmaline are 
large and opaque, and exhibit phenomena which I believe have not been recog- 
nised in any other mineral. The most interesting specimen of this kind I owe to 
Professor FLemine, who pointed out to me one of the peculiarities which it con- 
tains. This specimen is accurately represented, of the natural size, in Fig. 2. 
The largest of the five crystals is 0°28 of an inch broad, and the smallest 0:08 of 
an inch. Their thickness cannot greatly exceed the thousandth of an inch, and yet 
_ it is with difficulty that the strongest sun-light can be seen through them. The 
form of the smallest is a perfect hexagon, and in the rest the same form is more 
or less distinct. In'the oval crystal there are numerous holes, and in all of them 
_ there are numbers of rectilineal cracks parallel to the sides of the hexagon, and 
- some of them so narrow that light can scarcely pass through them. When we 
look at the sun through one of these crystals, a curious optical phenomenon 
is seen, a luminous hexagonal surface, composed of lines of light, parallel to the 






















* A fluid even may have thus escaped, and the’ circular hollow remained as before. In 
_ support of this opinion, see Edinburgh Transactions, vol. x., p. 11; but especially vol. xvi., p. 13; 
_ or Phil. Mag., vol. xxxi., p. 101, August 1847. 
