THE PRESIDENTIAL ADDRESS 7 



been dissipated in steam long before he had had a chance of dis- 

 playing the advantageous variation which was to make him the 

 ancestor of the human race.' When Perry read this pronouncement, 

 sweeping aside the firm convictions of biologists and geologists, he 

 was led to re-examine the evidence and soon found a flaw. The 

 heat of the earth had been calculated on the assumption of a con- 

 ductivity uniform through the whole mass, but Perry showed that 

 with a conductivity becoming higher with increasing depth the 

 Kelvin-Tait estimate of the time required for cooling to the existing 

 temperature — on which the age of the habitable earth had been 

 based — must be immensely lengthened. Perry told me of this 

 destructive criticism and very kindly helped me to make use of it 

 in the address to Section D at Liverpool in which I replied to Lord 

 Salisbury's amusing attack on the evolutionists. 



Lord Lister was our President at Liverpool in 1896, and I cannot 

 resist the temptation to digress for a moment and recall the address 

 in which one of the greatest benefactors of mankind told us, with the 

 utmost simplicity and modesty, the story of his life's work and the 

 success which, in spite of all opposition, had been achieved. To 

 hear him was an enduring inspiration. 



The year 1896 was also the Jubilee of Lord Kelvin's wonderful 

 half-century of achievement in research and teaching, and I could 

 not help feeling some regret that any criticism of his work 

 should appear at this particular time. But in the kindly spirit 

 of our Association such doubts were quite unnecessary. I well 

 remember how he came one day to our Sectional Committee-room 

 to bring me some volumes of his works, and how, as I have recorded 

 before, in the following year as we were travelling across Canada 

 after the Toronto Meeting and the chance of collecting insects for 

 a few minutes at each station could not be resisted, Lord Kelvin 

 said to his wife, ' My dear, I think we must forgive Poulton for 

 thinking that the earth is so very old when he works so hard in one 

 day out of all the endless millions of years in which he believes 1 ' * 



The one line of evidence which left some anxiety in 1896, was 

 suggested by Helmholtz who allowed the sun only eighteen million 

 years to have been giving out radiant heat at the present rate — a 

 period Lord Kelvin was willing to extend to 500 million — and this 

 estimated maximum was also accepted by Sir George Darwin, who, 

 in his address ^ at Cape Town in 1905, spoke of the new evidence 

 obtained by M. and Mme. Curie in their proof that radium gives 

 out heat, and, quoting in confirmation the work of R. J. Strutt, 

 W. E. Wilson, and G. H. Darwin, finally concluded that ' the 

 physical argument is not susceptible of a greater degree of certainty 



* Report, British Association, Centenary Meeting, 1931, p. 78. 

 5 Report, British Association, 1905, pp. 514-518. 



