SOCIAL SCIENCES 1rs9

Eighteenth Century Theories of Evaporation and Rain

DWAYNE R. MASON, University of Oldahoma, Norman
During the first half of the eighteenth century, theories of evapora-
tion and rain based upon a hydrostatic concept were dominant. These
hydrostatic theories were characterized by the common idea that very
small amounts of water are greatly increased in volume by heat. These
discrete particles of water vapor are thus of less specific weight than the
surrounding air and are therefore buoyed up by the weight of the air. An
early example of this type of theory is that of Edmund Halley (16rs6-1742).
Halley reasoned that if an atom of water were to be expanded to 10 times
its fonner diameter by heat, it would then be specifically 1000 times
lighter than before. Since liquid water is 800 times as heavy as air, the
expanded atom of water would be buoyed up hydrostatically in air and
would rise until it reached a level at which the air had the same specific
weight as the expanded water atom.' Wllhelm Homberg (16rs2-1715)
explained this expansion of water atoms as the result of particles of fire
attaching themselves to the particles of water and forming a conglom-
erate lighter than air.
s
William Derham (1657-1735) agreed that evap-
oration was a hydrostatic process but insisted, "Water Vapour is nothing
other than small Bubbles, or Vesiculae detached from water by Solar or
Subterraneous Heat, or Both." Derham considered the visible bubbles
formed in boiling water to be an extreme example of this process.'
These theories explained the formation of clouds and rain as the
reverse of the process of evaporation. This created a minor conceptual
difficulty. If the fire matter forms a union with, or Is encapsulated by,
water to form a body lighter than air, what is the mechanism by which
the fire matter is released, causing precipitation? Bernard Nieuwentijt
(1654-1718), a Dutch mathematician, offered a solution to this problem.
He had come to this conclusion by reasoning from experimental evidence.
Having subjected a cup of very hot water to the partial vacuum of an air
pump, Nieuwentijt observed that the water boiled whlle water of the same
temperature in a cup outside the air pump did not. He also observed that
after having boiled under low pressure for a period of time, the water was
sensibly cooler than than which had been left outside the air pump. From
this evidence Nieuwentijt inferred that air pressure prevents the escape
of fire matter attached to water particles whlle the reduced pressure in the
higher levels of the atmosphere permits it.
John Theophilus Desaguliers (1683-1744) argued that the existence
of fire matter had never been demonstrated. Particles of matter do not
actually expand upon being heated but the repulsive force of the particles
is increased by heating. A given amount of heat increases the repulsive
force of a water particle more than the same amount of heat increases
the repulsive force of an air particle. "For the same Heat which rarefies
Air only %, wlll rarefy Water very near 14000 times, changing it into
Steam or Vapour as it boils it: And in Winter that small degree of Heat,
Which in respect to our Bodies appears cold, will raise a Steam or Vapour
from Water at the same time it condenses Air.'"
In all of the foregoing hydrostatic theories of evaporation, water
vapor is visualized as particles moving up through air like bubbles rising
through water. The matter of dust and smoke, for example, was known
to have specific weights hundreds of times that of air. During the eigh-
teenth century the suspension of such particles in air was explained to be
the result of their having very large 8Urface areas in comparison to their
Weights. As a result, air resistance 18 able to greatly slow their fall. In
order to have a buoyant force only equal to the downward tendency of a
dust-mote, a particle of water vapor would be required to have a specific
weight hundreds of time. less than air. Some scIentista considered this
180 PROC. OF THE OKLA. ACAD. OF SCI. FOR 1965
physical model unaau.tactory and began to evolve theories of evaporation
and rain that were founded upon an entirely different conceptual frame-
work.
An early example of th!s new type of theory appears in an unsigned
item in the Hutoire de J'ACGdemje Royale du Bcietacea for the year
The author gave h1a interpretation ot the reasoning ot Jean Bouillet (1690-
1777). Water i8 known to abeorb some of the air which touches its sur-
tace even though the air ia much lighter than the water. Conversely, air
ro111ng along the 8urface ot water picks up some of the water, even though
the water ia much heavier than the air.'
In 1751. Charles Le Roy (1726-1779) presented a paper to the
l'Acadtmje Jl,t)yaJe des Sciences entitled sur Ie la sus-
pension de l'eau dana I'air Ie sur 1& RoHe." Le Roy stated flatly that air
d18solves water in the same manner as water dissolves most salts. He
used the term dusoJutWti in the same way it was used by re8 OhimiBt68.
Just as hot water can dissolve more salt than cold water, hot air can
di880lve more water than cold air. There is a limit to the amount of either
I&lt or water that can be dissolved in a solvent at any given temperature.
It a solution ia saturated and the temperature of the solution is reduced,
precipitation occurs. Thus, Le Roy substituted for the physical model ot
the hydrostatic theories a chemical model in which the mechanism of
evaporation was "l'union inUme des dernieres molkules des deux corps.'"
Benjamin Franklin (1706-1790) was elected to the Royal Society of
London in 1756. On June 3d of that year a paper written by Franklin and
entitled "Physical and Meteorological Observations, Conjectures, and Sup-
positions" was read to the Society. Franklin stated that, although par-
ticles of air are mutually repulsive, those of air and water attract one
another. "Hence water will dissolve in air, as salt in water.'" With "...
every particle of air assuming one or more particles of water; when too
much [water] is added, it precipitates in rain.'" The active agent of pre-
cipitation is the tendency of water to cohere. Warm air will hold more
than cold air because, "... its particles being by heat repelled to a greater
distance from each other, there by more easily keep the particles ot
water, that are annexed to them, from running into cohesions."lo Frank-
lin rejected the 8upposition that evaporation was analogous to boiling.
