Among crystalline bodies, rock-crystal, or silica, is the best
conductor of heat. This fact accounts for the steadiness of temperature
in one set district, and the extremes of Heat and Cold presented by
day and night on such sandy wastes as the Sahara. The sand, which is
for the most part silica, drinks-in the noon-day heat, and loses it by
night just as speedily.
The influence of the hot winds from the Sahara has been observed
in vessels traversing the Atlantic at a distance of upwards of
1100 geographical miles from the African shores, by the coating of
impalpable dust upon the sails.