RECENT ADVANCES IN SCIENCE 187 



There may also be an atom whose nucleus has mass i and zero 

 charge, i.e. one formed by the close union of a hydrogen nucleus 

 and an electron. Such an atom would have very novel pro- 

 perties ; it would, for example, have no external field except 

 at very short distances, and would therefore pass freely through 

 ordinary matter. 



Since the number and arrangement of the external electrons 

 which determine most of the chemical and physical properties 

 of an atom are fixed by the nuclear charge, it is to be expected 

 that, as long as this charge remains the same, the properties 

 also would be the same. This explains the existence of 

 isotopes having different atomic weights but the same 

 nuclear charge. It will thus be seen that a positive knowledge 

 of the nuclear charge is a matter of fundamental importance. 

 It is probably equal to the atomic number. Further in- 

 formation can be obtained by means of experiments on the 

 scattering of swift a- and yS-rays. Such experiments are at 

 present in progress, and preliminary results obtained by Mr. 

 Chadwick, who is working with a-rays, indicate that the number 

 of free unit positive charges in the nucleus does not differ 

 from the atomic number of the corresponding atom by more 

 than I %, i.e. by more than the experimental error. 



In the course of the lecture Sir Ernest indicated some 

 other lines along which the problem of atomic structure is 

 being attacked at the Cavendish laboratory. It is possible 

 that the impact of rapidly moving electrons may suffice to 

 cause the disintegration of oxygen, nitrogen, or other ele- 

 ments. This can be tested by observing the spectrum given 

 by one of these gases in a vacuum tube after an intense 

 bombardment of a suitable substance by cathode rays, and 

 experiments on these lines are being carried out by Dr. Ishida. 



Dr. Shimizu has devised a modification of C. T. R. Wilson's 

 expansion apparatus whereby several expansions can be pro- 

 duced in one second. By studying photographic records of 

 the tracks of the a-particles in these expansions, it is hoped to 

 obtain information as to the conditions which determine the 

 disintegration of the atoms, and also something about the 

 relative energies in the a-particle, the escaping atom, and the 

 residual nucleus. 



Prof. D. C. Miller, of Cleveland, U.S.A., has published several 

 papers containing the results of experiments on explosive 

 sounds carried out during the war. Using the standard ap- 

 paratus employed for sound-ranging, he finds {Science, June 18, 

 1920, pp. 619-20) that the velocity of sound at a distance of 

 100 ft. in front of a 10 in. gun is about 1,240 ft. per sec, or 

 22 % above the normal value ; at a distance of 200 ft. the ex- 



