ON CHEMISTRY. 
297 
immediately in contact with them. But as they are only the instru¬ 
ments, and not the causes of the phenomenon, some mighty agent 
induces, as before stated, a change in the aerial region, and renders 
that region the attracting surface by producing a stratum or body 
of clouds : the vapors then, are drawn upwards: the etherial matter 
in the opposite surface of the ground, under the clouds, is poured 
forth— still through the herbage as its conducting medium,—renders 
that surface warmer, attenuates the watery deposit upon the points 
of plants, and bears it upward in the form of vapors, which join, and 
congregate with the attracting stratum of clouds. 
The second part of the paragraph asserts that when warmth is 
compatible with clearness, the dew becomes very copious. This 
seems to be an assumj)tion of a fact that occasionally may be, and is, 
in conformity with the order of nature, but which is by no means 
generally so. In very dry summers, the dew rapidly diminished : 
in 1818, when the temperature at night ranged between 60 and 70 
degs. for weeks together, scarcely any dew was deposited. Confirmed 
drought, perfect clearness, and high temperature, by day and night, 
were unproductive of dews, though the evaporation must have been 
at its maximum. In fact, air, —heavy air,—was the concomitant; 
the barometer was almost constantly above 30 inches, and proved 
the weight of the atmospheric column. Even in the present arid 
spring, the dews amount (where my means of observation extend, at 
least,) to little or nothing; not to one-fourth of that quantity in 
which they are deposited in ordinary, showery springs, during the 
fine intervals. 
Dr. Wells observed that a very thin, and slight covering, even a 
muslin handkerchief, stretched at a few inches above the surface of 
the ground, retained much warmth;—thus, ‘ one night when the fully 
exposed grass was 11 degs. colder than the air,’ the sheltered grass 
was 3 degs. warmer:’—from these, and other facts, some philoso¬ 
phers—Dr. Wells particularly—have inferred that—the formation of 
dew is the consequence of radiation, —that cold is the cause of dew, 
and not dew of cold; and it is always found, during the formation 
of dew, that the surface of the ground is colder than the circumja¬ 
cent air, owing to its radiation of heat into the atmosphere. The 
best radiators are soonest dewed; hence, grass and vegetables are 
more quickly covered with dew than gravel stones or metals; and as 
the earth dissipates its heat by radiation, it will be seen that any 
slight awning spread over the ground will prevent radiation, and 
keep the earth warm.” “Bodies become colder than the neighbour¬ 
ing air before they are dewed; and as different bodies project heat 
