330 Prof. Bragg and Mr. Kleeman on 



number of ions produced by the particle, and that if the total 

 number of ions produced by an a particle were the same, no 

 matter the gas in which it completed its course, then the 

 product of the ranges and the ionization per cm. of path 

 would be a constant. Now Rutherford has made measure- 

 ments of the total ionization produced by the particle in some 

 substances, and found it the same in each case. But there 

 are other substances for which this is not true ; e. g., ether, ethyl 

 chloride, and methyl bromide ; and in consequence the second 

 of these suppositions is not justified. It is even possible that 

 the first may also prove incorrect. It is conceivable that the 

 a particle may spend energy on ionization whilst passing- 

 through a molecule, especially a complex molecule, and that 

 the products may never get away from the molecule so as to 

 be separated by the electrical field, and measured by the 

 electrometer. That this actually occurs certainly seems a 

 natural inference from some of the following results. 



Let us now consider the experimental evidence. In fig. 2 

 are drawn three curves, A, B, and C. The first of these repre- 

 sents the ionization curve of radium in air. Curve B shows 

 the result of placing a number of thin sheets of silver-leaf 

 over the radium, so that the rays had to traverse these before 

 reaching the ionization-chamber. Curve C shows the effect 

 of substituting thin silver-foil for the leaf. The product of 

 the density and the thickness was in the case of the leaf 

 •00213, and in the case of the foil '009(57. These figures 

 were readily obtained by weighing a measured area of the 

 material in each case ; the leaf was not easy to handle, and 

 the loss of range in its case not very great, so that the foil 

 experiment is the more reliable. 



The curves ABC are clearly similar in shape, and differ 

 only in their height above the zero line. The silver sheet 

 has not in either case caused any modification of the a stream, 

 except that it has cut off an equal amount from the range of 

 each particle. 



If some of the particles had been stopped by the metal, or 

 if some had lost more energy than others, there would have 

 been more or less distortion of the curve. But the former 

 condition does not occur, as has been already explained, and 

 the latter was also prevented in this case by the great 

 uniformity of the foil, which was polished on both sides. 



The loss of range in the case of the silver- foil is 33*5 mm. 

 If we take the density of the air to be *0012_, we may say 

 that a silver stratum for which thickness X density = '00967 

 is equivalent to an air stratum for which the same product 

 = 3-35 X '0012 = -004:02. The ratio of these quantities is 2*41 . 



