464 Mr Lusby, Some Experiments on lonisation in Dried Air. 



in the very last experiment carried out — namely, that with filtered 

 air — the effect of drying alone was tested, and neither the positive 

 nor the negative charge was affected. This result shows that 

 either the ions go through the U-tube intact, or else they drop 

 their water molecules and pass on; and their mobilities in the 

 latter case being presumably those they would possess in dry air, 

 they are still caught by the testing condenser. 



The secondary effect, accidentally discovered, is in some ways 

 more interesting than the main result, on account of the light 

 which it seems to throw on the nature of the large neutral centres 

 in the air, investigated recently by de Broglie {loc. cit). In the 

 atmosphere there are normally present three types of bodies which 

 are of importance in an ionisation theory, viz. small ions, neutral 

 centres, and large ions, the latter being a combination product 

 of the two former. When a gas is in statistical equilibrium, some 

 of the small ions are constantly combining with small ions of 

 opposite sign, others combine with neutral nuclei to form large 

 ions, and a proportion of these large ions combine with large ions 

 of opposite sign. Here two main coefficients of recombination 

 have to be considered, that of small ion with small ion, and that 

 of small ion with large ion. Now, other things being equal, the 

 coefficient of recombination between two ions depends on the sum 

 of their mobilities, hence the former coefficient is approximately 

 twice the latter. But as the large ions outnumber the small ones 

 by 50 to 1, the chief factor in recombination is the large ion ; 

 or — to go back one step — the neutral centre. Hence anything 

 that removes these large nuclei will tend to reduce recombination ; 

 this is the present-day method of accounting for the action of 

 so-called "dust." As to what happens to these nuclei at the 

 temperature of liquid air one can only speculate. They must 

 either be deposited in the U-tube or else dissociate into smaller 

 bodies, whose chances of combining with a small ion are rare. 



de Broglie {loc. cit.) states that high temperatures break up 

 the nuclei ; it is reasonable to suppose that a low temperature 

 could do the same. If we supposed them to consist partly of solid 

 and partly of gaseous matter, then the great difference in the 

 coefficients of expansion of the different constituents could con- 

 ceivably break up the complex system at a low temperature, on 

 the same principle that daiup rocks crack in cold weather. It is 

 important to note that water plays no part in this action ; the 

 effect occurs equally well in dry or damp air. Whatever may be 

 the composition of the neutral centre (and therefore of the large 

 ion) evidently water is not present to any large extent. 



In conclusion, I wish to thank Prof Sir J. J. Thomson for his 

 valuable suggestions and kindly interest during these experi- 

 ments. 



