240 Mr Blackadder on circumstances connected with the 
schist, with veins branching from it in all directions. At F, the 
limestones are separated by beds, G, of amygdaloid, and filled 
with nodules of Hint, agates, &c. 
The valleys are filled up with detritus, and are nearly level. 
In a climate so dry and so conservative, where it never rains, 
the rocks present a novel aspect to the eye of a European. In 
the Desert, however, there are evident traces of torrents, per- 
haps the effect of water-spouts. It is wonderful that mountains 
of their height should not attract a regular supply of humidity 
from the atmosphere. The wells in the centre of the tract may 
be about 100 feet deep, excavated in the schist, and are gene- 
rally brackish and sulphureous. 
Art. III. — On certain Circumstances connected with the Con- 
densation of Atmospheric Humidity on solid surfaces. By 
Henry Home Blackadder, Esq., F. R. S. E. &c. Surgeon. 
With a Plate. Communicated by the Author. (Concluded 
from p. 91.) 
4'* Solid bodies, which are at the same time the worst con- 
ductors of heat, and are possessed of a strong hygroscopic pro- 
perty, or an organization corresponding in effect thereto, are 
those which have their temperatures reduced most speedily, and 
to the greatest amount, when exposed on a clear evening after 
sunset. Solid bodies which have no hygroscopic property, 
and are the best conductors of heat, are those which have their 
temperatures the last, and the least reduced of all others. 
Of the class of substances which have no. hygroscopic proper- 
ty, those which have the least capacity for heat, and have the 
least conducting power, have their temperatures the soonest, and 
most considerably reduced. 
These positions might be shewn to be correct, by compara- 
tively recent experiments and observations, but this has been 
judged unnecessary, as they are fairly deducible from facts al- 
ready well known to every one, at all conversant with the subject. 
If then, with these positions in view, it can be shewn, that, on an 
evening productive of dew, polished metals may have moisture 
condensed on their surfaces, without radiation, or any thing, 
equivalent thereto being requisite to bring about that effect it 
