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POPULAR SCIENCE REVIEW. 
of quartz, and it frequently escapes in a pure state from mineral 
springs and fissures in the earth’s crust. 
A great number of rock sections, chiefly porphyries, granites, 
and basalts, have been examined for liquid carbonic acid, in 
but few cases, however, with success. 
In making these observations it is necessary to get rid of the 
influence of the mineral causing double refraction, by fixing a 
Nicol’s prism on to the microscope stage ; otherwise in small 
water cavities the appearances are very similar to those seen 
when two liquids are present, so that one may be easily deceived. 
An exceedingly useful little contrivance for applying heat to the 
specimens of crystals containing fluid-cavities, consists of a glass 
tube about of an inch in diameter and 12 inches long ; it is drawn 
out at one end to a jet of about of an inch aperture, the jet 
being bent at an obtuse angle at about an inch from the point. 
To prevent the glass being softened and bending when heated, 
it is covered for four inches in its central part by a piece of brass 
tube, which just slides on not too easily. The straight end of 
the tube is somewhat pointed, and passes through an india-rubber 
cork fitting into a, universal joint upon a stand having a sliding 
motion in the upright, so that it may be raised and lowered at 
will. The end of the glass tube which passes through the cork 
has a piece of india-rubber tube slipped over it, 15 inches in 
length, and connected with a ball-syringe, whereby air may be 
drawn in and discharged again. By heating the metal tube 
with a spirit-lamp or Bunsen burner, the air discharged by 
squeezing the ball-syringe will be heated, and may be directed 
on to the object while under the microscope without any dis- 
placement. 
Mr. P. J. Butler has employed a somewhat similar apparatus, 
but of more elaborate construction, for showing the evaporation 
and condensation of carbonic acid in some of his large speci- 
mens. The usefulness of this little instrument will be readily 
understood when it is mentioned that portions of liquid car- 
bonic acid, so small as not to be recognizable under a magni- 
fying power of 800 diameters, have been revealed by its aid. 
They could not have been greater than the 5 q* 00 of an inch in 
diameter, and their presence was made known by the instanta- 
neous change in the appearance of the cavities caused by warm 
air, and then again a reversion to their former appearance by 
subsequent cooling. 
Let us consider what would be the effect of an enormous 
pressure of rock on carbonic acid heated above its critical tem- 
perature : it would be in a condition, as far as we can tell at 
present, capable of unlimited compression, so that, taking into 
consideration the comparative incompressibility of water, it 
would be possible to convert it into a gas with a greater density 
