DUST, FOGS, AND CLOUDS. 363 
used. When the air in the receiver is expanded and cooled, this cloudy con- 
densation becomes thicker. 
When commercial hydrochloric acid is put in the test tube, its vapour does 
not give rise to fumes on mixing with the moist air in the receiver, and on ex- 
panding and cooling the air, no fumes appear, only the rainy form of conden- 
sation is produced. A quantity of very strong hydrochloric acid was prepared 
by keeping the solution in which the acid was condensed in a freezing mixture. 
This acid fumed abundantly in the air, but gave no fumes in filtered air, and 
only rainy condensation when the pressure was reduced. 
These two acids act very differently, the first condensing freely at many 
centres, and without nuclei, and giving a foggy condensation in pure and unsatu- 
rated air, while the hydrochloric acid only condenses with difficulty, and at few 
centres, and only gives the rainy form of condensation when supersaturated. 
The next experiments were made with commercial sulphuric acid, and also 
with some of the acid concentrated by boiling in a glassvessel. The, air which 
had passed through this acid gave no fumes, but on making the slightest ex- 
pansion a fog appeared. This fog is quite characteristic of sulphuric acid, and 
is quite different from any artificial fog I have seen. The particles are 
extremely small, and the display of colour remarkably brilliant, and when 
properly lighted rivalling in distinctness the colours of the soap bubble. This 
beautiful fog is only got when the acid is strong, and I think is best produced 
when the entering airis dry. This point, however, requires confirmation, though 
the result might be expected, as the surface of the acid will then be less 
weakened by moisture abstracted from the air. After the acid has absorbed 
much vapour, or if water has been added to it, the fogging decreases and gives 
place to the rainy form of condensation when expansion is made. This rainy 
condensation also disappears when the acid is very weak. If we heat the strong 
acid to a temperature of about 60° or 70° C., the vapour condenses and forms 
fumes in pure air without nuclei, and without being expanded. 
These experiments show that water vapour may be condensed without 
nuclei being present. The affinities which the vapours of the acids have for the 
water, causing the formation of new compounds, and these compounds being 
highly supersaturated, condense easily without nuclei, and in certain circum- 
stances this condensation may be determined in even unsaturated air. These 
water-acid nuclei once formed, continue to act as centres of condensation. In 
these cases the manufactured nuclei are liquid, but solid nuclei may be formed 
in a similar manner. This may be shown by the following experiment. Place 
hydrochloric acid in the receiver or flask, and pump out all the air and replace 
it with filtered air. If, after this is done, and the acid shows no sign of cloudi- 
ness, and nothing but rainy condensation on expansion, we take the stopper 
out of a bottle of ammonia and hold it near the filter, so that the escaping 
