924 



SCIENCE 



[N. S. Vol. XXIX. No. 754 



and Moulin^^ describe an instrument for 

 automatically registering the ionization of 

 the atmosphere caused by the small and 

 the large ions, with which they have ex- 

 perimented during the past few years; 

 from the use of such an apparatus most 

 important information will be derived. 



For some time observations of these 

 large ions, in the air at normal pressure, 

 have been made at the physical laboratory 

 of the University of Sydney. In this in- 

 vestigation I have been joined, at times, by 

 students whose names will be given in con- 

 nection with the mention of results they 

 have obtained, and throughout have been 

 most ably helped by my assistant, Mr. 

 Carl Sharpe. Owing to the variable char- 

 acter of the natural ionization, the work 

 has proved extremely tedious, as it is only 

 on somewhat rare occasions that a series of 

 observations is accordant enough to give a 

 definite measure of the mobility. The 

 ionization is more uniform after sunset 

 and we observe mainly in the night time. 



All our observations have been made 

 with apparatus constructed after the pat- 

 tern of that used with such success by Pro- 

 fessor Zeleny," in his determination of the 

 mobility of the small ions. In such an 

 instrument a uniform stream of air flows 

 through a metal tube which forms the outer 

 conductor of a cylindrical condenser, the 

 ions drifting on to an inner axial elec- 

 trode, due to the forces in the electric field 

 established between the tube and the axial 

 rod. The theory of the method of finding 

 the mobility with such an apparatus, as 

 given by Professor Zeleny, is well known ; 

 it has been followed without modification 

 in calculating the results of the present 

 series of experiments. Greater uniformity 

 in the ionization is obtained if the air, be- 

 fore reaching the measuring tube, is drawn 



" Langevin and Moulin, Le Radium, 4, p. 218, 

 June, 1907. 



"Zeleny, Phil. Trans., A, 195, p. 193, 1900. 



through a considerable length of piping. 

 We have not noticed any effect on the na- 

 ture of the ions due to the somewhat pro- 

 longed contact of the air with the metal 

 of the pipes, and in most of our experi- 

 ments several meters of iron or of gal- 

 vanized iron piping have been employed. 

 In all cases Dolezalek electrometers have 

 been used to measure the ionization cur- 

 rents. 



During the investigation some definite 

 results have been obtained, of which I pro- 

 pose to give a general account. 



In thinking of M. Langevin 's discovery 

 the idea must have occurred to many, and 

 is indeed suggested by Professor Ruther- 

 ford in his book on "Radio-active Trans- 

 formations," that the large ions may be 

 due to the presence of water vapor. My 

 efforts to elucidate this point have resulted 

 in finding that there is a definite relation 

 between the mobility of the ion and the 

 amount of moisture in the air. 



"When a current of air is passed over 

 hygroscopic substances, without mechan- 

 ical filtration, Mr. S. G. Lusby finds that 

 large ions are absorbed and has noticed a 

 loss in number amounting to 55 per cent., 

 after the air had flowed over a tray con- 

 taining phosphorus pentoxide. I find, in 

 addition, that after leaving the drying 

 agent, those large ions which still exist in 

 the air decrease in mobility with time, and 

 that when the relative humidity changes 

 from 80 to 4 per cent., at a temperature of 

 19° C, they are not in equilibrium with 

 the new vapor pressure conditions until 

 after the lapse of about twelve minutes. 

 Owing to the variable nature of the nat- 

 ural ionization, and perhaps to other 

 causes, the calculated mobilities exhibit 

 considerable irregularities, but show in an 

 unmistakable manner, when the equilib- 

 rium state is established, a dependence of 

 the mobility on the amount of water vapor 

 in the air, the relation between the two 



