734 JOHN JOHNSTON 



— that at the melting-point the volume of the liquid is greater than 

 that of the soHd. 



The best and most extensive experimental work is that of 

 Bridgman,^ who determined the melting-curve of a number of sub- 

 stances at pressures iip to 12,000 atm. — a pressure corresponding 

 to a depth below the surface of the earth of about 30 miles. Some 

 of his data are presented in Table I, which shows especially the 

 gradual diminution of the effect with each successive pressure 

 increment. Bridgman also made direct measurements of the 

 change of volume on melting (AF) throughout his pressure range, 

 and found that it decreases slowly and at a continuously decreasing 

 rate; the shape of the curve suggests that the change of volume 

 would not become zero at any finite pressure. Moreover, by com- 

 bining his data and using the foregoing equation he calculated AZ?; 

 it does not tend toward zero at higher pressures, but remains 

 approximately constant, showing if anything a tendency to increase 

 with the pressure. The general conclusion from this work accord- 

 ingly is that, up to pressures of 12,000 atm. at least, there is no 

 indication of a maximum melting-point, still less of a critical end- 

 point solid-liquid; on the contrary everything indicates that such 

 points, if indeed they exist at all, can occur only at pressures alto- 

 gether outside the range of possible experiment at the present time. 



Table I illustrates another interesting point: namely, that at a 

 pressure of about 5,800 atm. the melting-point of solid CO2 has 

 reached 31°, the critical temperature of the liquid; at pressures 

 higher than this it sublimes or passes directly into a gas. Conse- 

 quently it is in this case actually possible to pass directly from a gas 

 (not a vapor, for it is above the critical temperature) to crystalline 

 solid merely by change of pressure. This would indicate that appli- 

 cation of sufScient pressure would convert even a gas into a solid, 

 and suggests that the existence of any large body of gas deep in 

 the earth is as little likely as the existence of much liquid. 



All of this evidence then tends to corroborate the belief that 

 the central portion of the earth is substantially solid, though this 

 does not imply that this solid will not flow whenever opportunity 



^P. W. Bridgrtian, Proc. Am. Acad., XLVII (1911-12), 347; ihid., XLVIT 

 (1911-12), 439; Physic. Rev., Ill (1914), 126, 153. 



