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cavities containing fluids convinced him that, if pitchstone and 
glass are types of igneous-formed substances, granite must be of 
aqueous origin. In the fluid cavities so abundant in topaz, Cairn- 
gorum, beryl, tourmaline, and felspar, all constituents of granite, he 
found the same appearance prevailed. These cavities are seldom 
entirely filled with fluid, an air-bubble usually occupying more or 
less of the cavity. After many hundred experiments on such 
cavities, the author found that when exposed to a temperature of 
94° Fahr., the bubble disappeared, the fluid entirely filling the 
cavity, and at the temperature of 84° the bubble reappeared with a 
singular ebullition, showing that the air had formed an atmosphere 
round the fluid. He was thus led to infer that those cavities could 
not have been filled at a temperature above 84°, and certainly not 
above 94° of Fahrenheit. 
As another proof that these cavities could not have been filled 
when the temperature of the surrounding rock was higher than the 
temperature above indicated, the author drew attention to the fact, 
that the bubble of air occupied always a much smaller portion of 
the cavity than the fluid, a condition which could not obtain, if, as 
other writers hold, the fluids were enclosed under intense heat and 
pressure. 
For the purpose of accurately determining the temperatures at 
which the bubble vanished and reappeared, the author constructed 
an apparatus which he exhibited and described. It consists of a 
microscope with a hollow iron stage, having a tube in the centre to 
admit light from the reflector. At one side, and inserted into the 
stage, is a small tin retort with a stopper ; at the other side, a tube 
is inserted and attached to a reservoir of water, from which the 
hollow stage and retort are filled. On applying heat to the retort, 
by means of a spirit-lamp, any required temperature under the 
boiling-point of the water may be obtained in the stage and retort. 
Above the stage is placed an iron saucer, in the centre of which 
an iron tube is rivetted, through which the light is admitted ; this 
vessel is filled with mercury, and in it is placed an upright ther- 
mometer, with the bulb shielded with cork or any other good non- 
conductor ; by this means it indicates the actual temperature of the 
mercury bath. The cavity to be observed is cemented with Canada 
balsam to a plate of glass 3x1 inch, and is floated on the surface 
of the mercury, so that the glass and mercury are in absolute con- 
