227 
1916-17.] On some Nuclei of Cloudy Condensation. 
thoroughly mixed with the air in the room. This of course gave a great 
increase in the total number of particles, which required a great number of 
expansions to clear ; but it will be seen from Table II that there was no 
increase in the number of very small particles. From this it would appear 
that neither flames nor solids in combustion are the source of the very small 
particles. It may be pointed out that these tests are open to the objection 
that the very small particles may have been brought down by the great 
number of showers. I have already shown reason for supposing that they 
did not exist at the beginning of the test. That they can escape being 
brought down by the showers is evident from the results of test No. 3, 
Table III. In that experiment the nuclei were so numerous that a large 
amount of filtered air had to be added before beginning the sifting process, 
and a very great number of showers was required to clear out the larger 
particles, yet there remained nuclei enough for many showers up to very 
high expansions. In that case there was probably no electric charge, as 
the temperature at which they were produced was not high enough to 
cause its escape from the iron. 
Nuclei produced by Heat. 
As the very small particles found in polluted air do not seem to be 
produced by combustion, an investigation of the effects of heat on different 
substances was made. In a paper, “ On Dust, Fogs, and Clouds,”* read 
before this Society in 1881, I pointed out that when bodies are cleansed 
by heat enormous numbers of nuclei are produced, and that it was pos- 
sible easily to detect the impurity driven off a small piece of iron wire 
only o~^oo of a grain in weight. This suggested that some of the very 
small nuclei in the air might have their origin in the something driven off 
by the heat of the fire from the coal and other bodies exposed to it. As 
this early experiment gave no indication of the size of these nuclei, the 
subject was investigated with our new methods. 
In order to test the effect of heat in producing nuclei from different 
kinds of matter, it was necessary to have a vessel which would stand 
heating without giving rise to nuclei. Tubes of a number of different 
substances were therefore tested. To do this the flask V was removed 
and a tube introduced in its place, the air being drawn from the filter 
through the tube into the test-flask. The tubes tested were of iron, brass, 
copper, glass, porcelain, alundum, and two kinds of silica tubes. One of 
these latter is transparent and looks like glass, and the other opaque and 
looks as if composed of white silk threads stretched lengthwise. Two 
* Trans. Roy. Soc. Edin vol. xxx, part i. 
