902 Scientific Proceedings, Royal Dublin Society. 
of the pollen or the bloom from flowering plants, and this some- 
times is seen in hot dry weather on the moors and mountains of 
Scotland, when the heather is in full bloom. Sometimes a dry 
dust may be observed in the upper stratum of the atmosphere, 
which largely consists of the spores of fungi, and with a moist 
air at lower levels and a high temperature, such a cloud becomes 
blight. Dry dust may also arise from the smoke of towns, or the 
condensation of fume from metallurgical works, or from the finer 
particles of dust from the smoke-shafts of factories, but which 
differs from smoke in its comparative freedom from carbonaceous 
matter. Such dust is visible in the neighbourhood of manu- 
facturing towns. 
Wet Dust.—Wet dust, such as that I now refer to, accompanies 
rain, hail, or snow in its descent upon the Harth. The dust from 
hail and sleet, suddenly precipitated, differed in composition from 
any other form of dust which was collected from the atmosphere 
in or near Dublin, during the course of the investigation referred 
to above, by reason of the large proportion of lead present. In 
this respect it resembles dust from the flue of an assaying furnace, 
where lead ores are constantly melted in considerable quantities, 
and lead fume is seen to rise from the crucibles when they are 
uncovered. It also resembles the dust from the flue of a gas- 
- muffle furnace, in which lead is oxidised and volatilised when 
cupellation assays are made. 
We could imagine the possibility of dust being brought down 
by means of sleet shortly after its escape from the laboratory 
chimney ; but on careful consideration of the facts, this appears to 
have been impossible with the particular samples of dust which 
were obtained from sleet. With hail-stones, in which the mineral 
particles are the actual nuclei, and so are contained within the 
spherules of ice, such a source of fume as a neighbouring chimney 
cannot be considered as having provided them. At first sight 
the following fact appeared to be a possible explanation of the com- 
position of this dust. About five miles to the south-east of Dublin 
lie the disused Ballycorus lead mines, in the neighbourhood of 
which lead smelting to a very limited extent is still carried on. 
The flue for condensing the fume runs up the surface of a hill for 
about half a mile, and terminates in a vertical shaft on the 
summit. When this hail, which has been mentioned, was collected, 
