480 Proceedings of the Royal Society of Edinburgh. [Sess. 
water mark to the water’s edge, at different parts of the beach, and always 
directly up or down wind ; that is, testing the air as it arrived from off the 
water and the same stream of air after it had passed on to the shore. The 
result of the observations was that much lower numbers were observed in 
the air at the water’s edge than after it had passed over the foreshore. 
At the water’s edge the numbers varied from 15,000 to 20,000 per c.c., 
while back on shore they were from 50,000 to 150,000 per c.c. These 
observations show that the sun-born nuclei are produced by some changes 
taking place on the foreshore. 
It will be remarked that the lowest numbers here are very high, far 
above anything previously observed in the air of that district, but a slight 
account of the history of the air arriving at the water’s edge explains these 
high numbers. The wind was southerly at the time, and the air arriving 
at the point of observation, though coming off the water, had a short time 
previously travelled over the foreshore at Port Appin or Lismore to the 
south, and had been exposed to the same conditions as on the foreshore at 
Appin ; its sun-formed nuclei had thus been greatly added to before it 
arrived at the place of observation. 
After my return home I began an investigation of the physics and 
chemistry of these sun-formed nuclei. At first the explanation seemed 
simple enough, but on further study it was found to be very complicated. 
Though satisfactory results were sometimes obtained, they could not be 
repeated with certainty, showing a lack of knowledge of some of the condi- 
tions. As the discovery of the source of the sunshine nuclei had only been 
made on the last day of my visit, I determined to revisit Appin and see if 
further light could be obtained by a study of the subject on the spot. 
October 8th was selected for this visit, and I was extremely fortunate 
in getting a day in every way suited for the purpose. The morning was 
cloudy, and when I arrived at Appin about mid-day I found the wind 
was light and from W.N.W. The air was very clear, and while the tests 
were being made the tide was out, thus exposing a good deal of the fore- 
shore' to the action of the sun. When the test began the sky was still 
clouded, but showed signs of clearing; the number of particles observed on 
shore was low, being only 105 per c.c. Shortly afterwards there were 
bright gleams of sunshine, when the numbers rose at times to from 2100 to 
4000 per c.c. On going down to the water’s edge to test the air before it 
passed over the foreshore, I found that the numbers fell to 103 per c.c. On 
the sky again clouding, the numbers were 2240 on shore, while at the 
water’s edge they fell to 70 per c.c. When work was again started at 
2.10, the sky was only half clouded, and the conditions had considerably 
