205 
1911-12.] The Sun as a Fog Producer. 
spontaneous condensation without any outside influence. If, however, the 
products are mixed with air, they are incapable of forming nuclei in the 
dark ; but after they have been exposed to sunshine they give a very dense 
condensation on expansion. This condensation after sunning continued for 
long, with each sample taken from the flask, and after a great deal of 
pumping had been done and products in the flask had been reduced to a 
very weak mixture. Part of the persistency in the condensation would no 
doubt be due to some of the gases absorbed at first by the filter getting 
free when purer air was passing, as it is found that a filter through which 
strong products have been passed is difficult to cleanse by pumping air 
through it, and can never be used again for any other purpose. The 
products from the fire can be kept in the large flask for hours and retain 
their activity if kept in the dark. These tests show that the sulphur 
products of our fires react to light in the same way as the S0 2 was 
found to do. 
Other tests were made on the products of the combustion of coal, hut 
this time without filtering them. In these tests the filter was removed 
and the pipe conveying the products was connected with the sunning-flask. 
The tests showed that the products gave rise to a considerable amount of 
spontaneous condensation in the test-flask, where they met with saturated 
air. If the filter was now introduced it stopped nearly all the spontaneous 
condensation, because it stopped not only the nuclei hut all the oxidising 
gases in the products, as they cannot pass through the filter ; and if the 
gases were sunned a dense form of condensation took place on expansion, 
but very little spontaneous condensation. If, however, peroxide of hydrogen 
was added to the filtered products of the fire in the flask, a very dense 
condensation took place on expansion, and also a good deal of spontaneous 
condensation, just as was found when testing S0 2 . 
Let us now try to picture to ourselves what takes place in the atmo- 
sphere in which the products of our fires are mixed. During the burning 
of the coal the sulphur in it is oxidised to sulphurous acid, and much of it 
soon changes to sulphuric acid. The latter, having a strong affinity for 
water, will form a number of nuclei. Some of these acids will also combine 
with the ammonia formed during combustion and produce many nuclei. 
Further, if there is any ozone formed during the combustion it also will 
help to make more nuclei. But it would appear that, in spite of all these 
different gases produced during combustion acting on the sulphur acids and 
tending to change them from the gaseous to the solid or liquid state, there 
still seems to remain an enormous amount of these sulphur products in the 
atmosphere in a gaseous form, and it is probably on these that the sun acts, 
