488 Proceedings of the Royal Society of Edinburgh. [Sess. 
an active fog-producer, yet it exerted some influence on the impurity 
which made the gases sensitive to sunshine. Further, even with the aid of 
the impurity it had no condensing effect by itself, and it was inactive so 
long as it was kept in the dark. 
A few other experiments were made with newly prepared gases to see 
if they deserved the character usually given them of being powerful fog- 
producers. The gases were prepared by means of a small battery, the gases 
being given off at platinum electrodes, dipped either in tap water or very 
weak acid ; but neither hydrogen nor oxygen, nor both combined, showed 
any tendency to cause fogging. In all the tests only a few drops were seen 
falling in the test flask, such as one expects from the bubbling of the gases. 
Other methods were used for preparing the gases, but so far I have not 
found them either fog-producers or even nucleus-producers, so long as they 
are not prepared from strong acids nor dried in sulphuric acid — a common 
practice. Now, in passing gases through that acid they become contami- 
nated with one of the worst fog-producers we have, and the stronger the acid 
the worse it is. As I showed long ago, if we wet a glass rod with sulphuric 
acid and heat it we get a stream of as fine a fog-producing vapour as any 
experimenter could desire ; and if we bubble ordinary air through sulphuric 
acid into moist air it soon makes a fog. The density of this fog seems to 
depend on the acid used. Some samples give denser fogs than others. The 
strength of the acid also affects the density of the fog produced by it ; but 
all the samples tested were fog-producers, and some gave very bad fogs 
even with filtered air, though the fogs are always less with filtered air than 
with unfiltered. 
Another test of the condensing power of newly prepared gases was made 
by placing some iron wire and sulphuric acid in the flask S and drawing in 
filtered air. When the newly prepared gases entered the test flask they 
only gave a slight condensation, due to the bubbling gases ; and this was 
stopped if the gases were passed through a glass-wool filter before they 
entered the test flask, and the gases showed no sign of spontaneous condensa- 
tion, which constitutes a true fog-producer. 
Oxidising Gases. 
The experiments on salt and iodine showed that, though these are 
occasionally active nucleus-producers, their activity is not constant ; and as 
this activity is always at the beginning of a test, it is not therefore likely 
to be due to the composition of the air entering the flask, but the composi- 
tion of the air may have some effect and may make them active. The 
