FUTURE HABITABILITY OF THE EARTH CHAMBERLIN. 375 



consonant. Most of the former were of the unobtrusive sort and 

 awakened little questioning, but some of the facts were startling, 

 some were indeed apparently quite incredible, and as a matter of 

 fact were long subject to the* suspicion of being the offspring of illu- 

 sion or inaccuracy. Only very slowly under the influence of repeated 

 confirmation did they gain credence. The gathering of this incon- 

 sonant data gradually weakened the hold of the inherited concept and 

 prepared the way for a reconsideration. 



Meanwhile, in the progress of physics, a serious source of doubt 

 had arisen respecting the tenability of the gaseous basis of the con- 

 cept. The older hypotheses of the origin of the earth were framed 

 before the kinetic theory of gases came into currency. After the 

 kinetic theory had been accepted, it was urged, notably by Johnstone 

 Stoney, that the velocities of some of the molecules of the outer air 

 must be such as to give rise to their escape, and thus to put a limit 

 to the amount of atmosphere which the planet could hold. When a 

 test of this type was brought to bear on the vast hot atmosphere 

 assigned the primitive earth, it gave rise to doubt as to the physical 

 tenability of the concept. 



Weakness also arose in another quarter. One of the main props 

 of the gaseous and quasi-gaseous hypotheses of the earth's origin, was 

 the conclusion that a condensation from any other dispersed state 

 than the gaseous or quasi-gaseous would lead to revolutions and 

 rotations in directions opposite to those actually possessed by most 

 of the planets and satellites. A closer examination of this deduction 

 under the stimulus of the doubt that had arisen from the kinetic 

 test showed weakness here also, and even a reversal of probabilities, 

 for it appeared that a slow ingathering of matter from a scattered 

 disk-like orbital state would give revolutions and rotations even 

 more consonant with the actual facts than would centrifugal evolu- 

 tion for a gaseous globe, as previously postulated. 



Thus, toward the close of the last century, there arose from differ- 

 ent quarters cogent reasons for a restudy of the whole subject. 

 Further scrutiny added new sources of doubt, and in the end the 

 tenability of all the gaseous and quasi-gaseous hypotheses was chal- 

 lenged and a new genus of hypotheses, based on orbital dynamics, 

 in contradistinction from gaseous dynamics, was offered instead. 



It is not appropriate for me to say that this challenge was suc- 

 cessfulh^ supported, or that the older concepts of the earth's origin 

 are to be laid on the shelf. As an advocate of the method of multi- 

 ple working hypotheses, it belongs to me rather to beg of you to 

 keep in use — so far as you find in them any working quality — all 

 hypotheses that yield any wholesome stimulus to inquiry. 



Much less would it be appro])riate for me to affirm that any form 

 of the newer concepts is entitled to take the place of the older in 



