THE PROGRESS OF SCIENCE 



617 



rising to give a hearty vote of thanks 

 to the lecturer. 



The speaker began by enumerating 

 some of the better known properties of 

 radium and the radioactive products, 

 such as the rate of decay of these sub- 

 stances, ranging from five thousand mil- 

 lion years for the half period of 

 uranium down to a few minutes for 

 some of its descendants. He mentioned 

 also the enormous energy of radioactive 

 disintegration, showing that one pound 

 of a radioactive product, if one could 

 gather so much, has the explosive energy 

 of ten million pounds of nitroglycerin. 

 He dwelt also upon the experimental 

 methods developed by Mr. I'. T. R. Wil- 

 son by which the expulsion of alpha 

 particles and beta particles can be made 

 visible and even photographed. The 

 first part of the path of the alpha par- 

 ticles is nearly straight, but as they 

 lose energy, contact with molecules of 

 other substances deflects them in a char- 

 acteristic manner. The beta particles, 

 on account of their greatly inferior 

 mass, pursue very irregular courses. 



Less well known are the results ob- 

 tained by Moseley on the interference 

 spectra of X-rays produced by reflection 

 from crystals, especially that of rock 

 salt. These spectra are capable of be- 

 ing photographed and are vastly more 

 simple and more regular than those ob- 

 tained from visible light. These spec- 

 tra evince a regularity among the ele- 

 ments which does not appear in the 

 more familiar light spectra, and these 

 regularities tend to elucidate the nature 

 of the atom. 



Lord Kelvin conceived an atom as 

 composed tf negative electrons included 

 ivitnin a space charged with positive 

 electricity holding the electrons to- 

 gether in a single body. This, how- 

 ever, appears to be inconsistent with 

 the researches of Rutherford, who lias 

 developed a theory of nuclear atoms, ac- 

 cording to which a central nucleus of 

 extremely high potential is surrounded 

 by negative electrons whose motion it 

 controls. If so, the electrons are eon- 

 trolled by the nucleus very much as the 

 planets are held to the solar system by 



VOL. LXXXIV.— 42. 



gravitation, and indeed there appears 

 strong reason to suppose that the force 

 involved is really inversely proportional 

 to the square of the distance as in the 

 case of gravitation. From this point of 

 view, the various elements are charac- 

 terized by the number of electrons in 

 the atom. Each electron carries a single 

 negative charge, arrd the nucleus carries 

 as many positive charges as there are 

 electrons to be controlled. This theory 

 of the atomic constitution explains the 

 irregularity in the movement of alpha 

 particles through a gas. When an 

 alpha particle approaches a nucleus 

 carrying a charge of millions of volts, 

 it is sharply deflected and may appear 

 even to rebound in the direction from 

 which it came. Sir Ernest illustrated 

 this by a fine experiment. Similarly, if 

 there were a small hole drilled through 

 the center of the earth, a ball dropped 

 from the surface would go straight 

 down and come straight back almost as 

 if it had been infinitely elastic and re- 

 bounded from an impenetrable surface. 

 It is posible to determine the number 

 of positive charges contained in each 

 one of the elements from hydrogen to 

 uranium, and it seems also that if the 

 elements are appropriately arranged the 

 charges increase by unit steps, so that 

 hydrogen contains a single positive 

 charge and uranium 92. This assump- 

 tion corresponds to the actual elements 

 with a small but very important excep- 

 tion. In the series of 92 possible 

 charges, there are just three gaps, cor- 

 responding, presumably, to three un- 

 known elements, and at the same time 

 the relationship of these unknown ele- 

 ments to the knowu elements is made 

 clear, so that the chemists have prelim- 

 inary information to guide them in the 

 search for the missing links. This is a 

 wonderful advance on the periodic sys- 

 tem of Mendeleef which has itself been 

 fruitful in the discovery of elements. 



TEE SMALL COLLEGE AND LIS 

 PRESIDENT 



The writer of the article on "The 

 Small College and its President' 

 which appealed in the May number of 



