October 13, 1921] 



NATURE 



21^ 



What does the poet or the artist tell us? Does 

 he not claim that what springs up within him — if 

 it be. in truth (he may add) in any valid sense his 

 — is quite inexplicable on what he regards as 

 psychological principles? And if psychological 

 principles deal only with conscious integration he 

 is right. His poetry, or his art, is not in its 

 essential nature the outcome of perceptive or re- 

 flective integration. Its well-springs lie deeper 

 than that in the unconscious. He rightly affirms 

 that the real thing in all true art is beyond his 

 conscious control, though the means by which it 

 is expressed must be learnt and may be bettered 



by taking thought. This is enshrined in the pro- 

 verb : Foeta nascitur non jit. And even of those 

 who can only appreciate his work, may it not be 

 said, with a touch of paradox, that enjoyment in 

 art becomes reflectively conscious in criticism? 

 This need not mean that the critic enjoys poetry 

 any the less for the combination in higher inte- 

 gration of unconscious and conscious enjoyment. 

 What it does mean is that the glad newness and 

 glory of surprise lies in the poetry and not in the 

 criticism. Once again it must be said that it is 

 the fresh unexpectedness that is still the hall-mark 

 of the unconscious. 



The Age of 



ALTHOUGH it cannot be claimed that the 1 

 joint discussion on the " Age of the Earth," 

 at the meeting of the British Association at Edin- 

 burgh on September 13, led to the complete 

 reconciliation of the views of the various sections 

 of the Association represented, there could be no 

 doubt concerning the extraordinary interest taken 

 in it. Members desiring admission over- 

 taxed the capacity of one of the largest lecture 

 theatres in Edinburgh, and shortly after the all 

 too short discussion one might overhear in the 

 streets of the city the remark, " They haven't 

 settled it yet-" It was quite evident that it was 

 a good thing, if merely for the dissemination of 

 modern views on the subject, that authoritative 

 representatives of each science should address 

 the same composite audience of physicists, geo- 

 logists, biologists, and many who would claim 

 none of these descriptions. 



It was not surprising that the starting point 

 of all the speakers was the inadequacy of Lord 

 Kelvin's estimate of twenty million years for the 

 age of the sun. Lord Rayleigh, whose lucid 

 of>ening of the discussion will long be remem- 

 bered, evidently believed that Kelvin had covered 

 his estimate sufficiently with the proviso concern- 

 ing sources of solar energy other than gravita- 

 tion. Such sources, i.e. radio-active materials, 

 Kelvin was unaware of, but we now know them 

 to exist in the earth, and must presume them 

 also to exist in the sun. Lord Rayleigh pro- 

 ceeded to develop his argument for arriving at 

 the age of uranium-bearing rocks from considera- 

 tions of the uranium-lead and helium which they 

 now contain. The order and rate of radio- 

 active disintegration through the series from 

 uranium to lead are known with considerable pre- 

 cision ; helium also is evolved at a definite ascer- 

 tained rate. An examination of the amount of 

 lead now present in uranium minerals enables 

 the time when disintegration commenced to be 

 specified, for the lead in the rocks in question 

 proves to be not ordinary lead but wholly that 

 isotope of atomic weight 206 which is necessarily 

 associated with the decay of uranium. Thus 

 broggerite found in the pre-Cambrian rocks at 

 Moss, Norway, contains lead of atomic weight 

 NO. 271 1, VOL. 108] 



the Earth. 



20606; the lead-uranium ratio is 01 13, and this 

 points to an age of 925 million years, upon the 

 assumption that uranium and its products have 

 always decayed as they do now. Estimates of age 

 can also be made by measuring the content in 

 rocks of that other product of disintegration — 

 helium — although leakage of this gas makes the 

 calculation less reliable. Allowing for this, how- 

 ever, the indications by helium content are 

 generally confirmatory of those given by the lead- 

 uranium ratio. These methods can be applied to 

 younger formations of rocks, thus obtaining the 

 approximate age of each. 



Lord Rayleigh pointed out that Prof. H. N, 

 Russell, by applying the argument statistically to 

 the earth's crust as a whole, arrived at the period 

 8 X 10^ years as an upper limit— this being six 

 times longer than that of any individual rock 

 yet examined. He concluded by giving a period 

 amounting to a moderate multiple of 1000 million 

 years as the probable duration of the earth's 

 crust in a condition suitable for the habitation of 

 living- beings. The radio-active investigations 

 leading to this conclusion are supported by other 

 physical and astronomical evidence. 



Prof. Sollas, who followed, made merry on 

 behalf of the geologist, "newly enriched " from 

 a " bankrupt " with a " mere score of millions 

 of years " to a " bloated capitalist— w^ith more 

 millions in the bank than he knew how to dispose 

 of." Within broad limits, he said, geologists 

 were ready to leave to the physicists the precise 

 calculation of geological time. Some geologists, 

 notably the brilliant and lamented Barrell, had 

 already begun to rebuild their science on the mag- 

 nified scale. For himself, he preferred first " to 

 make sure that the new radio-active clock was not 

 as much too fast as Lord Kelvin's was too slow." 

 In this connection Prof. Sollas directed attention 

 to Prof. Joly's examination of the " pleochroic 

 haloes " occurring in uranium-bearing black mica. 

 These haloes, which are formed by the a-ravs ex- 

 pelled by uranium in the various stages of its 

 disintegration, are found generally to have ranges 

 consistent with those obtaining in modern times. 

 The two inner rings, however, form a notable ex- 

 ception, indicating ranges greater than normal 



