These cavities, which occurred in _sapphire_, _chrysoberyl_, _topaz_, _beryl_, _quartz_, _amethyst_, _peridot_, and other substances, were sometimes sufficiently large to be distinctly seen by the naked eye, but most frequently they were so small as to require a high magnifying power to be well seen, and often they were so exceedingly minute, that the highest magnifying powers were unable to exhibit their outline. [Illustration: _Fig. 80._] The greater number of these cavities, whether large or small, contained two new fluids different from any hitherto known, and possessing remarkable physical properties. These two fluids are in general perfectly transparent and colourless, and they exist in the same cavity in actual contact, without mixing together in the slightest degree. One of them expands _thirty_ times more than water, and at a temperature of about 80° of Fahrenheit it expands so as to fill up the vacuity in the cavity. This will be understood from the annexed figure, where A B C D is the cavity, _m n p o_ the highly expansible fluid in which at low temperatures there is always a vacuity V, like an air-bubble in common fluids, and A _m n_, C _o p_, the second fluid occupying the angles A and C. When heat such as that of the hand is applied to the specimen, the vacuity V gradually contracts in size, and wholly vanishes at a temperature of about 80°, as shown in Fig. 81. The fluids are shaded, as in these two figures, when they are seen by light reflected from their surfaces. [Illustration: _Fig. 81._] [Illustration: _Fig. 82._] When the cavities are large, as in Fig. 82, compared with the quantity of expansible fluid _m n p o_, the heat converts the fluid into vapour, an effect which is shown by the circular cavity V becoming larger and larger till it fills the whole space _m n o p_.