DUST, FOGS, AND CLOUDS. 359 
apparatus is complete. The glass tube must now be thoroughly cleansed. 
This is done by highly heating it in a BuNsEN flame, while air is being drawn 
through it. The end of the glass tube next the filter is now opened, and three or 
four small pieces of iron wire introduced into it. The pieces of wire are placed 
some distance from each other, and near one end of the tube. The tube is 
now closed, and the Bunsen flame placed under the other end of the tube, and 
far enough away from the pieces of iron so as not to heat them. The air in 
the apparatus is now thoroughly cleansed by pumping out the air and admitting 
filtered air, till no cloudiness appears. During this process the height of the 
flame has been reduced, so as the temperature may not be high enough to drive 
anything off the glass tube. When the air is quite pure, and all rainy 
condensation ceased, the flame is reduced to about one-half, so as to leave a 
good margin of safety. After this is done, one of the small pieces of iron wire 
is drawn from the cold part of the tube by means of a magnet, and dropped 
- in the hot part, and two or three strokes of the pump are made, to cause a 
current of air to pass through the tube and bring whatever impurities are 
driven off the iron into the flask. The stop-cock at the filter is now closed, 
and a slight vacuum made. The amount of nuclei given off by the wire is 
indicated by the amount of cloudy condensation which now takes place. 
To make further certain that the impurities came from the wire, the piece of 
iron is now removed by means of the magnet, when the filtered air is now found 
to come into the flask without any nuclei, the air remaining cloudless on 
expansion. To make still further certain of the result, another of the pieces of 
wire is drawn into the hot part of the tube, when the cloudiness again appears, 
and again disappears after its removal, or after it has been highly heated. 
The pieces of iron wire experimented on weighed from zo55 tO gogq Of a 
grain. With pieces so small as this, so abundant and evident is the cloudiness 
produced, that I feel certain that if I could have manipulated, say the z5,o55 
of a grain, the effect would have been perfectly definite and decided. 
Thousands of particles driven off the zo55 of a grain, and the wire not 
perceptibly lighter afterwards, indicates almost molecular dimensions. It 
seems probable that some of the nuclei in these experiments are driven off as 
gases or vapours. These gases and vapours will afterwards condense when 
cooled in the receiver. It is not necessary that these gases should have nuclei 
on which to condense, as they will be highly supersaturated when cooled to the 
temperature of the receiver, and we know that it is only when supersaturation 
is slight that nuclei are necessary. These gases will, according to their com- 
position, condense either to solid or liquid nuclei, on which the water vapour 
will condense. 
In the first part of this paper attention has beenjcalled to the importance of 
the composition of the atmospheric dust. It was pointed out that some kinds 
