RECENT ADVANCES IN SCIENCE 207 



matter itself. Th departure of Dr. Marsden on active service 

 interrupted the work ; but Prof. Rutherford, in view of the 

 importance of this latter result if substantiated, has contrived, 

 despite considerable pressure of war work, to continue and 

 extend the experiments in greater detail during the past five 

 years. It is difficult to give a decisive answer on the possibility 

 that hydrogen is a product of radio-active change, on account of 

 the numerous factors involved. Firstly, it is difficult to be sure 

 of the absence of hydrogen as a contamination in the source 

 and absorbers of the radiation. Secondly, in experiments on 

 impact between a particles and hydrogen atoms in the apparatus 

 referred to above, the number of scintillations due to atomic 

 nuclei set into swift motion by colliding a particles is much 

 greater than is to be expected on simple theory. Thirdly, 

 both nitrogen and oxygen atoms are also set in such swift motion 

 by collision with a particles that they cause scintillations outside 

 the range of the a particles. In fact, in Marsden 's early experi- 

 ments a large number of scintillations were probably not due 

 to H atoms at all, but to high-velocity N and O atoms produced 

 from the air between the source and the screen. It is on the 

 latter two points that Rutherford concentrates in these four 

 papers ; for a satisfactory analysis of them is essential before 

 any safe opinion can be formed on the radiation of H particles 

 from radio-active matter. 



At the time when Marsden was engaged on his experiments, 

 Dr. C. Darwin, another worker in the Manchester laboratories, 

 subjected the question of collision between a particles and atoms 

 to mathematical analysis. Assuming that the forces involved 

 arise from the charges carried by atomic nuclei which are to be 

 regarded as points, and also assuming the inverse square law, 

 Darwin was able to derive a formula for the number of swift 

 velocity H atoms produced by a given stream of a particles 

 with a given range, per cm. of their path. There will be, for 

 instance, a certain maximum range for these H atoms, a range 

 attained, doubtless, by such atoms as suffer a direct " head-on " 

 impact from an a particle. For example, this is 28 cms. in air 

 for H atoms set in motion by a particles with a range of 7 cms. 

 in air (those from radium C). (If the a particles have a smaller 

 range, the maximum range of the resulting swift H atoms is 

 also smaller, but always about four times the range of the a 

 particles). Extremely few of the H atoms will, however, be 



