36 president's address — SEcnox a. 



To test the theory, Mr. Sutherhind applies the equation to the 

 experiments of ^[r. Pliillips* on tlie negative ion, takings A = 0'1764, 

 C' = loOo and 0' = 70, with the following results : — 



S 411 399 383 373 348 333 285 20i1 94 



^ calculated 2-48 2-42 2-33 227 213 2()5 1-75 1-22 -235 

 )fc observed 2-49 240 230 221 2125 200 178 1-23 -235 



As will be noticed, the comparison of the mobility calculated 

 from the above expression with the results of Mr. Phillips' valuable 

 series of observations shows an accordance w^ll within the limits of 

 experimental error, over the whole i-ange of temperature from 95° to 

 411° absolute. The apparent decrease in the size of the ion with rise 

 of temperature, as discovered by Mr. Phillips, is thus shown to be due 

 to an eflfect 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 concludes 

 from his determination that most probably the small gaseous ion is 

 the ordinai'y ion of electrolysis. 



Mr. Sutherland's expression for the mobility of the ion, by con- 

 taining a symbol representing the boiling point of the gas substance at 

 the pressure of the experiment, indicates a dependence of the mobility 

 on the pressure of the gas ; the comparison of the values given by it 

 have yet tD be compared with the results of experiment. t 



The idea of the small ion as a cluster of a few molecules, founded 

 on insecure assumptions, was perhaps chiefly characterised by its 

 riumerical vagueness ; its replacement by a definite theory cannot but 

 be regarded as marking a great advance in our knowledge of ionic 

 stiticture. 



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 represented by 

 the mere collection of the results of experimental investigations. The 

 large ions were discovered by M. LangevinJ in 1905, who found that 

 their movement, in an electric field with a potential gradient of one 

 volt per centimetre, is only at the rate of one three-thousandth of a 

 centimetre per second, but that, under natural conditions, tlieii- number 

 is about fifty times as great as that of the small ions. In a later 

 communication MM. Langevin and Moulin§ describe an instrument for 

 automatically registering the ionisation of the atmosphere caused by 

 the small and the large ions, with which they have experimented, 

 during the past few years ; from the use of such an apparatus most 

 important information will be derived. 



For some time obseiwations of these large ions, in the air at 

 normal pressure, have been made at the Physical Laboratory of the 

 University of Sydney. In this investigation I have been joine<l,' at 

 times, by students whose names will be given in connection with tlie 



* Phillips loc. cit. 



t Langevin. Ann. de Chimie et de Physique, t. 28, \>. 2Sit. 1;>0.S, 



:t Langevin. C.K., t. 140, p. 232, 1905. 



§ Langevin and Moulin. Le Radium, 4, p. 21S, June 1907. 



