Septembee 3, 1909] 



SCIENCE 



293 



had the same average kinetic energy of 

 movement as the molecules of the solutions 

 in which they were suspended ; in fact, the 

 particles in suspension behaved in all re- 

 spects like molecules of very high molecular 

 weight. This is a very important result, 

 for it indicates that the law of equiparti- 

 tion of energy among molecules of different 

 masses, which is an important deduction 

 from the kinetic theory, holds, at any rate 

 very approximately, for a distribution of 

 particles in a medium whose masses and 

 dimensions are exceedingly large compared 

 with that of the molecules of the medium. 

 Whatever may prove to be the exact ex- 

 planation of this phenomenon, there can 

 be little doubt that it results from the 

 movement of the molecules of the solution 

 and is thus a striking if somewhat indirect 

 proof of the general correctness of the kin- 

 etic theory of matter. 



From recent work in radioactivity we 

 may take a second illustration which is 

 novel and far more direct. It is well 

 known that the a rays of radium are de- 

 flected by both magnetic and electric fields. 

 It may be concluded from this evidence 

 that the radiation is corpuscular in char- 

 acter, consisting of a stream of positively 

 charged particles projected from the ra- 

 dium at a very high velocity. From the 

 measurements of the deflection of the rays 

 in passing through magnetic and electric 

 fields the ratio e/m of the charge carried 

 by the particle to its mass has been de- 

 termined, and the magnitude of this quan- 

 tity indicates that the particle is of atomic 

 dimensions. 



Rutherford and Geiger have recently de- 

 veloped a direct method of showing that 

 this radiation is, as the other evidence indi- 

 cated, discontinuous, and that it is po.ssible 

 to detect by a special electric method the 

 passage of a single a particle into a suit- 

 able detecting vessel. The entrance of an 

 a particle through a small opening was 



marked by a sudden movement of the 

 needle of the electrometer which was used 

 as a measuring instrument. In this way, 

 by counting the number of separate im- 

 pulses communicated to the electrometer 

 needle, it was possible to determine by 

 direct counting the number of a particles 

 expelled per second from one gram of 

 radium. But we can go further and con- 

 firm the result by counting the number of 

 a particles by an entirely distinct method. 

 Sir William Crookes has shown that when 

 the a rays are allowed to fall upon a screen 

 of phosphorescent zinc sulphide, a number 

 of brilliant scintillations are observed. It 

 appears as if the impact of each a particle 

 produced a visible flash of light where it 

 struck the screen. Using suitable screens 

 the number of scintillations per second on 

 a given area can be counted by means of a 

 microscope. It has been shown that the 

 number of scintillations determined in this 

 way is equal to the number of impinging 

 a particles when counted by the electric 

 method. This shows that the impact of 

 each a particle on the zinc sulphide pro- 

 duces a visible scintillation. There are 

 thus two distinct methods— one electrical, 

 the other optical— for detecting the emis- 

 sion of a single a particle from radium. 

 The next question to consider is the nature 

 of the a particle itself. The general evi- 

 dence indicates that the a particle is a 

 charged atom of helium, and this conclu- 

 sion was decisively verified by Rutherford 

 and Royds by showing that helium ap- 

 peared in an exhausted space into which 

 the a particles were fired. The helium, 

 which is produced by radium, is due to the 

 accumulated a particles which are so con- 

 tinuously expelled from it. If the rate of 

 production of helium from radium is meas- 

 ured, we thus have a means of determining 

 directly how many a particles are required 

 to form a given volume of helium gas. 

 This rate of production has recently been 



