231 
1916-17.] On some Nuclei of Cloudy Condensation. 
a cotton-wool filter wetted with caustic potash, a method previously found 
effective in clearing out impurities ; it had no effect, however, in reducing 
the number of nuclei produced by the red-hot platinum. The presence of 
water vapour was suspected of playing a part in the action. As it was im- 
possible to use chemical methods of drying the air for experiments of this 
kind, since the cure is worse than the disease, a physical one was adopted. 
The air after passing the caustic and cotton-wool filters was drawn through 
a coil of pipe placed in a freezing mixture ; but again no decided decrease 
could be observed in the condensation. These experiments are somewhat 
unsatisfactory, because the density of the condensation depends greatly on 
the temperature of the tube, and it is not easy without elaborate apparatus 
to keep this constant ; again, the rate at which the air is drawn through 
the tube affects its temperature, so that any effect of the purification of 
the air might easily be lost in the differences of temperature. 
As already stated, all the tubes became active producers of nuclei when- 
ever their temperature was raised high enough to cause discoloration of the 
bunsen flame. The platinum tube also complied with this rule. The flame 
above the platinum tube was always brighter than below it, and showed 
the D lines more brilliantly. Other samples of platinum gave varying 
amounts of discoloration, but all gave some. After the platinum tube had 
been highly heated to thoroughly cleanse it, the nuclei produced at a low 
red heat were all small, requiring high expansions to make them active, 
similar to what was found with iron. They are so unstable that they all dis- 
appeared in ten minutes, probably owing to their having an electric charge. 
This quick disappearance takes place whether the nuclei are kept in the 
damp test-flask or in an intermediate dry one. If, however, we heat the 
tube to a high temperature, the nuclei are almost all large, just as was found 
with iron, and are as stable as the ordinary dust in the atmosphere, some 
remaining in suspension for more than a day. 
A few tests were made with platinum wire placed in the flask V and 
heated red by electricity. These gave the same result as the tube. The 
results are entered in test No. 7, Table III, from which it will be seen that 
there were very few small particles, though a great quantity of nuclei. 
Porcelain Tube. 
The effect on this tube was interesting. At a good red heat it gave 
great quantities of very small nuclei, which were very unstable. All 
disappeared in three minutes, but by keeping the highest temperature up 
for about ten minutes the particles grew to be large enough to condense 
with the slightest expansion. 
