166 
MAGAZINE OF SCIENCE AND ART. 
heat, it will tend to expand with the rise of tern 
perature at exactly the same rate that air would ex¬ 
pand, or, as it is confined, will exert an elastic 
force, increased at the same constant ratio as hap¬ 
pens with air, or any othor permanent gas. This 
is, however, a very different thing from the rapid 
rate at which the force rises when additional 
water is present, because in this case a much 
larger quantity of water is really converted into 
vapour, instead of tho first taken portion of va¬ 
pour exerting its own increased expansive force 
alone. By the elastic force of vapour at suoh-and- 
sueh a temperature, is, therefore, always meant., 
the force with which it is thrown off from the 
surface of water at that temperature, which, is 
the same, of course, as tho maximum force which 
it will ultimately exert against the sides of any 
confining vessel. 
It may be well to distinguish the two senses of 
elastic force * as the actual elastic force, and the 
possible elastic force , the latter being, of course, 
the maximum force, due to a giveu temperature. 
19. But what if we take .a quantity of gaseous 
vapour, and cool it 1 If its elastic force were 
originally 1<;S3 than the maximum, which it might 
and would be, were water present, it may for 
some time remain in the gaseous state. B at as 
the temperature falls, the maximum force due to 
such temperature also fallB very rapidly, while 
the actual elastic force of any given quantity of 
vapour only diminishes, at the slow and oonstant 
rate of a permanent gas. It is, therefore, evi- 
evident, that at eomo determinate temperature, 
the actual force will equal, and afterwards begin 
to exceed the maximum possible force. The result 
which inevitably follows is, that a determinate 
portion of the aqueous vapour is condensed into 
the liquid form, and the elastic pressure of the 
remainder being, as it were, thereby relieved, this 
remainder may continue gaseous, exerting of 
course the maximum pressure duo to the tempera¬ 
ture, uutil a further reduction of the latter neces¬ 
sitates a further condensation. 
20. All these effects are produced solely by 
heat ; but we must also notice how much heat is 
required to produce them. When water is con¬ 
verted into aqueous vapour, a large quantity of 
heat is absorbed and reduced to a latent or imper¬ 
ceptible condition. The exact amount of this 
latent heat varies somewhat according to the tem¬ 
perature at which the conversion takes place, but 
it is sufficient for ns to remember that it is a very 
large amount, and that, in fact, as .much heat 
would be required to convert a cubic inch of wa¬ 
ter into vapour as would raise it, if it could be 
retained in the liquid form, by 1000 degrees of 
Fahrenheit’s scale. 
When a body of water throws off aqueous va¬ 
pour, it must supply this latent heat by the 
reduction of its own sensible temperature. And 
in order that vapour may again be condensed, it 
is necessary that there should be some cold body 
present to withdraw this heat. To show the im¬ 
portant bearing of this point, I will just remark 
that, when a given quantity of rain water falls to 
the surface of the earth, it must have left behind 
it in the air, a very large but determinate quan¬ 
tity of heat, the effects of which aro interesting 
and important to the subject. 
So long as vapour retains its gateous form, the 
quantities of heat required to raise its tempera¬ 
ture are expressed, relatively to other gases, by 
its specific heht, or capacity for heat; hut we ntci 
only remember, that when aqueous vapour is ex¬ 
panded or compressscd, the specific heat does not 
alter so much as with air, which likewise causes 
the sensible temperature not to fail or rise so 
rapidly. For instance, if the whole utmosphere 
were eompoaed of aqueous vapours, it is Mid its 
temperature would not fall more than 3 degrees 
for every 5000 feet of ascent, instead of 1 degree 
for every 300 feet, as is the caso with air. (Daniell.) 
21. Having now a sufficient comprehension of 
the changes which air aud water undergo by the 
influence of heat, we have only to consider their 
mutual relation. 
Of course, when water is either in the liquid 
or the solid state, it is of so much higher a speei- 
tic gravity than air, that it will always tend to 
sink rapidly through the latter. If the liquid 
water be divided, however, into very minute par¬ 
ticles, these will subside so slowly, as to appear 
suspended, and almost stationery in the air. This 
is, I think, the whole mystery of the suspension 
of clouds in tho air, though some eminent persons 
havo thought it necessary to propound various 
peculiar theories on the subject. Indeed, any 
slight difficulty of conception whioh may be felt 
is completely got over, if a diligent examination 
of the clouds discloses, as, I think, many appear¬ 
ances which prove that cloud-particles do subside 
wherever there are no counteracting forces. 
22. It is again not an easy task to make dear 
the relation of gaseous air to gaseous vapour. It 
is generally said that they are independent of each 
other, and that the one does not fill the room of 
the other. This is no doubt true, ultimately. Thus 
almost the same quantity of vapour will rise into 
a cubic foot of space, whether that spjee be pre¬ 
viously vacuous or filled with air of auy density; 
in both cases the vapour will ultimately press 
against the sides of the contailung vessel, with its 
own maximum force appropriate to the temperature. 
In one case this elastic force will constitute the 
whole pressure within the vessel; in the other case 
it will be added to the original clastic force of the 
air, however great that may be. 
But though it would thus appear that air and 
vapour do not in the least intercommunicate their 
pressures, they have the power of impeding each 
other’s motions to an indefinite extent. If water 
he placed in a vacuum, vapour will rise from it, 
and instantaneously fill the whole space; and 
although nearly the same quantity of vapour would 
undoubtedly Tise were air > present, the evaporation 
would, in this case, go on comparatively slowly, 
and the more slowly, the greater the density of the 
air. Particles of air, we may suppose, impede the 
moving particles of vapour, much like particles of 
sand in a filter impede but do not actually restrain 
the water flowing between them. 
23. The relation of air and vapour in the 
atmosphere is best expressed, I think, by saying 
that they are entangled together j and that, though 
quite independent so far as ultimate uniform 
diffusion and equilibrium are concerned, they, 
impede each other in arriving at this condition of 
equilibrium. 
The general effect of this in the.atmosphere is, 
that whatever change befalls a portion of air, tile 
same change must likewise affect the vapour it 
contains, because the latter cannot escape Irom its 
entanglement. 
