422 
MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
luminous rays. The scattering of the ultra-violet rays by the cloud particles is 
probably also the cause of the fog (without expansion) spreading further and 
reaching a greater density on the side of the focus next the source. 
If the exposure to the rays of the arc he continued after the fog has become 
visible, this often assumes very remarkable shapes, resembling those obtained by 
Tyndall with air which had been passed through a solution of hydrobromic or hydio- 
iodic acid.'^'' The fog may, for example, develop dark strise which may he straight 
and vertical, or may have quite complicated forms. They sometimes produce a 
regular cone-in-cone structure {cf. Tyndall, loc. cit.), or the fog may become divided 
up into rounded clouds often connected by a thread of fog along the axis of the 
tube. These complicated forms were noticed most frequently in some of the earlier 
experiments, in which a longer tube (60 centims.) of the same cross-section was used, 
Avith a quartz lens Avhose focal length was 25 centims. The light was thus less concen¬ 
trated and the fog took much longer to become visible, generally about ten minutes. 
There can be no doubt that all these complicated cloud forms o\ve their origin to 
air currents in the tube. A dark stria may ahvays be produced at will at any part 
(d’ the fog by Avarming the loAver edge of the tube at that point, by holding one’s 
finger against it. A stream of air, free from fog, rises at this point in a narroAv 
layer, and a dark vertical stria is produced,! the brightness of the fog immediately 
on each side of it being also increased. Such a dark hand persists for a long time 
after the exciting cause has been removed. 
No condensation could be produced in pure steam even Avith prolonged exposure. 
The air AA^as expelled from the tube by alloAving a rapid current of steam to pass 
through the apparatus for an hour at Ioav pressure. The tube AA^as alloAved to cool to 
the temperature of the room (15° C.) before exjDosing to the ultra-violet rays. The 
failure to j^roduce any Ausible condensation in the steam Avas not due to the quartz 
becoming dimmed through drops of Avater condensing on it, for on letting in a small 
(.[uantity of air the effect Avas readily obtained. Fogs Avithout expansion AA^ere 
obtained Avithout difficulty Avith an air pressure of 5 centims. of mercury. 
To determine to Avhat extent Avater vapour Avas necessary for the production of these 
fogs, experiments Avere made Avith a much smaller apparatus than that just described. 
I’his is shown in fig. 6. The tube Avas 16 centims. long and 4 centims. in diameter. 
A solution of potash, or of sulphuiic acid, AA'as placed in the bottom of the tube. 
The stop-cocks Avere lubricated Avith H2SO4. A sIoav current of filtered air AA’as draAvn 
through the apparatus. This Avas then alloAved to stand for some time to enable the 
equilibrium vapour pressure to be attained before the exposui’e to the ultra-violet 
rays Avas begun. 
* Tyndall, ‘ Ifoy. Soc. Proc.,’ \'ol. 17, p. 92, 1869. 
t The production of dark Imiids of this kind in fogs Avas observed by Ta'NDALI, (‘ Roy. Inst. Proc.,’ 
vol. 6, p. 1, 1870), and further studied by Lord R.AYLEIGII (‘Roy. Soc. Proc.,’ a’oI. 34, p. 414, 1882), 
Lodge (‘ Nature,’ aoI. 28, p. 297, 1883), and Aitkex (‘Trans. Roy. Soc.,’ Edin., vol. 32 (1), p. 239). 
