What is the nature of the liquids which are thus imprisoned in these cavities contained in the crystals of lavas and granites? Careful experiments have given a conclusive answer to this question. In many cases the liquid is water, usually containing considerable quantities of saline matter dissolved in it. Sometimes the saline matters are present in such abundance that they cannot all pass into solution, but crystallise out, as in fig. 7--Nos. 3, 4, 5--where cubic crystals of the chlorides of sodium and potassium are seen floating in the liquid; in other cases the liquid is a hydrocarbon like the mineral oil which is present in great abundance in deep-seated rocks in many parts of the globe. But in some other cases the liquid contained in the cavities of crystals is found to be one which could scarcely be anticipated to occur under such circumstances--the gas known as carbonic add, which under extreme pressure can be reduced to a liquid condition. In cavities containing liquefied carbonic acid, if the rock be warmed up to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with an appearance like ebullition or boiling, as represented in fig. 9. Now the temperature which we have indicated is the 'critical point' of carbonic acid, and above that temperature it cannot exist in a liquid condition, however great may be the pressure to which it is subjected. The liquid has been converted into a gas which completely fills the cavity. The carbonic acid in the cavities of crystals has frequently been isolated and its nature placed beyond doubt by spectroscopic and ordinary chemical tests. The presence of these liquids in the cavities of crystals clearly proves that the latter must have been formed under enormous pressure--a pressure sufficiently great to reduce, not only steam, but also volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a liquid. [Illustration: Fig. 9.--Cavity in Crystal containing Carbonic-Acid Gas at a temperature of 86° F., and passing from the liquid to the gaseous condition.]