324 Scientific Intelligence. 



tation of y-rays by the /?-rays coming from the source, and also 

 to show that the remaining y-rays were in excess of the primary 

 y-radiation. 



The method of experimenting may be briefly described as fol- 

 lows : The glass tube was first surrounded by some material of 

 low atomic weight, such as aluminium. The whole was then 

 enclosed in a tube composed of a metal of relatively high atomic 

 weight, for example, gold. The ionization produced by the y-rays 

 coming from this system was next measured by the balance 

 method of Rutherford and Chadwick. After this, the positions 

 of the layers of aluminium and gold were interchanged so that the 

 more dense metal was next to the glass tube and was bombarded 

 by the a-rays. The ionization was again measured and it was 

 found to be greater than in the first instance by about 1 part in 

 300 when the face of the ordinary ionization-chamber consisted of 

 a sheet of aluminium l m:n thick. In order to obtain more conclu- 

 sive results a special form of air-tight ionization-chamber was con- 

 structed in such a manner as to readily admit of evacuation and 

 subsequent filling with the saturated vapor either of carbon bisul- 

 phide or of methyl iodide. In this way the ionization produced 

 by the excited y-rays was increased relative to the ionization due 

 to the primary y-rays. In other words, advantage was taken of 

 the difference in penetrating power of the y-rays emitted by the 

 radio-active material and the y-rays excited by the a-particles. 

 When carbon bisulphide was used, the ionization was about 5 per 

 cent greater for the glass-gold-aluminium order than for the glass- 

 aluminium-gold sequence. In the case of methyl iodide the dif- 

 ference was a little larger. By covering the glass tube with a 

 sufficiently thick layer of aluminium foil to stop all the arrays, 

 and by showing experimentally that the ionization no longer 

 altered its value when the aluminium and gold tubes were inter- 

 changed, it was demonstrated that the excess of y-rays found in 

 the previous experiments could not be due to the primary /?-rays. 

 The remaining possibilities were exhausted by employing a mag- 

 netic field in an obvious way to deflect the soft /?-rays, etc. 



Taking the ionization observed when the inner tube was alu- 

 minium as 100, the ionization obtained with gold as the inner 

 tube was 104*8, with silver 102*5, with copper 101 -2, and with 

 paper 99'7. Consequently the excitation of y-rays by a-rays has 

 been thoroughly demonstrated. A brief account of further exper- 

 iments by J. Chadwick and A. S. Russell is given in "Nature" 

 for December 26, 1912. Due attention will be given to the 

 results of the latest investigations as soon as they are published 

 in extenso. — Phil. Mag., January, 1913, p. 193/ h. s. u. 



10. Direct Determination of the Mean Free Path of Gas 

 Molecules. — Up to the present time no direct method for the 

 determination of the mean free path of uncharged gas molecules 

 has been devised An ingenious solution of this problem has been 

 worked out by J. Franck and G. Hertz. The key-note to the 

 process consists in making measurements on the lengths of the 



