364 JOHN AITKEN ON 
gaseous ammonia may pass along with the air through the filter, the ammonia 
on arriving in the flask will combine with the hydrochloric acid and form a 
dense cloud of sal-ammoniac. When the ammonia and the hydrochloric acid 
combine in the filtered air, the tension of the sal-ammoniac vapour so formed 
is enormously greater than that due to the temperature, and it easily condenses 
without nuclei. This experiment suggests that part of the rainy condensation 
given by hydrochloric acid may be due to the ammonia in the air combining 
with the acid and forming sal-ammoniac nuclei on which the vapour condenses. 
These experiments show how nuclei may be formed from gases in the air, 
and these nuclei may have so great an affinity for water vapour as to cause 
it to condense on them from an unsaturated atmosphere. 
Returning again to the action of the products of combustion of sulphur in 
air, we have seen that these products alone can determine the condensation of 
water vapour from unsaturated air. There are, however, many substances in 
the air with which this acid will tend to combine. It would be impossible 
to go over all the substances in the air which have affinities for this acid, 
and consider the effects of these new compounds, in moist air. I have, how- 
ever, selected one, which from the magnitude of its effects deserves special 
notice. That substance is ammonia, another of the products of combustion of 
our coal fires. If we take an open-mouthed receiver wetted on the inside, 
and hold it over a little burning sulphur for a few seconds, as in the previous 
experiment, we will get a thin haze, which we know tends to thicken. But if 
on placing the receiver on its tray, we put a drop of ammonia on a piece of 
glass and introduce it into the receiver, the result is very striking. Dense 
fumes will be seen to rise from the ammonia, and in a few minutes the receiver 
will be full of a fog so thick it will be impossible to see an object in the middle 
of the receiver. In this case there are evidently formed solid nuclei, composed 
of sulphite and sulphate of ammonium, in a very fine state of subdivision. 
The intense cloudiness is only in part due to this solid, the greater part is due 
to the condensation of water vapour. If the experiment is made in dry air the 
fogeing is not nearly so intense as in moist air. By burning a larger amount 
of sulphur in the moist air of the receiver, we can easily make a fog so very 
intense that it is impossible to see through an inch of it. This fog is found to 
be very suitable for experiments on vortex rings, as it is easily prepared, and 
the “ dead ” rings dissolve, and do not thicken the air of the room to the same 
extent as the usual sal-ammoniac rings. 
Experiments were also made in the cellar with this fog-producer. The 
wet and dry bulb thermometers at the time showed a difference of fully one 
degree. Yet by burning a few grains of sulphur, and dropping on a piece of 
paper a little ammonia, the cellar became filled with a most intense fog, — 
many times more intense than would be produced by the sulphur alone. 
