2i8 The Afnerican Geologist. October, i898 
this little instrument Mr. Sorby found that the liquid, which, 
as shown, filled one-half of the cavity at ordinary temperatures 
such as 50° F., expanded so rapidly that its volume was 
doubled and the cavity was full at 89° F. This increase of 
bulk within so narrow limits of temperature at once excluded 
all ordinary liquids, and by further investigation and com- 
parison Mr. Sorby was able to decide that the substance was 
nothing less than liquid carbonic acid, the only known liquid 
whose rate of expansion was equally great. Here was a solid 
fact contributed by the microscope toward the solution of 
some of the difificult and complicated problems presented by 
the physics of the earth's crust, and, again, we shall find from 
this study of a drop of liquid almost infinitely little, contained 
in an instrument equally minute, may flow results of great 
moment and far-reaching consequence. It is not the size but 
the solidity of the premises that authorizes the conclusions. 
Granting, as we must, that the little drop is carbon-dioxide 
in the liquid form, we can safely advance by reasoning on the 
known properties of this substance somewhat as follows: 
The critical temperature of carbon-dioxide is about 88" F. 
(87.6°), that is to say, above this it exists only as a gas, and can 
by no pressure be liquified. Now it is in the highest degree 
improbable that at the time and in the conditions when the 
crystalline rocks were formed the surface of the earth was 
below this point. On the other hand, we may confidently rely 
on its having been far above this critical temperature. Obvi- 
ously then the carbon-dioxide must have been scaled up in the 
crystals in a gaseous state — a bubble of carbonic anhydride. 
Here the problem becomes indeterminate. Both the original 
temperature and pressure are unknown. But arguing from 
what we know of the physics of this substance we may deduce 
the following conclusion: At present ordinary temperature, 
50° F., the pressure in the microscopical registering ther- 
mometer of Mr. Sorby, must amount to about forty-eight at- 
mospheres or 720 pounds on the square inch. This little in- 
strument was exactly filled at 89° F., very near the critical 
temperature. At this point the minimum pressure which will 
enable the carbonic anhydride to retain the liquid state is 
seventy-three atmospheres or 1,100 pounds on the square inch. 
Consequently, if as inevitable, we assume a higher tempera- 
