55 
The number of small crystals of other minerals included in rubies is 
often very great. There must be at least four different kinds ; but it 
would be difficult to determine what minerals they all are. Some are very 
well characterized octahedrons, variously modified ; and, as .shown by fig. 
5, their planes are very generally arranged parallel to planes of the ruby, 
and to the small plate-like crystals already mentioned in describing 
sapphire. These octahedrons have no influence on polarized light, and in 
general form and character correspond so closely with spinel, that it seems 
very probable that they are that mineral For some time we thought 
they were angular fluid-cavities filled with liquid ; but when cut across in 
the sections, they are clearly seen to be solid, though less hard than ruby. 
Many of the other included crystals are of such very rounded forms that, 
if it were not for their action on polarized light, they might easily be 
mistaken for cavities filled with some fluid. Most of these rounded 
crystals are colourless; but some are of more or less dark orange-red 
colour, and are certainly not the same mineral as the colourless or the 
octahedral crystals ; and in all probability the thin and flat are a fourth 
kind. Occasionally alternating plates of ruby with their axes in different 
positions, gave rise to a beautiful series of coloured stripes when examined 
with polarized light. 
Spinel. 
The ruby spinels from Ceylon sometimes contain fluid- cavities which 
differ in a striking manner from those of any other mineral that has come 
under our notice. One of these is shown in fig. 7. They are, to a great 
extent, filled with a yellow substance, indicated by the shading, which 
seems to be either a solid or a very viscous liquid. It incloses transparent, 
sometimes well-defined cubic crystals, which have no action on polarized 
light; transparent, prismatic, or plate-like crystals, which strongly 
depolarize it ; and black opake crystals, either in larger pieces or mere 
grains. The rest of the cavity is in each case about one-third full of a 
colourless liquid, which seems to contract on the application of heat, 
because it passes entirely into vapour, as occured in some of the cavities 
in topaz described by Brewster. In this change it must expand about 
six hundred times less than when water passes into steam. Spinel also 
incloses crystals of several other minerals which we have not yet been 
able to identify. 
Aquamarina. 
The most striking peculiarity of this mineral is the occurrence of numbers 
of fluid-cavities containing two fluids and a vacuity, as shown by fig. 6. 
