426 MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
not due to mercury vapour was 2 :)roved by making the exposure immediately after the 
stream of oxygen had stopped before the mercury va|:)Our in the gauge could have had 
time to diffuse into the rest of the aj^paratus. Before the final exj)eriment, also, the 
tubes leading to the gauge and the jjei'manganate tube were sealed off. After the 
cloud aiDjoeared the exjoosure was continued for three-quarters of an hour and the 
apparatus left till the next day. The fog was found to liave disaj^i^eared by that 
time, as was seen on exposing to the light of the arc with a plate of mica interjDosed; 
in less than two minutes after the removal of the mica, however, a new fog was 
produced. The last ex 2 :»eriment was performed with the object of determining 
whether the formation of the cloud dej^ended on the presence of minute traces of some 
sulrstance, which might all be used iqD to form fog if the exjDosure was sufficiently 
j^rolonged. After the fog had settled to the bottom of the vessel, it was thought 
that the complete or jDartial removal of the active substance would he made manifest 
in an increased difficulty in ^^roducing a second fog. No such effect was found; the 
exjijosure was perhajos, however, not sufficiently 2 )rolonged for any very great weight 
to be attached to the result. 
The experiments described leave little room for doubt that pure oxygen and water 
vapour alone are sufficient to enable a cloud to he jDroduced under the influence of 
ultra-violet light. 
In hydrogen, fogs could not Ije obtained under' the influence of ultra-violet light 
without exjransion. Experiments were made with an exiransion ajrjraratus to see if 
nuclei of any kind were produced in this gas when exjrosed to ultra-violet light. 
Fig. 8. 
A T-shaped cloud-chamber (fig. 8) was used, the rest of the apjjaratus being that 
shown in fig. 1. The horizontal tube (fig. 8) was 9 centims. long and 1‘8 centims. 
in diameter. The ends were closed by quartz jdates, fixed like those in the large 
apparatus which was used for exj^eriments on the clouds irroduced in moist air by 
ultra-violet light without exjransion. 
The arc was used as the source of ultra-violet rays. These were made to converge 
by means of a quartz lens to a ])olnt slightly beyond the middle of the tube. The 
