198 Proceedings of the Koyal Society of Edinburgh. [Sess. 
window of the room where the tests were being made. The air was very 
clear and pure. The number of particles on these days was from 182 to 
207 per c.c. With this air the S0 2 gave no condensation after being sunned ; 
it acted exactly like the artificially purified air. To make sure that there 
was no mistake in these tests, and that the failure of the pure air to give 
any cloudy condensation with S0 2 was not due to any lack of other 
conditions, the following experiments were made. The window as well 
as the door of the room was closed, and a wax match was burned in the 
room to make some of the pollution of inhabited areas. This raised the 
number of particles in the room from 200 to 50,000 per c.c. This air was 
drawn into the apparatus and sunned ; on testing, it gave a very dense 
condensation, showing that the sun was strong enough, and that there was 
enough S0 2 in the flask ready to form an alliance so soon as its partner put 
in an appearance, though unable to form nuclei in the pure air. 
Some experiments were made in this pure air with the products of 
burning sulphur to see if the sulphur produced the something necessary to 
make its products active with sunshine. A little sulphur was burned and 
the products mixed with the air in the room and drawn into the sunning 
flask, from which the S0 2 solution had been removed. After exposure to 
light this always gave a slight condensation, showing that some action had 
been produced by the light. It was found, however, that if a match was 
burned in the room at the same time, or a little after, a very much denser 
condensation took place, which goes to show that the burning sulphur does 
not produce sufficient of the something necessary to make all the S0 2 active ; 
and it seems possible that the small amount of condensation produced by 
burning sulphur alone may have been due to the combustion of organic 
matter in the sulphur, or produced in the act of lighting it. Though this 
was done in the passage outside the room, some of the products may not 
have been excluded. 
The experiments with solution of S'0 2 were continued on the 27th and 
28th of the month ; but by this time the wind was getting westerly and 
south-westerly, and the air was losing its transparency, while the number 
of particles had risen to from 1000 to 2000 per c.c. With this air there 
was always some condensation in the test-flask, though never so dense as 
that given by the air at Falkirk ; if, however, a match or a piece of paper 
was burned in the room the density became very great. This semi-pure 
air could also be purified by passing it through the caustic filter, showing 
that the slight showers given by it and S0 2 are probably due to the same 
cause as the denser condensation given by the air of Falkirk. 
