234 Proceedings of the Royal Society of Edinburgh. [Sess. 
it will be seen that the few particles produced were extremely small. 
The numbers given probably do not give the correct rate of production, 
as there was not time for the water vapour to get diffused through all 
the flask. The similarity of these tests to those made by chemical action 
at high temperatures is evident. When the action begins, and is slight, 
the nuclei in both cases are very small, and as the action increases in 
activity they become larger. Time also has an important effect. It was 
also noticed that the magnesium was most active if not highly polished 
but only drawn two or three times through the emery cloth. 
The magnesium was removed from the flask Y and a piece of ordinary 
commercial zinc 5 cm. x 2 cm. put in its place. The zinc was polished 
immediately before testing, but it proved much less active than magnesium. 
In dry air only a few nuclei were observed, and in damp air all the nuclei 
were small, none being active with less than 8 per cent, expansion (see test 
No. 13). The time given for the zinc to act was only three minutes, because 
it was found that if left for an hour all the particles had disappeared. 
In the next tests amalgamated zinc was used. The metal in this 
condition gave far more and larger particles than when simply polished. 
As will be seen from test No. 14, Table III, it gave many showers requiring 
only an expansion of 2 per cent, and many more showers at 4 per cent., but 
none requiring more than 12 per cent, expansion. As it is possible that 
others may wish to repeat this experiment with amalgamated zin£, it will 
save them trouble if I give an account of my experiences with it. Some 
time after I had completed the above results, I wished to investigate a 
point which had been observed but not understood at the time. It was 
noticed that the zinc after a time had become inactive, and if left overnight 
there were no nuclei remaining, all being deposited and no new ones made. 
Occasionally the action could be started by supplying a fresh supply of air ; 
at other times this had no effect. The total cessation of all activity while 
in the same air looked as if this depended, not simply on water vapour, 
but on the presence in the air of some impurity which got used up ; 
just as was found when investigating the action of radio-activity on 
sulphurous acid in producing large nuclei, when it was found to depend on 
some impurity in the air.* 
On beginning to investigate this point, the amalgamated zinc previously 
used was tried, but its action was now weak and uncertain. A new piece 
of zinc was therefore amalgamated, but on testing was found to be quite 
inactive. As this piece was cut from a different sheet from the first 
one, it was thought the failure might be due to some difference in the 
* “ The Sun as a Fog-Producer,” Proc. Roy. Soc. Edin ., vel. xxxii, part ii, No. 16. 
