82 



NATURE 



[March 17, 192 1 



value which is equal to the frequency of the X-rays 

 multiplied by the same constant Iti. It is to be 

 observed that we cannot measure all the electron 

 velocities as soon as they exist because some of 

 the motions begin in the body of the substance, 

 into which the X-rays have penetrated, and have 

 lost speed on the way out. Again, therefore, there 

 is nothing against the hypothesis that the energy 

 of every electron set going by waves of given 

 frequency is originally the same, and is deter- 

 mined by the standard condition already given. 



Not only in the case of ^-rays are these effects 

 observed, but also in the case of light. The only 

 difference is that the frequencies of light vibrations 

 are some 10,000 times less than those of X-rays, 

 and the electron energies correspondingly 

 smaller. When the light waves produce the elec- 

 trons we have what is known as the photo-electric 

 effect. The production of light by electrons has 

 been much studied recently in experiments to find 

 " resonance-potentials " — that is to say, the magni- 

 tudes of potentials which must act on electrons so 

 as to give them enough energy to excite certain 

 particular radiations from atoms on which thev 

 fall. 



Exactly how this strange transfer of energy 

 from one form to another takes place we do not 

 know : the question is full of puzzles. The mag- 

 nitudes involved are hard to realise ; it helps if 

 we alter their scale of presentment. Suppose that 



the target of the X-ray bulb were magnified in 

 size until it was as great as the moon's disc — that 

 is to say, about a hundred million times. The 

 atoms would then be spheres a centimetre or so 

 in diameter. But the electrons would still be in- 

 visible to the naked eye. The distance from earth 

 to moon would correspond roughly to the distance 

 that ordinarily separates the bulb from an ob- 

 server or his apparatus. We now shoot the en- 

 larged electrons at the moon with a certain velo- 

 city ; let us say that in every second each square 

 yard or square foot or square inch, it does not 

 matter which, receives an electron. A radiation 

 now starts away from the moon which immediately 

 manifests itself (there is no other manifestation 

 whatever) by causing electrons to spring out of 

 bodies on which it falls. They leap out from the 

 earth, here one and there one ; from each square 

 mile of sea or land, one a second or thereabouts. 

 They may have various speeds ; but none exceed, 

 though some will just reach, the velocity of the 

 original electrons that were fired at the moon. 

 That, reduced again to normal size, is the process 

 that goes on in and about the X-ray bulb : which 

 is part of a universal natural process going on 

 wherever radiation, electron or wave, falls on 

 matter, and which is clearly one of the most im- 

 portant and most fundamental operations in the 

 material world. 



{J^o he continued.) 



Obituary. 



The Rt. Hon. Lord Moulton of Bank, F.R.S. 



THE news of the sudden death of Lord Moulton 

 on March 9 came as a shock to all who had 

 been associated with his many activities. Notwith- 

 standing his advanced age — he was in his seventy- 

 seventh year — he was so full of vigour that all his 

 friends had looked forward to some further years 

 of activity for the good of the country he loved 

 so well, and for which he rendered such mag- 

 nificent services. He died in the midst of his 

 work; the very day before his death he was 

 engaged in hearing an appeal at the House of 

 Lords. A short time before, he delivered a speech 

 on behalf of the chemical industries of the country 

 with all his customary lucidity and vigour, and 

 again on February ig he showed his delightful 

 personal charm as chairman of a " Saturday Even- 

 ing " at the Savage Club. These random incidents 

 rnight almost be taken as typical of the outstand- 

 ing qualities of the man — the brilliant judge and 

 lawyer, the man of science and patriot, and the 

 genial companion whose sympathy and humour 

 helped to brighten many a life, and never 

 more than in the dark days of the war, when he 

 was always ready to cheer and inspire those 

 around him and to lead the way in meeting one 

 difficulty after another. 



After his brilliant career previous to the war, in 

 which he had shown himself an adept at science, 

 classics, law, and politics, as well as an athlete 

 NO. 2681, VOL. 107] 



and a linguist. Lord Moulton might well have 

 been content to rest upon his laurels, but unques- 

 tionably his greatest achievements were for the 

 cause of his country, when, at the age of seventy, 

 he took up a burden which would have taxed the 

 endurance of the strongest man, and set himself 

 to organise the resources of the country to obtain 

 the explosives necessary for the war. Looking 

 back upon his earlier career, it might almost seem 

 that his numerous activities were directed by 

 destiny towards the great climax of his life. Cer- 

 tainly they formed a unique training which fitted 

 him for his supreme task in a way which could 

 scarcely have been paralleled. 



Lord Moulton was born on November 18, 1844, 

 at Madeley, his father being the Rev. James Egan 

 Moulton, a Wesleyan minister. After passing 

 through the Wesleyan school at New Kingswood. 

 near Bath, he entered St. John's College, Cam- 

 bridge, and had a brilliant career as a student. 

 In 1868 he became Senior Wrangler and first 

 Smith's prizeman, and took a gold medal at 

 London University. He was elected a fellow and 

 lecturer at Christ's. His academic career was 

 not of long duration. In 1874, at the age of about 

 thirty, he was called to the Bar, and speedily 

 became famous as a specialist in patent cases. 

 His scientific training gave him a great advantage 

 in dealing with such subjects, and he was en- 

 trusted with many cases involving very large 



