SYMPOSIUM ON RADIOACTIVITY. 67 



here by alternative hypotheses.. The extrapolation of the known 

 fact under experimental pressures is of course entitled to prece- 

 dence and this alternative was used as the basis of the first 

 approximation to the heat curve of the earth's interior. For the 

 other factors, such as specific heat, necessarily taken into account 

 in the computation, assumptions as near to known facts as possi- 

 ble were made. On these assumptions it was foimd that the heat 

 generated between the surface and the center of the earth may 

 be represented by a curve which rises at a very low rate near the 

 surface and is followed by a slowly increasing rate for about one- 

 third the distance to the center, beyond which it rises at a decreas- 

 ing rate to the center ; or, if traced from the center outward, this 

 computed curve of temperature declines faster and faster at everj' 

 step for about two-thirds of the distance and then declines less 

 and less rapidly to a vanishing point near the surface. Hence, if 

 conductivit)- be assumed to be the same at all depths, the outward 

 flow of heat on such a gradient would increase in rate from the 

 center to the two-thirds point and then grow slower toward the 

 surface, from which it follows that, on these assiunptions of uni- 

 form compressibility and uniform conductivity- taken by them- 

 selves, the internal heat should have been progressively lowered 

 in the deep interior and raised in the more superficial parts. The 

 conductivit}- of rocks is so ver}- slow, however, that its effects at 

 the surface under the conditions named caimot have been large 

 up to the present unless the earth is much older than even radio- 

 activity seems to imply. 



This first approximation to a theoretical curve of heat, even 

 when modified by conduction, has not been supposed to represent 

 the actual distribution of heat at the present time, for reasons 

 that follow. 



There is ground to think that compressibilit}- falls ott as 

 increased degrees of compactness are attained. In working out 

 the curs-e which was published in Vol. I of Chamberlin and Salis- 

 bury's Geolog}-, p. 566, Dr. Limn used as a guide the Laplacian 

 law of densit}' which postulates that density varies as the square 

 root of the pressure. This distribution of density- harmonizes 

 fairly well with such astronomical tests as are available and gives 

 a mean density for the earth which is near that required by the 

 earth's total weight. The assimiption that the increased density 

 of the interior is all due to compression, however, makes no 

 allowance for the probable transfer of lighter matter to or toward 



