JlTNE 11, 1909] 



SCIENCE 



923 



November 9, 1908, and communicates a 

 paper on the same subject to this section. 



Mr. Sutherland, to our regret, is unable 

 to be present at this meeting of the associa- 

 tion, but he allows me to communicate to 

 the section a letter of his on the theory of 

 the small ion written to me on February 

 6, 1908, and permits me to mention the re- 

 sults of his investigation at this stage of 

 our proceedings. 



Amplifying the discussion developed in 

 his viscosity paper by the addition, in the 

 energy expression, of a term representing 

 the electrical potential energy of ion and 

 molecule when in contact, Mr. Sutherland, 

 in his letter, proceeds to investigate the 

 relation between the mobility and tempera- 

 ture and deduces for the mobility of the 

 ion the simple expression, 



^ e — d' 

 where J. is a constant, 6 the absolute tem- 

 perature, 6' the absolute boiling point, 

 under the experimental pressure, of the 

 substance of the gas in which the ions are 

 formed, and C a constant similar to that 

 represented by C in his now well-known 

 viscosity formula. 



To test the theory Mr. Sutherland ap- 

 plies the equation to the experiments of 

 Mr. Phillips" on the negative ion, taking 

 A = 0.1764, 0' = 150.5 and 6' = 10, with 

 the following results: 



a 411 399 383 373 348 333 285 209 94 



i calculated 2.48 2.42 2.33 2.27 2.13 2.05 1.75 1.22 .235 

 t observed 2.49 2.40 2.30 2.21 2.125 2.00 1.78 1.23 .235 



As will be noticed, the comparison of the 

 mobility calculated from the above ex- 

 pression with the results of Mr. Phillips's 

 valuable series of observations, shows an 

 accordance well within the limits of ex- 

 perimental error, over the whole range of 

 temperature from 95 degrees to 411 de- 

 grees absolute. The apparent decrease in 



•Phillips, loo. ait. 



the size of the ion with rise of temperature, 

 as discovered by Mr. Phillips, is thus shown 

 to be due to an effect of the ionic charge 

 similar to that of molecular force which 

 accounts for the apparent shrinkage of the 

 molecules in the viscosity problem. 



Mr. Sutherland shows, in addition, how 

 his investigation enables an estimate to be 

 made of the diameter of the ion, and con- 

 cludes from his determination that most 

 probably the small gaseous ion is the ordi- 

 nary ion of electrolysis. 



Mr. Sutherland's expression for the 

 mobility of the ion, by containing a symbol 

 representing the boiling point of the gas 

 substance at the pressure of the experi- 

 ment, indicates a dependence of the mobil- 

 ity on the pressure of the gas; the com- 

 parison of the values given by it have yet 

 to be compared with the results of experi- 

 ment.^" 



The idea of the small ion as a cluster of 

 a few molecules, founded on insecure as- 

 sumptions, was perhaps chiefly character- 

 ized by its numerical vagueness; its re- 

 placement by a definite theory can not but 

 be regarded as marking a great advance 

 in our knowledge of ionic structure. 



Turning now to the consideration of the 

 larger ions in the air, it may be said at once 

 that our knowledge is as yet but repre- 

 sented by the mere collection of the results 

 of experimental investigations. The large 

 ions were discovered by M. Langevin^^ in 

 1905, who found that their movement, in 

 an electric field with a potential gradient 

 of one volt per centimeter, is only at the 

 rate of one three-thousandth of a centi- 

 meter per second, but that, under natural 

 conditions, their number is about fifty 

 times as great as that of the small ions. 

 In a later communication MM. Langevin 



" Langevin, Arm. de Chimie et de Physique, 

 t. 28, p. 289, 1903. 



"Langevin, 0. R., t. 140, p. 232, 1905. 



