666 Dr. A. S. Eve on the Effect of 



as the dust particles are in all probability very large compared 

 with the ions ; thus, if a positive ion strikes against a dust 

 particle and sticks to it it forms a large system which is much 

 more likely to be struck by a negative ion and neutralized 

 than if the positive ion had remained free ; in this way the 

 presence of dust will facilitate recombination of the ions ' 

 (Sir Joseph Thomson, ' Conduction of Electricity through 

 Gases/ 2nd edition, p. 20). 



It may be objected to this explanation that pairs of small 

 mobile ions will recombine more readily than pairs of ions, 

 one large and one small. In fact, if the rate of production of 

 ions is constant, the rate of recombination a may be expected 

 to decrease, and, therefore, the total number of ions will 

 increase, if dust or smoke is present. It is true that the 

 small ions tend to disappear, but large ions replace them and 

 recombine more slowly, so that there is on the whole an 

 increase. 



Further experiments on this point will be described later 

 in this paper ; for the present it is sufficient to state that the 

 variations recorded from day to day in Table I. are attributed 

 to the relative presence or absence in the atmosphere of dust, 

 smoke, or other centres, which cause the many small ions 

 rapidly to become large ions and thus pass the testing vessel 

 without detection. 



Moreover, it is probable that the small negative ions, owing 

 to their greater mobility, pass in larger numbers to the 

 particles % of dust or other centres than do the positive ions, 

 and on this account an excess of small positive ions is 

 indicated by the electroscope, whilst an excess of large 

 negative ions passes through the testing vessel. The degree of 

 humidity, and diffusion to the parts of the apparatus may be 

 contributory to the observed excess. 



Ions from Flames. 



In order to ascertain if humidity affects the ratio of the 

 number of positive and negative ions, a kettle v\as boiled 

 vigorously in the room (7x4x3 in. 3 ) over a two-ringed 

 Bunsen burner, at a distance of 3 metres from the apparatus. 

 With the radium in position it was found that the ionization 

 current was diminished about 50 to 70 per cent., but this 

 diminution was produced also when the kettle of water was 

 absent. The effect was due to the ions from the flame alone. 



A remarkable result may be recorded. If the ionization 

 current was measured at B, with ladium near Y (fig. 2), 

 with the Bunsen flame burning in the room, and if an 

 increasing potential was given to T, then the readings 



