lonisation in Dried Air. 463 



former case not affect Erikson's coefficient at any temperature and 

 in the latter case would increase it. In either case, the effect 

 of temperature on recombination would give a result quite the 

 opposite to that observed. Still, in order to verify this theory, 

 the position of the ionising agent was altered, and it was placed 

 between the U-tube and the testing condenser. Obviously, if 

 recombination were causing the phenomenon, the current received 

 by the electroscope would depend on the distance between the 

 uranium and the testing condenser. The result was surprising; 

 the ionisation in the dried air was increased to 10 times that in 

 ordinary air, instead of only twice, as before. This seemed to 

 point to the fact that reduced recombination occurred in the dried 

 air, although I am not aware that any previous experimenter 

 ever suspected or allowed for this action. To make sure of the 

 point, the air before entering the apparatus was passed first 

 through a tube 3 decimetres long, containing calcium chloride, 

 and then through another tube 4 decimetres long, containing 

 pumice, which had previously been treated with sulphuric acid 

 in the ordinary way. On immersing the U-tube in liquid air as 

 before the ionisation was still increased tenfold, and the negative 

 action of water vapour on recombination confirmed. 



The only other thing that remained to be tried was the effect 

 of dust. All early experimenters on recombination found that, in 

 order to obtain consistent results, dust must be carefully excluded 

 from the gas experimented on — a result clearly illustrated by 

 Owens'* experiments with tobacco smoke. Although it seemed 

 extremely improbable that drying (or cooling) a gas could remove 

 its dust particles, still the result of filtering the air was tried. 

 At first, only a decimetre of cotton wool was used, with the result 

 that, on drying the air as before, the increase in ionisation was 

 only about 100 7^ ; when the length of cotton wool was increased 

 to 5 decimetres the phenomenon vanished altogether, and was 

 therefore due solely to recombination. 



(3) Discussion of Results. The negative result obtained in 

 the primary investigation, namely the absence of any preponder- 

 ance of charge of one sign in dried air, is certainly disappointing. 

 As was pointed out earlier, practically all the known properties 

 of ions are modified by water vapour, such modification being 

 more pronounced in the case of negative ions ; still one must not 

 forget that the positive ion is also affected by moisture, though to 

 a lesser degree. It may be objected that the effect sought for is 

 masked by the recombination effect ; but that is not so, for in the 

 first method employed the electroscope was quite sensitive enough 

 to easily record a 5 7o difference between the two charges. Further, 



* Owens, Phil. Mag. Oct. 1899. 



30—2 



