VAP 
Valve, in anatomy, a lliin membrane, 
applied on several cavities and vessels of 
the body,' to afford a passage to certain luv- 
moiirs going one way, and prevent their re- 
flux towards the place from wlience they 
came. The veins and lymphatics are fur- 
nished with valves, which open towards the 
heart, but keep close towards the extremi- 
ties of those vessels ; that is, they let the 
blood and lymph pass towards the heart, 
but prevent their returning towards the ex- 
treme parts from whence they came. 
VAN, in naval affairs, the foremost divi- 
sion of a naval armament, or that part which 
leads the way to battle, or advances first in 
the order of sailing. 
VANDELLIA, in botany, a genus of the 
Didynamia Angiospennia class and order. 
Natural order of Peisonatae. Scrophnla- 
riae, Jussieu. Essential character: calyx 
four-parted ; corolla ringent ; filaments the 
two outer from the disk of the lip of the co- 
rolla ; anthers connected by pairs ; capsule 
one-celled, many-seeded. There are two 
species, viz. V. diffusa, and V. pratensis. 
VANGUERIA, in botany, a genus of 
the Pentandria Monogynia class and order. 
Natural order of Aggregat®. Rubiace®, 
Jussieu. Essential character: calyx five- 
toothed ; corolla lube globular, with a 
hairy throat ; stigma bilamellate ; berry in- 
ferior; four or five seeded. There is but 
one species, viz. V. edulis, supposed to be a 
native of China. 
VAPOUR, in meteorology, a thin humid 
matter, which, being rarefied to a certain 
degree by the action of heat, ascends to a 
particular height in the atmosphere, where 
it is suspended, until it returns in the form of 
dew, rain, snow, &c. On this subject we 
refer our readers to the articles Evapora- 
tion and Meteorology, and shall make 
■"a few additional observations on dew, which 
is a phenomenon proper to clear weather. 
It begins to be deposited about sun-set, is 
most constant in vallies, and on plains, near 
rivers, and other collections of waters, and 
abounds on those parts of the surface which 
are clothed with vegetation. It is often 
suspended when rain is approaching, as like- 
wise in windy weather^ and before thunder 
storms; an unusually copious deposition 
however sometimes precedes rain. The 
following is said to be the usual appearance 
in the valley through which the Thames 
passes. After a clear warm day tliere is 
gradually formed on the horizon a conti- 
nuous haze, rising sometimes to a conside- 
rable height, and often tinged by the set- 
VAP 
ting sun with a fine gradation of red and 
violet shades. This is the precipitated wa- 
ter become faintly visible in its descent. 
Dew is always to be found on the grass by 
the time that this haze has become conspi- 
cuous, and its abundance is propcvtioned to 
the density and permanence of She latter. 
The following facts are deserving of no- 
tice. 
In this country the dew is observed more 
copiously in the mornings of spring and 
summer than at other times in the year. 
Sometimes, however, in autumn and winter, 
an abundant dew is deposited in the night. 
In countries nearer the equator, the dews are 
generally observed in the morning through- 
out the whole year ; and in some places 
they are so very copious as in a great mea- 
sure to supply the deficiency of rain, which 
seldom falls in tliose places. The conden- 
sation of the vapour which forms the dew 
mostly takes place while the sun is below 
the horizon ; the greatest deposition taking 
place soon after the setting of the sun. In 
cloudy weather there is little or no dew 
deposited: the most considerable quantity 
is observed in a morning, subsequent to a 
clear, still, and cool night, which has fol- 
lowed a pretty warm day. The lower parts 
of bodies that are exposed to the ambient 
air are first covered with dew. The most 
singular circumstance is, that dew is not 
deposited upon all kinds of substances in- 
discriminately : it falls upon certain bodies 
much more abundantly than on others, and 
upon some even not at all. The drops of 
dew attach themselves to glass, crystals, 
and porcelain, much more readily than to 
other bodies; next to these come tlie leaves 
of vegetables, wood, especially when var- 
nished, and common earthen ware, but the 
dew adheres least of all to all sorts of me- 
tallic bodies. We may now notice Mr. 
Dalton’s observations, which are the result 
of a variety of well conducted and very ac- 
curate experiments on this subject. 1. That 
aqueous vapour is an elastic vapour sui ge- 
neris, difffiisible in the atmosphere, but form- 
ing no chemical combination with it. 2. 
That temperature alone limits the maxi- 
mum of vapour in the atmosphere. 3. That 
there exists at all times, and in all places, 
a quantity of aqueous vapour in the atmos- 
phere, variable according to circumstances. 
4. That whatever quantity of aqueous va- 
pour may eixst in the atmosphere at any 
time, a certain temperature may be found, 
below which a portion of that vapour would 
unavoidably fall, or be deposited, in the 
