Kennedy — The large Ions in the Atmosphere. 5 



was determined separately is less than 1,000 as compared with the mean value, 

 16,000, obtained in the experiments conducted in the city atmosphere. The 

 large values of ions on Jan. 12 are explained by the fact that on that day the 

 place of observation was enveloped in air coming from Dublin, and bringing 

 with it the artificially produced large ions of the city. In fact, on that day 

 it was easy to see that Dalkey was in the track of the smoke brought by the 

 light breeze from Dublin. The only other occasion, as has been pointed out 

 above, on which values of N were found comparable with those in Dublin was 

 on Feb. 13, on the occasion of an exceedingly thick sea-fog. Even this value 

 is small compared with the number, 53,000, obtained in a similar fog in Dublin. 



It woidd seem, therefore, from the Dalkey observations that by far the 

 greater part of the large ions existing in the atmosphere of the city are dire to 

 local combustion products. The great variations in the number per cc. on 

 different occasions are probably to be attributed to changes in magnitude and 

 direction of air-currents, sometimes, as may happen on clear days, making the 

 gases from chimneys rise to great heights, and sometimes producing the 

 opposite effect, and so tending to concentrate the large ions near the ground. 



The results of experimental work on large ions, as far as it has gone at 

 present, bear a very striking analogy to those furnished by the classical 

 researches of Aitken on condensation-nuclei in the atmosphere. In the first 

 place, as has been pointed out above, ions are produced from flames which 

 have ultimately a mobility the same as that of the large atmospheric ion. 

 Aitken found that myriads of condensation-nuclei are produced from the same 

 sources. For example, in a room lighted by gas-jets he found that the number 

 of such nuclei per cc. increased from 26,000 to 3,000,000 on lighting the gas j 

 and that only half of these had disappeared next morning. The numbers got by 

 him in various cities varied from 40,000 to 300,000, while in pure air he 

 obtained values as low as 200. Extremely low values were found in the "West 

 Highlands of Scotland and on the Swiss mountains, the air in these regions 

 being remarkably free from artificial contamination. The Dalkey experiments 

 furnish the corresponding parallelism in the case of the large ions. At one 

 station in the South of France, Aitken got values the mean of which was 

 3,500, except on one day when the wind blew from Toulon, nine miles distant. 

 The number of nuclei per cc. then rose to 25,000. Exactly similar to this is 

 the effect observed at Dalkey on the one day in which there was a wind from 

 the city. Aitken got exceedingly high values on the occasion of a snowfall in 

 Glasgow. The number of large ions during a snowstorm in Dublin was as 

 high as 54,000. During some observations on the top of the Eiffel Tower the 

 number of nuclei per cc. on one occasion fell from 104,000 to 226 during an 

 exceptionally heavy downpour of rain. This suggests a comparison with the 

 very great values, on one occasion as high as 60,000, obtained by us in Dublin 



