54 
polarization so as to coincide with one of the axes of the crystal. High 
powers may then be used with perfect definition ; and they show many 
small cavities, sometimes of most irregular forms, like fig. 4 ; and very 
often their sides are so inclined, that they totally reflect transmitted light, 
and appear black and opake. In some specimens most of the cavities 
have lost their fluid. 
Besides fluid-cavities, there are many small crystals of other minerals 
included in sapphires, but not so many as in rubies. The most striking 
are small plate-like crystals, often of triangular form, with one angle very 
acute. They are very thin, and give the colours of thin plates; so that 
when viewed by reflected light they look something like the scales from a 
butterfly. Seen edgewise, they appear as mere black lines, and are 
arranged parallel to the three principal planes of the sapphire, as shown 
by fig. 5. These small crystals and the minute fluid-cavities cause many 
sapphires to appear milky by reflected, and somewhat brown by trans- 
mitted light; and being arranged in zones related to the form of the 
crystal, they often show, as it were, lines of growth. 
Rubies. 
Though the ruby and the sapphire are of course essentially the game 
mineral, yet their structure is in many respects as characteristically 
different as their colour. The number of fluid- cavities in rubies is far less, 
and the larger cavities are very rare, and only contain what appears to be 
water or a saline aqueous solution, as is shown by the amount of expansion 
when the specimen is heated to the temperature of boiling water. Those 
containing a similar fluid to that included in sapphires do occasionally 
occur ; and when they are minute they are extremely interesting, since 
they show the spontaneous movement of the bubbles to greater perfec- 
tion than any mineral that has come under our notice. This is perhaps, 
to some extent, due to the nature of the liquid, which is more mobile 
than the saline aqueous solutions contained in the cavities of the quartz 
of granite and syenite. It is manifestly a molecular movement analogous 
to that seen in all matter when very minute particles are suspended in a 
liquid, so as to allow freedom of motion ; and the rapidity of the move- 
ment is certainly dependent on the size of the particles. It is not seen 
to advantage if the diameter of the bubbles is more than T _J__of an 
inch; but when it is about ^-J-^ they move to and fro in the most 
surprising manner, with such rapidity that the eye can scarcely follow 
them. 
