DUST, FOGS, AND CLOUDS. 365 
Using the same apparatus as was used for determining the fuming power 
of the different acids in filtered air, it is found that when experimenting on 
sulphuric acid and vapour of ammonia, that sulphate fumes are formed in the 
receiver if the acid is slightly heated, thus showing that this sulphate dust can 
form without nuclei. It, however, seems in the highest degree probable that 
when dust is present the dust particles will form the centres on which the 
sulphate will condense. 
Almost all salts when heated in a Bunsen flame produce nuclei which 
determine condensation in unsaturated air. The condensing power of the 
different products, however, differ greatly. The bicarbonate of soda gives but 
little effect, while chloride of calcium and bromide of potassium are much 
more powerful. But by far the most powerful artificial fog-producing substance 
when used in this way is the chloride of magnesium. If we put a small 
quantity of this salt on a piece of wire-cloth, and heat it with the BuNSEN flame, 
and collect the products in a wetted receiver, the fog will be seen rapidly 
forming and showering down the sides of the receiver. As rapidly as the 
water is evaporated from the sides of the receiver it is condensed by the active 
nuclei in the gases. After the receiver has been placed on the table for a few 
minutes it will be found full of a fog so dense it is only possible to see through 
a depth of five centimeters of it. When a little of this chloride was heated in 
an alcohol flame in the cellar the result was a fog many times more dense than 
that produced by sulphur alone. The fog-producing power of the heated 
chloride of magnesium would appear to be due to the salt beig decomposed 
by the heat, and free hydrochloric acid being driven off in a highly concentrated 
state. The amount of hydrochloric acid is, however, small considering the 
density of the resulting fog. The density of this fog is very much greater than 
the fog produced by hydrochloric acid prepared from chloride of sodium and 
sulphuric acid. 
In all these cases the reactions are excessively difficult to trace. Other 
experiments in which the action is much simpler were made by burning a little 
sodium in the receiver. The combustion of this substance gives rise to its 
oxides in a fine state of division. This fine soda-dust when mixed with dry 
air gives but little cloudiness, but when mixed with damp air a dense fogging 
results. Potassium when burned gives a similar effect, but the fog is not so 
intense. 
We may conclude from these experiments—1. That as regards cloudy con- 
densation of vapour in our atmosphere there is dust and dust. Some kinds of 
dust have such an affinity for water that they determine the condensation of 
vapour in unsaturated air, while other kinds of dust only form nuclei when the 
air is supersaturated, that is, they only form free surfaces on which the vapour 
may condense and prevent supersaturation. In many of the experiments it 
