296 



SCIENCE 



[N. S. Voi,. XXX. No. 766 



of N on the theory by using more recent 

 and more accurate data, and found it to 

 be 2.47 X 10^®. Since in the simple theory 

 no account is taken of the additional scat- 

 tering due to fine suspended particles which 

 are undoubtedly present in the atmosphere, 

 this method only serves to fix an inferior 

 limit to the value of N. It is difficult to 

 estimate with accuracy the correction to be 

 applied for this effect, but it will be seen 

 that the uncorrected number deduced by 

 Lord Kelvin is not much smaller than the 

 most probable value 2.77 X 10^* given 

 later. Assuming the correctness of the 

 theory and data employed, this would in- 

 dicate that the scattering due to suspended 

 particles in the atmosphere is only a small 

 portion of the total scattering due to mole- 

 cules of air. This is an interesting example 

 of how an accurate knowledge of the value 

 of N may possibly assist in forming an 

 estimate of unknown magnitudes. 



It is now necessary to consider some of 

 the more recent and direct methods of esti- 

 mating N which are based on recent addi- 

 tions to our scientific knowledge. The 

 newer methods allow us to fix the value of 

 N with much more certainty and precision 

 than was possible a few years ago. 



"We have referred earlier in the paper to 

 the investigations of Perrin on the law of 

 distribution in a fluid of a great number of 

 minute granules, and his proof that the 

 granules behave like molecules of high 

 molecular weight. The value of N can be 

 deduced at once from the experimental re- 

 sults, and is found to be 3.14 X 10'^ The 

 method developed by Perrin is a very 

 novel and ingenious one, and is of great 

 importance in throwing light on the law 

 of equipartition of energy. This new 

 method of attack of fundamental problems 

 will no doubt be much further developed in 

 the future. 



It has already been shown that the value 

 iV^ = 2.56 X 10^" has been obtained by the 



direct method of counting the a particles 

 and determining the corresponding volume 

 of helium produced. Another very simple 

 method of determining N from radioactive 

 data is based on the rate of transformation 

 of radium. Boltwood has shown by direct 

 experiment that radium is half trans- 

 formed in 2,000 years. From this it fol- 

 lows that initially in a gram of radium 

 .346 milligram breaks up per year. Now 

 it is known from the counting method that 

 3.4 X 10" a particles are expelled per sec- 

 ond from one gram of radium, and the evi- 

 dence indicates that one a particle accom- 

 panies the disintegration of each atom. 

 Consequently the number of a particles 

 expelled per year is a measure of the num- 

 ber of atoms of radium present in .346 

 milligram. From this it follows that there 

 are 3.1 X 10^^ atoms in one gram of 

 radium, and taking the atomic weight of 

 radium as 226, it is simply deduced that 

 the value of N is 3.1 X 10". 



The study of the properties of ionized 

 gases in recent years has led to the develop- 

 ment of a number of important methods of 

 determining the charge carried by the ion, 

 produced in gases by a rays or the rays 

 from radioactive substances. On modern 

 views, electricity, like matter, is supposed 

 to be discrete in structure, and the charge 

 carried by the hydrogen atom set free by 

 the electrolysis of water is taken as the 

 fundamental unit of quantity of electricity. 

 On this view, which is supported by strong 

 evidence, the charge carried by the hydro- 

 gen atom is the smallest unit of electricity 

 that can be obtained, and every quantity 

 of electricity consists of an integral multi- 

 ple of this unit. The experiments of Town- 

 send have shown that the charge carried by 

 a gaseous ion is, in the majority of cases, 

 the same and equal in magnitude to the 

 charge carried by a hydrogen atom in the 

 electrolysis of water. From measurement 

 of the quantity of electricity required to 



