237 
1916-17.] On some Nuclei of Cloudy Condensation. 
wards answer to a much lower one. They act just like “penny dips”: 
after each coating they come out larger. It is thus possible to bring all the 
very smallest nuclei down with only a 2 per cent, expansion. First give 
a quick expansion of 8 or 10 per cent., and immediately return the pressure ; 
after that dip many will be found to respond to a 2 per cent, expansion. 
When these are all down, another high expansion will grow more of 
the remainder to the size that will be active at 2 per cent, expansion, 
and so on. It should be noted that the air in this experiment is never 
very dry, but it will not be saturated, owing to the sudden increase of 
25 or 30 per cent, in the pressure due to the return of the piston, which 
provides more than enough heat to dissolve the particles which have not 
fallen out. 
The activity of the different metals after polishing declines at different 
rates. Zinc is soon inactive. Magnesium falls off* somewhat rapidly at 
first, but keeps up its action, though feebly, for a long time, as even 
an old unpolished piece gives some nuclei ; these, however, may be due 
to the broken ends, or to clean surfaces produced in the handling. 
Amalgamated zinc remains fairly active for days. 
It may be as well to state that the figures in Table III do not 
represent anything definite ; they only indicate the nature of the results. 
A slight change in the temperature in the first seven tests would alter 
all the figures, higher temperatures tending to increase the size and the 
number of the nuclei. Similarly with the last seven tests : anything that 
increased or diminished the activity of the action would change all the 
figures. It should also be noted that all these tests were made in darkness, 
so that the effects of ultra-violet light on the zinc and other metals 
are excluded. 
After the above was written I found that Professor C. T. R. Wilson * 
had made a number of experiments with his apparatus on the nuclei pro- 
duced by metals. He tested zinc, amalgamated zinc, lead, copper, and tin. 
His conclusion was : “ In no case were the metals found to produce nuclei 
requiring only slight expansions to catch them.” He did find, however, 
that most of them have an effect. When the expansion was great enough 
to cause condensation on the ions — that is, when v 2 /v 1 was between 1*25 
and 1*38 — the condensation in the presence of the metals was denser 
than without them. The great difference in the results of our tests seems 
to receive its explanation in the conditions of the two methods of experi- 
menting. In Professor Wilson’s apparatus the metals were in the test- 
chamber, and therefore exposed to saturated air, and their surfaces would 
* Phil. Trans., A, vol. 192, pp. 403-453. 