In boi11ng heat reverses water's normal tendency to attract and cohere.
Steam from bo1l1ng water does not mix with air; on the contrary, it
expels air from the space it had occupied.
The solution theory of evaporation assumed that moisture-laden air
ia heavier than dry air, just as a solution of salt in water has a greater
8pecific weight than that of water alone. Franklin insisted that a par-
ticle ot air loaded with adhering water, although heavier than a dry air
particle, could be supported and held aloft by the interaction of several
particles of dry air in a system of mutual repulsion. He explained that
heavy, loaded particles in descent would repel air particles in their way.
forcing them nearer to other particles, which in turn, repel them back
into the path of the descending particles. The repulsive force of the
particles immediately below the descending ones would be augmented by
that of their neighbors until it was sufficient to hold the loaded particles
suspended,11
Thus. during the mld-elghteenth century, there were two distinctly
different types of theories of evaporation and rain. The hydrostatic the-
ories were ba8ed upon the concept of the matter of water and air having
volume. weiCht. and lmpenetrabillty. Of theee characteristics, only vol-
ume is mutable. Heat 18 the apnt of change In volume. This conceptual
modelia phyBlcal, analogous to bubble. rlBlDg through water. The solution
theories of evaporation and rain were based upon the concept that the
SOCIAL SCIENCES 161
matter of air is self-repulsive, the matter of water is cohesive, and that
water and air have an affinity for one another. Heat is an agent only
Inasmuch as it increases air's repulsion for itself. This conceptual model
is chemical, analogous to salt dissolving in water and to precipitates
forming in supersaturated solutions.
The choice of later eighteenth century scientists between these two
types of theories was probably based upon their commitment to a partic-
ular conceptual scheme. Some scientists could not subscribe to a concep-
tual framework which seemed to deny the well established hydrostatic
and hydrolic theories of the day. On the other band, many scientists
found the concepts of saturation and precipitation so satisfying that they
would not accept any theory that did not include them.
NOTES AND LITERATURE CITED
'E[dmund] Halley, "A Estimate of the Quantity of Vapour raised out
of the Sea by the warmth of the Sun; derived from an Experiment shown
before the Royal Society, at one of their late Meetings," Phil. Trans. Roy.
Soc. London No. 189 (Sept & Oct 1687), pp. [366]-[370].
2"Vapours" in John Harris, Lexicon technicum, or a Universal Bng-
lish Dictionary of Arts and Sciences, Bxplaining Not only the TeTm3 Of
Art, but the Arts Themselves (London: Printed for Dan. Brown, et al.,
1704), pp. 7F[lr]-[7Flv].
SW[illiam] Derham, Physico-Theology: or, a Demonstration of the
Being and Attributes of God from, His works of Oreation, Being the Su1J-
stance of XVI Sermons Preached in St. Mary Ie Bow-Ohurch, London, at
the Hon" Mr. Boyle's Lectures, in the years 1711 and 171', With large
Notes, and many curious Observations Never Before Publi8hed (London:
W. Innys, 1713), pp. 20-21; 48-49.
Bernard Nieuwentijt, The Religious Philosopher: or, the Right Use
of Oontemplating the Works of the Oreator. 1. In the wonderful Structure
of Animal Bodies, and in particular Man, 11. In the no less wonderful and
wise Formation of the Elements and their various Bllects upon Animal
and Vegetable Bodies, And 111. In the most amazing Strocture Of the
Hea'OCns, with all its Furniture. Designed for the Oonviction Of Atheists
and Infidels. Throughout which, all the late Discoveries in Anatomy,
Philosophy, and Astronomy, together with the various Bxperiments made
use of to illustrate the same, are most copiot181y handled by that learned
Mathematician Dr. Nieuwentijt. To which i8 prefix'd a Letter to the
Translator by the Reverend J. T. Desagulier8. Translated by John Cham-
berlain (3d ed.; 2 vola.; London: J. 8enex, E. Taylor, W. and J. Innys,
and J. Osborne, 1724), vol. 1, pp. 241-263.
'John Theophilus Desaguliers, "An Attempt to Solve the Phaenomenon
of the Rise of Vapours, Formation of Clouds and Descent of Rain," PhU.
Trans. Roy. Soc. London XXXVI (1729), 6-22.
'''Sur des liquides," HiBtoire de I'Academte Royale de8
Sciences. Annee M.DCOXLl1. Avec le8 memoires de matMmGtjque cG de
physique, pour la meme annee. des regiBtr68 de cette Acadhnfe
(Paris: de l'Imprimeris Royale, pp. 18-21.
'Charles Le Roy, lIur It la lIuspenaion de l'eau
dans l'air, It sur la Histoire de l'Academfe Royale de.tJ Scfences.
Annee M.DOOLI. Avec le8 memoir68 de mat1lbn4tlque cG de phyfique,
poIIr la meme annee. Tiru des regwtr68 de cette AcacUmfe (Parie: de
l'Imprimerie Royale, 1755), pp.
'Benjamin Franklin. uPhysical and Meteorological Observations, Con-
jectures, and Suppositions," Phil. TrelM. Roy. Soc. London LV (1766),
182-192.
p. 182 -lind., p. 183, JlIbfd., p. 186.