SCIENCE 



Friday, I^ovember 21, 1919 



CONTENTS 

 Atomic Projectiles and their Collisions with 

 Light Atoms: Sir Ernest EuTHERroKD. . . . 467 



Second Award of the Elliot Medal 473 



Proposed Constitution and By-laws of the 

 American Association for the Advancement 

 of Science 474 



Scientific Events: — 



The Southwestern Geological Society; The 

 Americaai Physical Society; The History of 

 Science and the American Historical So- 

 ciety; The Section of Zoology of the Ameri- 

 can Association ; The Deflection of Light hy 

 Gravitation and the Theory of Relativity.. . iTJ 



Scientific Notes and News 479 



University and Educational News 485 



Discussion and Correspondence: — 

 A Helium Series in the Extreme Vltra-violet : 

 Professor Theodore Lyman. Double Use 

 of the Term Acceleration: Professor C. 

 M. Sparrow 481 



Notes on Meteorology and Climatology : — 

 Aerological Worh — Winds; Airplanes and 

 the Weather : Dr. Charles F. Brooks .... 483 



Special Articles: — 

 A Preliminary Note on Foot-rot of Cereals 

 in the Northwest : B. F. Dana 484 



The New Haven Meeting of the National 

 Academy of Sciences 486 



The American Mathematical Society: Pro- 

 fessor F. N. Cole 487 



Meeting of the Committee on Policy of the 

 American Association for the Advancement 

 of Science : Dr. L. O. Howard 487 



MSS. intended for publication and books, etc., intended for 

 review should be sent to The Editor of Science, Ga 

 Hudson, N. Y. 



ATOMIC PROJECTILES AND THEIR 



COLLISIONS WITH LIGHT 



ATOMSi 



The discovery of radio-activity has not only 

 thrown a flood of light on the processes of 

 transformation of radio-active atoms ; it has at 

 the same time provided us with the most pow- 

 erful natural agencies for probing the inner 

 structure of the atoms of all the elements. 

 The swift a-particles and the high-speed elec- 

 trons or ;8-rays ejected from radio-active bod- 

 ies are by far the most concentrated sources of 

 energy known to science. The enormous 

 energy of the flying a-particle or helium atom 

 is illustrated by the bright flash of light it 

 produces when it impacts on a crystal of zinc 

 sulphide, and by the dense distribution of ions 

 along its trail through a gas. This great store 

 of energy is due to the rapidity of its motion, 

 wihich in the case of the a-particle from radium 

 C (range 7 cm. in air) amounts to 19,000 km. 

 per second, or about 20,000 times the speed 

 of a rifle-bullet. It is easily calculated that 

 the energy of motion of an ounce of helium 

 moving with the speed of the ct-particle from 

 radium C is equivalent to 10,000 tons of solid 

 shot projected with a velocity of 1 km. per 

 second. 



In consequence of its great energy of motion 

 the charged particle is able to penetrate deeply 

 into the structure of all atoms before it is 

 deflected or turned back, and from a study of 

 the deflection of the path of the a-particle we 

 are able to obtain important evidence on the 

 strength and distribution of the electric fields 

 near the center or nucleus of the atom. 



Since it is believed that the atom of matter 

 is, in general, complex, consisting of positively 

 and negatively charged parts, it is to be antici- 

 pated that a narrow pencil of a-particles, after 

 passing through a thin plate of matter, should 



1 An address before the Royal Institution of 

 Great Britain, June 6, 1919. 



