DEVELOPMENTS IN ELECTROMAGNETISM BLOCH. 237 



under the least trace of oxygen; it is brought down to 1.7 cm. by 

 1.5 per cent of oxygen. The tendency to associate with the oxygen 

 molecules is therefore much greater than with the argon atom. 

 Nitrogen shows a behavior analogous to argon. 



(4) The study of the charge carried by the ions has led also to 

 important results. The method used for measuring the charge e 

 is based upon the condensation of water-vapor upon the ions 

 (Townsend and J. J. Thomson) and has been further perfected by 

 Millikan^ and his pupils. By means of a microscope a single drop 

 of oil or other material charged by the ionized gas is observed be- 

 tween the horizontal plates of a condenser. Its rates of rise or fall 

 due to the combined electrical and gravitational fields are followed, 

 and from these rates the charge e may be computed. Thus by ob- 

 serving the sudden changes in the rates the new charges can be noted 

 as they are added or taken away from the drop. It is found that 

 these modifications of the charge of the drop always occur in whole 

 multiples of the same elementary charge, e. The mean of the num- 

 bers found for e was 4.89 XlO"^** electrostatic units. This number 

 accords with that deduced by Rutherford from his measures with 

 the rays although J. Perrin found somewhat smaller values from his 

 study of emulsions and of the Brownian movement. 



An important fact was noted by Townsend ^ and his students : Ions 

 of double charge, 2e, or multiples of this, were found in ionized gases. 

 This was noted in the experiments made in 1890, by means of which 

 Townsend, measuring the diffusion coefficient Z> by a method using 

 a gaseous current and comparing it with the mobility h was able to 

 determine the product Ne of the charge of the ion by the Avogadro's 

 number (the number of atoms per atom-gram). This was a static 

 method and permitted the evaluation directly of the quotient h/I) 

 which equals the product Ne. This result was dependent upon the 

 method of ionization used. At mean pressures and with the a rays 

 from radiiun in air or the secondary rays due to X rays produced 

 upon polished brass in hydrogen or oxygen, slightly moist, ions of 

 opposite sign were both found to give nearly the value 1.24X10^". 

 However, if the secondary rays are produced in air at a sheet of 

 brass, oxidized or covered with vaseline, or in other gases (hydrogen, 

 oxygen, carbonic acid) upon the same strip polished and covered 

 vaseline, the value of Ne is much greater for the positive ions. It 

 may be found as high as 2.4X10^°. We conclude therefore first, that 

 certain positive ions carry a charge 2e; secondly, that such ions are 

 produced by the more penetrating secondary rays which are not 



1 Millikan, Radium, p. 345, 1910 ; Phys. Rev., vol. 32, p. 349, 1911. 



2 Townsend, Proc. Roy. Soc, vol. 80, p. 207, 1908 ; vol. 81, p. 464, 1909 ; vol. 85, p. 25, 

 1911 ; Haselfoot. Proc. Roy. See, vol. 82, p. 18, 1909. 



