1911-12.] 
197 
The Sun as a Fosr Producer. 
responded to the action of sunshine, just as it ceased to respond when the 
same air was frequently used in the tube apparatus. This experiment 
showed that there was something in ordinary air having an affinity for 
soda which was necessary for making the S0 2 active as a nucleus-producer 
with sunshine. 
Pure Country Air. 
Having found that sunshine has no effect on S0 2 in air purified either 
by successive showers of S0 2 nuclei, or by being passed through cotton- 
wool soaked in caustic soda, it was thought it might be interesting from a 
meteorological point of view to know whether there was any air in nature 
so pure, or at least having that kind of purity that it will not form nuclei 
of condensation with S0 2 after being sunned. When visiting Loch Awe, 
Argyllshire, in June last, some tests were made of the air of that place for 
the purpose of getting information on this point. At that place, or indeed 
anywhere in the Highlands, we cannot expect to find pure air when the 
wind blows from impure directions. At Loch Awe only the winds from 
west to north are pure; from all other directions they bring impure air 
from inhabited areas of the surrounding country. These impurities are 
carried great distances. The air from the Lowlands of Scotland sends vast 
quantities of impurities to the wilds of the Highlands. At Loch Awe, with 
north-westerly winds, the number of dust particles is low, from 100 to 200 
particles per c.c. ; but when the wind comes from any other direction the 
number goes up to at least ten times as many, and often to much 
higher figures. 
The apparatus used at Loch Awe was the same as that already described 
and shown in fig. 1, with the exception that the lamp for illuminating the 
test-flask was not used, because the products of combustion would have 
polluted the air. For illumination the light of the sky was used. The 
test-flask being enclosed in the blackened box already described, the light 
of the sky was found to give ample illumination for the purpose. 
The tests were begun on 24th June, and though the wind was from a 
northerly direction the air was never very pure , owing to local pollution. 
The number of dust particles was very variable, owing to the irregular 
mixing of this pollution. With this air the S0 2 always gave a slight 
condensation after sunning, but the amount varied greatly. The effect, 
however, was very different from what I had ever seen before. Instead 
of the dense fog given by the air of Falkirk, sometimes only a slight 
shower would fall in the flask. On the 25th and 26th the direction of 
the wind shifted slightly, and the local pollutions no longer came to the 
