Hartisy—On Haze, Dry Fog, and Hail. 551 
dust was steel-grey and it was magnetic. It contained iron, 
copper and sodium, lead calcium, potassium, manganese, nickel, 
silver, thallium (a trace), gallium, and rubidium (a trace), 
doubtful. 
As II. contained thallium, a substance found in pyrites flue- 
dust, it is evident that it might be precipitated from the 
atmosphere in a neighbourhood where sulphuric acid is made ; 
but as the flue-dust from vitriol works in Dublin contains a 
notable proportion of indium, the absence of this element pre- 
eludes the possibility of I. and II. having come from them. 
Volcanic dust is distinguished by the very small proportion of 
the heavy metals, lead ‘and iron for example, and the principal 
constituents being lime, magnesia, and the alkalies. 
Dust from the clouds shows a certain regularity in composi- 
tion, whether it has fallen directly, or been carried down by rain, 
sleet, snow, or hail. Hach specimen, out of several, appeared to 
contain the same proportions of iron, nickel, calcium, copper, 
potassium, and sodium. ‘The proportion of carbonaceous matter is 
small. The samples collected from hail, sleet, and snow differ 
from one another, and differ widely from other specimens, by 
reason of the large proportions of lead which they contain. 
It need scarcely be mentioned that different specimens of 
soot differ very widely in composition, even when taken from 
different chimneys in the same house. ‘The quantity of the 
metals, such as iron, lead, copper, silver, &ec., differs with the 
amount of carbon in the soot. 
Having pondered over the subject, I will now proceed to 
state the conclusions arrived at after collecting together the 
results of numerous and varied observations. 
Dry Dust.—Fume of whatever kind, condensing to solid matter, 
causes the formation of a dry dust, provided such condensation 
takes place in a dry air. Then the proportion of moisture 
condensed on the solid matter is small; the result, as Aitken has 
shown, is a mist or fog; if large, it is a cloud; if still larger it 
may become’ hail, rain, or snow, according to the temperature. 
Shortly, however, it may be stated that what appears as mist low 
down on a mountain, exists as snow at a greater elevation where 
the air is colder. Dry dust may be seen in the air as a distinct 
discoloration of the sky. It may arise from the blowing away 
