i9o8.| THE PHYSICS OF THE EARTH. 187 



dently appeared to Arrhenius five years before) that matter under 

 these extreme conditions must be essentially gaseous ; and as it is 

 above the critical temperature, it is made to behave in confinement 

 as an elastic solid. Now in all gaseous masses the density is pro- 

 portional to the pressure so long as the gas remains perfect ; and 

 the gas does not cease to be perfect when the temperature is above 

 the critical value, though it may acquire in confinement the property 

 of an elastic solid if the pressure be great enough to bring the 

 molecules within a distance at which the molecular forces become 

 effective in spite of the high temperature. Thus while the property 

 of rigidity in cold solids depends wholly on molecular forces 

 which prevent deformation, this property for gaseous matter in 

 confinement under such pressure that it acquires the property of an 

 elastic solid, is due wholly to the pressure. The molecular forces 

 giving effective rigidity must increase in proportion to the pressure, 

 or in a higher ratio. 



If according to hypothesis the matter is made solid by pressure, 

 then the molecular forces resisting deformation in the imprisoned 

 matter thus solidified cannot resist deformation in a less degree than 

 the direct proportion to the pressure on which the solidification de- 

 pends. And any ratio higher than the direct proportionality to the 

 pressure would most likely depend on the temperature. Now the 

 temperature in the earth is supposed to be everywhere such as to 

 make the density conform essentially to Laplace's law; and the pres- 

 sure resulting from this law of density gives the matter everywhere 

 the property of an elastic solid, and therefore its molecular proper- 

 ties must correspond to the physical state determined by the laws of 

 density and pressure. 



It is of course conceivable that some parts of the globe might 

 be relatively more rigid than is required to give solidity, but the 

 effect of this would only increase the average rigidity of the earth 

 as a whole. And since seismological and other observations seem 

 to show that the globe is solid throughout, except a thin layer just 

 beneath the crust, the hypothesis of a rigidity proportional to the 

 pressure will give a true minimum value of the earth's rigidity. 



Now on the hypothesis that the density follows Laplace's law, 



