235 
1916-17.] On some Nuclei of Cloudy Condensation. 
samples. Another piece cut from the same sheet as the first was therefore 
prepared, but on trial it also failed to give any nuclei. Much time was 
spent in trying to discover the cause of the difference : flasks, tubes, etc., 
were all changed, but without result, and at one time it looked as if the 
first results would have to be scrapped and put down to some unknown 
cause — not an infrequent occurrence in experiments of this kind, where 
very minute quantities of impurities produce very observable results. On 
thinking over all the conditions of the experiments, I remembered that 
the last two pieces of zinc were very carefully amalgamated, and that 
they looked more brilliant than the first piece. Reflecting on what that 
meant, it appeared that in the two last samples I had been testing not 
zinc but mercury. Taking one of the inactive pieces, I rubbed it with 
a piece of emery cloth, to expose the zinc. On now testing, it was found 
to be as active as the first, and in some tests gave more nuclei than shown 
in Table III. 
Having obtained the conditions for making the zinc active and depend- 
able, it was possible to proceed with the inquiry as to whether or not the 
activity was due to impurities in the air. To purify the air the caustic filter 
was used. We cannot, however, draw the purified air directly into the flask 
containing the zinc, because the filter alters the vapour contents of the air, 
and vapour has an effect on the activity ; so if we wish to compare the 
condensation given by pure air with that given by impure air, we must 
have the same amount of vapour in both cases. To accomplish this a coil 
of pipe was introduced between the filter and the flask. The coil was 
wetted inside and placed in cold water, some 15° F. below the temperature 
of the room. By this means the damp air from the purifying filter had 
some of its water taken out, and the dry air from the other filter was 
brought to the same humidity. By attaching the purifying filter and the 
ordinary one alternately to the coil, we obtained the supplies of ordinary 
or pure air at the same humidity. After a number of trials there was no 
evidence that the impurities played any perceptible part in the action. 
The purified air, like the other, gave great quantities of nuclei. The 
explanation of the stoppage of the action seems to be that the zinc gets 
coated with a film of water, which stops the action, since its activity can 
be restored by heating and drying its surface. The tests of lead gave 
negative results. 
Calcium, Potassium, and Sodium. 
Attention was now directed to the most active of all the metals, to see 
if they also produced nuclei while oxidising. Calcium was first tested. 
