230 Proceedings of the Royal Society of Edinburgh. [Sess. 
much filtered air had to be added before clearing out the larger nuclei, while 
after these were all down there were many small nuclei requiring higher 
expansions to make them active. The result of a second heating is shown 
in test No. 3. The condensation is still very dense, and there are still 
many very small nuclei. Test No. 4 shows the effect of a slightly higher 
temperature. There is now a great decrease in the number of the larger 
particles, but a great increase in the number of the very small ones, 
some requiring an expansion of 20 per cent, to make them active. Test 
No. 5 gives the result of the next heating, when, as will be seen, there were 
almost no large nuclei, but many very small ones — some so extremely small 
that they required the highest possible expansions. In this test there were 
still smaller nuclei than shown in the table ; 2 showers were obtained 
with an expansion of 24 per cent., showing the presence of nuclei requiring 
a supersaturation almost as great as that required by ions, so indicating 
almost molecular dimensions. It will be noticed that in these tests the 
number of large nuclei was greatest at first and decreased with each 
successive heating. The probable explanation is that these large nuclei 
are due to the effect of heat on the impurities on the surface of the iron, 
and that as these are driven off the large nuclei disappear; there now 
being less impurity, the nuclei become smaller, and some of these small 
nuclei may be due to the oxidation of the iron as the temperature rises. 
In test No. 6 is given the effect of heating the iron to a red heat. The 
nuclei are again very abundant and large, and the air had to be greatly 
diluted before it could be tested for small nuclei ; but scarcely any of 
these were found, less than a shower requiring more than 2 per cent, 
expansion. These large nuclei are probably the result of chemical action 
combined with the escape of electricity at the high temperature. 
A piece of thick copper wire was also tested in the tube. Like the iron, 
it gave very dense condensations when heated much below red heat. 
When the temperature was raised the number and size of the nuclei de- 
creased, but at red heat it was not so active as iron, though it continued 
to give small nuclei. On afterwards examining the copper it was found 
to be blackened by the chemical action. 
Platinum Tube. 
Some tests were made with a small platinum tube. It was found that 
a good deal of heating did not stop its producing nuclei at a low red heat. 
Catalysis was suspected as the cause of these nuclei, as oxidation is not 
likely to take place. Some experiments were therefore made to see if its 
action could be checked by purifying the air. The air was passed through 
