NA TURE 



[November 5, 1908 



by the electric power companies has already reduced 

 their beauty, and if the existing projects are carried 

 into effect the American Fall will be reduced to a 

 few threads of water and the Canadian Fall rendered 

 comparatively insignificant. The second danger is the 

 possible tilting of the area of the Great Lakes, which 

 would, at the rate of movement estimated by Dr. 

 G. K. Gilbert, bring Niagara to a close in about 3500 

 or 5000 years. This theory is of great interest, as it 

 has been generally advanced as the best established 

 case of a still progressing uplift of a large area of the 

 earth's crust. Dr. Spencer, however, rejects this con- 

 clusion, and though he lays great stress on recent 

 earth-movements in the region to the north-east of 

 the Great Lakes, he claims that the lake region itself 

 has been quite stable, and that no earth-movements 

 are now taking place there. The facts advanced to 

 prove the supposed uplift he holds can be explained by 

 seasonal and meteorological changes. 



matter which are ejected from radio-active matter at 

 a speed of about 10,000 miles per second. The great 

 number of o particles which are projected from 

 radium is well illustrated by the multitude of scintilla- 

 tions observed when the a particles from a trace of 

 radium fall on a screen of zinc sulphide. We shall 

 see later that 136 million a particles are expelled 

 every second from one milligram of radium in radio- 

 active equilibrium. From the point of view of modern 

 theory, the appearance of an a particle is the sign 

 of a violent atomic explosion in which a fragment 

 of the atom — an a particle — is ejected at a high speed. 

 In the majority of the known active substances, the 

 expulsion of an a. particle accompanies the transforma- 

 tion of one substance into another, and the decrease 

 of atomic mass consequent upon the loss of an a 

 particle at once offers a reasonable explanation of the 

 appearance of an entirely new kind of matter in 

 place of the old. 



Map of the Pre-Glacial Valleys of the Great Lake Reg 



The Geological Survey of Canada is to be con- 

 gratulated on this interesting, well illustrated, and 

 important memoir. Its value renders all the more 

 regrettable the inclusion of a series of personal 

 charges against one of the most respected of American 

 geologists, which are quite out of place in an official 

 publication. J. W. Gregory. 



TUE NATURE AND CHARGE OF THE a PAR- 

 TICLES FROM RADIO-ACTIVE SUBSTANCES. 



'T^HE development of our knowledge of radio-activity 

 -•- has emphasised the primary importance of the 

 a particles, which are projected in great numbers 

 from most of the active substances. As Rutherford 

 showed in 1903, the a particles are veritable atoms of 



NO. 2036, VOL. 79] 



Space does not allow us here to discuss the very 

 interesting facts that have been brought to light by 

 the work of Bragg and Kleeman and others in 

 regard to the character of the absorption of the a 

 particle by matter. It suffices to say that it has been 

 found that the o particles from one kind of active 

 matter are all projected initially at an identical speed, 

 but that this initial velocity varies within comparatively 

 narrow limits for different kinds of matter. The a par- 

 ticle, in consequence of its great energy of motion, 

 plunges through the molecules of matter in its path, 

 leaving in its train a large number of dissociated 

 or ionised molecules. Some important questions at 

 once arose when it was found that the a particle was 

 an atom of matter of mass comparable with the 

 hydrogen atom, viz., Are the a particles expelled 

 from different kinds of matter identical in constitu- 



