294 



SCIENCE 



[N. S. Vol. XXX. No. 766 



measured accurately by Sir James Dewar. 

 He has informed me that his final measure- 

 ments show that one gram of radium in 

 radioactive equilibrium produces 0.46 cubic 

 millimeters of helium per day, or 5.32 X 

 10"° cubic millimeters per second. Now 

 from the direct counting experiments it is 

 known that 13.6 X 10" a particles are shot 

 out per second from one gram of radium 

 in equilibrium. Consequently it requires 

 2.56 X 10" o particles to form one cubic 

 centimeter of helium gas at standard pres- 

 sure and temperature. 



From other lines of evidence it is known 

 that all the a particles from whatever 

 source are identical in mass and constitu- 

 tion. It is not then unreasonable to sup- 

 pose that the a particle, which exists as a 

 separate entity in its flight, can exist also 

 as a separate entity when the a particles 

 are collected together to form a measurable 

 volume of helium gas, or, in other words, 

 that the a particle on losing its charge 

 becomes the fundamental unit or atom of 

 helium. In the case of a monatomic gas 

 like helium, where the atom and molecule 

 are believed to be identical, no difficulty of 

 deduction arises from the possible combina- 

 tion of two or more atoms to form a com- 

 plex molecule. 



We consequently conclude from these 

 experiments that one cubic centimeter of 

 helium at standard pressure and tempera- 

 ture contains 2.56 X 10" atoms. Knowing 

 the density of helium, it at once follows 

 that each atom of helium has a mass of 

 6.8 X 10"-* grams, and that the average 

 distance apart of the molecules in the 

 gaseous state at standard pressure and 

 temperature is 3.4 X 10"' centimeters. 



The above result can be confirmed in a 

 different way. It is known that the value 

 of e/m for the a particle is 5,070 electro- 

 magnetic units. The positive charge car- 

 ried by each a particle has been deduced 

 by measuring the total charge carried by a 



counted number of a particles. Its value 

 is 9.3 X 10"" electrostatic units, or 3.1 X 

 10"-° electromagnetic units. Substituting 

 this number in the value of e/m, it is seen 

 that m, the mass of the a particle, is equal 

 to 6.1 X 10"" grams— a value in fair agree- 

 ment with the number previously given. 



I trust that my judgment is not preju- 

 diced by the fact that I have taken some 

 share in these investigations; but the ex- 

 periments, taken as a whole, appear to me 

 to give an almost direct and convincing 

 proof of the atomic hypothesis of matter. 

 By direct counting, the number of identical 

 entities required to form a known volume 

 of gas has been measured. May we not 

 conclude that the gas is discrete in struc- 

 ture, and that this number represents the 

 actual number of atoms in the gas 1 



"We have seen that under special condi- 

 tions it is possible to detect easily by an 

 electrical method the emission of a single 

 a particle— i. e., of a single charged atom 

 of matter. This has been rendered pos- 

 sible by the great velocity and energy of 

 the expelled a particle, which confers on it 

 the power of dissociating or ionizing the 

 gas through which it passes. It is obvi- 

 ously only possible to detect the presence 

 of a single atom of matter when it is en- 

 dowed with some special property or prop- 

 erties which distinguishes it from the mole- 

 cules of the gas with which it is sur- 

 rounded. There is a very important and 

 striking method, for example, of visibly 

 differentiating between the ordinary mole- 

 cules of a gas and the ions produced in the 

 gas by various agencies. C. T. R. Wilson 

 showed in 1897 that under certain condi- 

 tions each charged ion became a center of 

 condensation of water vapor, so that the 

 presence of each ion was rendered visible 

 to the eye. Sir Joseph Thomson, H. A. 

 Wilson and others have employed this 

 method to count the number of ions present 



