edges his indebtedness to Dalton
for the theory of evaporation; yet he still clings to the idea that
the vapor, though independent of the air, is combined with particles of
caloric. He holds that clouds are composed of vapor that has previously
risen from the earth, combating the opinions of those who believe
that they are formed by the union of hydrogen and oxygen existing
independently in the air; though he agrees with these theorists that
electricity has entered largely into the modus operandi of cloud
formation. He opposes the opinion of Deluc and De Saussure that clouds
are composed of particles of water in the form of hollow vesicles
(miniature balloons, in short, perhaps filled with hydrogen), which
untenable opinion was a revival of the theory as to the formation of all
vapor which Dr. Halley had advocated early in the eighteenth century.

Of particular interest are Howard's views as to the formation of dew,
which he explains as caused by the particles of caloric forsaking the
vapor to enter the cool body, leaving the water on the surface. This
comes as near the truth, perhaps, as could be expected while the old
idea as to the materiality of heat held sway. Howard believed, however,
that dew is usually formed in the air at some height, and that it
settles to the surface, opposing the opinion, which had gained vogue
in France and in America (where Noah Webster prominently advocated it),
that dew ascends from the earth.

The complete solution of the problem of dew formation--which really
involved also the entire question of precipitation of watery vapor in
any form--was made by Dr. W. C. Wells, a man of American birth, whose
life, however, after boyhood, was spent in Scotland (where as a young
man he enjoyed the friendship of David Hume) and in London. Inspired,
no doubt, by the researches of Mack, Hutton, and their confreres of
that Edinburgh school, Wells made observations on evaporation and
precipitation as early as 1784, but other things claimed his attention;
and though he asserts that the subject was often in his mind, he did not
take it up again in earnest until about 1812.

Meantime the observations on heat of Rumford and Davy and Leslie had
cleared the way for a proper interpretation of the facts--about the
facts themselves there had long been practical unanimity of opinion. Dr.
Black, with his latent-heat observations, had really given the clew to
all subsequent discussions of the subject of precipitat