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Proceedings of the Royal Society of Edinburgh. [Sess. 
Concluding Remarks. 
Let us now see if these experiments help us to understand what these 
small particles are which in my early papers are called dust particles, which 
form the nuclei of cloudy condensation in the atmosphere, and which are 
now called large ions and have been investigated by means of the electric 
held. In many of the papers defending the latter view there seems to 
be considerable misunderstanding, as some of the writers associate these 
very fine particles with the dust raised by winds, etc. Now, though one 
particle of that kind of dust may weigh more than thousands of the finer 
particles, their number is quite negligible. It is certainly very difficult to 
understand how there can be such enormous numbers of solid and liquid 
particles in the air. That there are such numbers is evident, however, and 
we must do our best to understand the conditions and not simply say it is 
impossible. The whole difficulty seems to lie in the size of the particles. 
If they are extremely small the whole difficulty vanishes. 
For illustration of the minuteness of the particles let us take the experi- 
ment with the 20V0 °f a grain of iron wire heated to a temperature much 
below red. From its contaminated surface the heat drove off many 
thousands of particles capable of causing condensation with but slight 
supersaturation, though it would have required a very fine balance to 
detect the loss. And yet it had lost something, as it ceased to be active after 
a short time unless it was heated higher or touched with something not 
purified by heat, after which it would be again active. Or take another 
example, in the experiment made with pure hydrogen burned in dustless 
air, previously referred to, where it was found that hydrogen burning under 
these conditions produced no nuclei that caused condensation with slight 
expansion. This, so far as I know, is the only example of chemical action 
in gases not accompanied by the formation of particles. While the burning 
hydrogen gave no particles, yet if a minute speck of cotton — so small as to 
be carried by the gentle current in the tubes — passed through the hydrogen 
flame there were always produced thousands of particles which caused con- 
densation with slight expansion. There was no dubiety in this observation, 
since it could be repeated by simply tapping the filter, and after a short 
time the passage of the speck of cotton was indicated by a bright flash 
in the almost invisible hydrogen flame, followed by the presence of con- 
densation in the test-flask. The extremely small amount of matter in 
condensation nuclei is evident from these and other experiments. 
If one might venture an opinion as to the difference in this case where 
the combustion of the hydrogen gave no particles, while the burning of 
