﻿Chemistry and Physics. 399 



eight to five.* With Langley's value Thomson's estimate of time 

 would have to be reduced by the factor five-eighths. 



It has been suggested by Croll that the primitive solar nebula 

 may have been hot. This heat must have arisen from the col- 

 lision of two or more masses ; if their relative velocity before 

 collision was that due simply to their mutual attraction, the heat 

 so generated is already counted in the heat generated by the con- 

 centration of the sun from a state of infinite dispersion. But if 

 the relative velocity existed otherwise than from their mutual 

 attraction, then the total heat in the sun might exceed that due 

 simply to concentration. Sir William Thomson considers the 

 hypothesis very improbable. The term improbability seems, how- 

 ever, almost to lose its meaning in these speculations, and at least 

 we know by the spectroscope that actual nebulae do consist of in- 

 candescent gases. 



In considering these three arguments I have adduced some 

 reasons against the validity of the first argument, and have en- 

 deavored to show that there are elements of uncertainty surround- 

 ing the other two ; nevertheless they undoubtedly constitute a 

 contribution of the first importance to physical geology. While 

 then we may protest against the precision with which Professor 

 Tait seeks to deduce results from them, we are fully justified in 

 following Sir William Thomson, who says that " the existing 

 state of things on the earth, life on the earth, all geological his- 

 tory showing continuity of life, must be limited within some such 

 period of past time as 100,000,000 years." 



If I have carried you with me in this survey of theories bearing 

 on geological time, you will agree that something has been ac- 

 quired to our knowledge of the past, but that much more remains 

 still to be determined. Although speculations as to the future 

 course of science are usually of little avail, yet it seems as likely 

 that meteorology and geology will pass the word of command to 

 cosmical physics as the converse. At present our knowledge of a 

 definite limit to geological time has so little precision that we 

 should do wrong to summarily reject any theories which appear 

 to demand longer periods of time than now appear allowable. 



In each branch of science hypothesis forms the nucleus for the 

 aggregation of observation, and as long as facts are assimilated 

 and coordinated we ought to follow our theory. Thus even if 

 there be some inconsistencies with a neighboring science we may 

 be justified in still holding to a theory, in the hope that further 

 knowledge may enable us to remove the difficulties. There is no cri- 

 terion as to what degree of inconsistency should compel us to give 

 up a theory, and it should be borne in mind that many views have 

 been utterly condemned, when lal er knowledge has only shown 

 us that we were in them only seeing the truth from another side. 

 — Nature, Sept. 2. 



* Langley (Ann. Rep. JR. A. S., 1885) estimates that 3 calories per minute are re- 

 ceived by a square centimeter at distance unity. This gives for the total annual radi- 

 ation of the sun 4*38 x 10 33 calories. Thomson gives as Pouillet's estimate 6 x 10 30 

 times the heat required to raise 1 lb. of water 21° Cels., or 2*7 x 10 33 calories. 



