920 



SCIENCE 



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



and on the resistance which is offered to 

 the movement; under the influence of the 

 electrical forces the ions drift, as it were, 

 in a definite direction, the positives travel- 

 ing to the negative electrode, and vice 

 versa, a motion in which the uncharged 

 molecules have no part. Other things 

 being equal, it is assumed that this drift 

 velocity of the ions is directly proportional 

 to the electric intensity and following the 

 suggestion of M. Langevin, the term 

 "mobility" has been adopted for the 

 average velocity acquired by an ion under 

 the influence of unit electric force. At the 

 present time the mobility of a class of 

 ions is its most readily determined prop- 

 erty, and it is principally to observations 

 of the mobility of the ions in different 

 gases and under various conditions that 

 we must look for a clue to the nature of 

 the ionic structure. In all cases I shall 

 state the value of the mobility as that of 

 the velocity, in centimeters per second, 

 due to an electric force represented by a 

 potential gradient of one volt per centi- 

 meter, that is, in practical units. 



Two types of ion are recognized as ex- 

 isting naturally in the air, the small ion, 

 with a mobility of about one and one half 

 under normal conditions, and another, 

 discovered by M. Langevin,^ and called by 

 him the large ion, which is characterized 

 by the very small mobility of only %ooo- 

 To these I now add a third, which has a 

 mobility of about %oo under normal cir- 

 cmnstances. It may be called, for the 

 present at least, the ion of intermediate 

 mobility, or the intermediate ion. 



M. Bloch' finds in air bubbled through 

 water ions of mobility of the order of one 

 or two tenths ; these seem to form a fourth 

 class of ions and it would be interesting 

 to know if they exist in air not specially 

 treated. 



' Langevin, C R., t. 140, p. 232, 1905. 

 ' Bloch, C. R., t. 145, p. 54, 1907. 



The small atmosphere ions are identical 

 with those artificially produced in air by 

 ionizing agents Avhich have been made the 

 subject of such numerous researches as 

 described by Professor Bragg in his ad- 

 dress. There is now considerable knowl- 

 edge, resumed in the beautiful kinetic 

 theory of gases, of molecular movements 

 and dimensions, and when it is thought 

 that an ion moves more slowly in an elec- 

 tric field than would a single molecule if 

 charged, as the ion must be made of the 

 stuff of the gas in which it is formed, what 

 more natural than to consider it a cluster 

 of a few molecules? This idea has been 

 generally adopted. The small ions are 

 thus assumed to be of somewhat greater 

 size than their fellow molecules, but as the 

 mobility notably increases with decrease of 

 pressure, and with rise of temperature, 

 their diameter is apparently not a constant 

 quantity. 



The direct argument, which is used to 

 support this view, considers that in the 

 numerous collisions which occur between 

 the charged and uncharged molecules, in 

 many cases the kinetic energy of the latter 

 will not be great enough to carry them 

 away, after impact, from the attraction 

 of the charge. The charged molecule will 

 thus collect other molecules around it, but 

 as the effect of the charge on the outer 

 members of the cluster diminishes as the 

 collection of molecules increases, the 

 growth will cease when the size is such 

 that the attraction of the charge at the 

 surface of the cluster, in grazing impact of 

 ion and molecule, is just insufficient to 

 hold the latter as a permanent member of 

 the ionic system. The principle involved, 

 in calculating the value of the limiting 

 radius, is similar to that which determines 

 Avhether a comet, in its close approach to 

 the sun, shall become a permanent member 

 of the solar system or wander into the 

 space from which it came. The calcula- 



