SIGNIFICANCE OF THE THREEFOLD STATE. 65 



to 365, the original pressure need be only approximately 1,040 atmospheres, 

 a pressure which corresponds to 13,000 feet of rock approximately. 



In this critical case the meniscus appears as soon as the temperature 

 falls below 365. Since pressure exerts but little influence on the volume 

 of liquids, the shrinkage of the water in the cavity, and hence the growth of 

 the gas-bubble, is largely a function of the fall in the temperature, and, with 

 a knowledge of the varying coefficient of expansion, the relation between the 

 size of the bubble and the volume of the liquid could be computed for any 

 temperature. The correction for the constriction of the cavity between 

 365 and 20 amounts to a little more than one per cent, which is to be 

 added to the size of both the cavity and the vacuole in computation. 



If these principles be true, a vapor-bubble relatively smaller than the 

 critical vacuole may be interpreted to mean that the meniscus did not appear 

 in the cavity until the crystal had cooled below the critical temperature, 

 i. e., that at this temperature the water was more than normally condensed, 

 owing to a pressure exceeding the critical pressure. On the other hand, 

 a vapor-bubble relatively larger than the critical vacuole means that, 

 although it did not appear until below 365, it began as a sizable vesicle 

 when it did start, owing to the lower pressure and more rarefied condition 

 of the water-gas. 



On the basis of his experiments, Sorby estimated that a vesicle amount- 

 ing to 28 per cent of the volume of the liquid in the cavity would vanish 

 when the water was heated to 340 C. According to this figure, a vacuole 

 occupying in the neighborhood of 30 or 35 per cent of the volume of the 

 liquid should correspond to a shrinkage of the water from the critical 

 point to ordinary temperatures. But this figure has not been confirmed 

 by other investigators. Unfortunately the figures obtainable for the ex- 

 pansion of water up to the critical point vary within such wide limits that 

 it does not seem advisable at the present time to attempt to calculate the 

 relative sizes of the critical vacuole arid the inclosing liquid. 



The difficulties involved in applying these principles are considerable. 

 Zirkel has pointed out that, even in cavities within the same crystal, there 

 is much variation in the relative volume of the vapor-bubble and the 

 liquid, from which the inference is drawn that the vapor-bubbles are due 

 to causes other than contraction on cooling. 1 Before this conclusion can 

 be accepted with confidence, due consideration must be given to the loca- 

 tion of the cavities within the crystal, and also to the evidence that they 

 are all primary inclusions. In an ascending lava subject to a steadily 

 diminishing pressure, those cavities formed during the early stages of 

 crystallization may be developed under conditions quite different from the 

 cavities later inclosed in the outer parts of the crystals. If systematic 

 differences in the cavities can be found to correspond with variations in 

 their location, something might be learned of the history of the lava during 

 the period of crystallization. Secondary fluid inclusions, formed subse- 

 quent to the solidification of the magma, must obviously be recognized 

 and avoided, whenever possible, in attempting to estimate the conditions 

 under which crystallization took place. 



1 Zirkel, cited by Geikie, Textbook of Geology, 1, p. 145. 



