constituents of the air, but condensible again to 
water by cold; their presence can easily be proved 
by reducing artificially its temperature. Thus, 
if a pitcher be filled with ice, the vapours of the 
air will be seen to condense on its outer surface, 
as a dew, and collect into drops of water. It has 
been asserted that, by the above evaporation, the 
water dissolves in the air as a salt in a liquid. 
But it has been found that evaporation takes 
place equally well in vacuo, and that here at 
equal temperatures the same amount of vapour 
will form in a given space as if it were filled with 
air. If, therefore, no other atmosphere existed, 
the earth would still be surrounded by an atmo- 
sphere of watery vapour; the quantity of it, 
therefore, only depends on the temperature. The 
atmosphere, therefore, so far from facilitating 
| the evaporation of water, is rather an obstacle to 
| its diffusion, since, in a vacuum, it would expand 
| itself with the utmost velocity. That wind and 
other commotions of the air facilitate the evapo- 
ration, depends upon the removal of the vapour 
_ formed from the surface of the water, whereby 
further evaporation would be prevented. 
The amount of vapour varies according to the 
temperature of the atmosphere and other circum- 
stances, from 4 per cent. to more than 4 per cent. 
by vol. Verner found as a mean of 50 experi- 
ments, in 1,000 parts of air, 8'47 parts of vapour, 
the maximum being 10°18, the minimum 61. In 
the forenoon, and before 2 o’clock, the mean was 
7:97, after 2 o'clock till evening, 8°85. In gen- 
eral, the higher the temperature, the greater is 
the absolute quantity of vapour. In the same 
‘way it is greater in summer than in winter, in 
day-time than in the night, in warm and low 
countries than in cold and mountainous, and in 
the proximity of seas and rivers, greater than 
farther inland. Much depends also on local cir- 
cumstances, as the peculiar property of the soil, 
the facility of drainage, the prevalence of certain 
winds. The atmosphere, however, rarely con- 
tains at any temperature the full amount of va- 
pour which it is capable of taking up. If this be 
the case, all further evaporation from the surface 
|-of the earth into it must of course cease; while, 
on the other hand, if its content of vapour be 
considerably below its capacity for it, evapora- 
tion is much more rapid, and the air is then said 
| to be dry. Hence the different influence between 
dry and damp air on the animal body, which 
must continually throw off a certain amount of 
| moisture by evaporation from the skin, in order 
to keep it of a certain moisture and temperature, 
-which is necessary to the preservation of health. 
If we compare the adsolute quantity of vapour 
with the quantity which the atmosphere at the 
same temperature is capable of taking up, we ob- 
tain the relative quantity of the vapour or the 
degree of moisture. It will thus be seen, that 
with the same absolute quantity of vapour, the 
moisture of the air depends on the temperature, 
and that at a higher temperature, the air may 
ATMOSPHERE. 
still be drier than at a lower temperature, al- 
though the absolute quantity of vapour be larger 
in the former than in the latter case, and that 
air which at a certain temperature feels damp and 
uncomfortable, by a slight increase in the tem- 
perature may become comfortable. In summer, 
and in warmer climates, the absolute quantity of 
vapour is generally larger, but the relative quan- 
tity smaller. The relative quantity of moisture 
in the atmosphere, may be determined by ascer- 
taining how far the air may be cooled down be- 
fore it deposits any moisture on a cooled body, 
which temperature is called its dewpoint. See 
Hyerometry.—When the atmosphere becomes 
perfectly saturated with vapour, the slightest 
cooling will cause the vapour to condense and as- 
sume the liquid form. When this takes place, the 
condensed vapour separates first as fine vesicles 
floating about in the air of from ga/55 to 4a'5o 
inch in diameter, thus constituting the common 
phenomena of clouds and fogs, which by collect- 
ing into drops or freezing, form rain or snow. 
Clouds.—So long as aqueous matter remains 
in the state of vapour, it is transparent. On its 
first condensation a cloud appears. ‘The manner 
of the formation of clouds is as follows :—Water 
on its first condensation tends to unite in the 
form of hollow globules, or vesicles containing 
air; as it parts at the same time with its latent 
heat, the air, as well within the vesicles as be- 
tween them, is rarefied, and the united mass of 
water and rarefied air may remain as light as an 
equal bulk of atmospheric air, or even lighter. 
Clouds may, therefore, remain floating in the at- 
mosphere, or even rise. As this heat is dissipated, 
the clouds grow heavier and fall, while the air in 
the vesicles losing its elasticity, permits them to 
be broken by the internal pressure. The water 
then runs into drops, which, being many times 
heavier than atmospheric air, descend, forming 
rain. 
Clouds may be formed in all cases where the 
temperature of the ground is lower than that at 
which the vapour, mixed with atmospheric air, 
can remain permanent. Thus, whenever a warm 
wind flows over a cold surface, mists and fogs 
take place; and if the difference of temperature 
be considerable, they may break into rain. For 
an equal difference of temperature between the 
ground and air, it may be shown that the greatest 
quantity of precipitation will take place, when 
the two unequal temperatures are both high. 
Thus the causes that would produce heavy rains 
in warm climates, may produce no more than 
fogs, or dense mists, in those that are colder. 
Clouds may also be formed on sudden changes of 
wind, when two masses of air are mixed that are 
both nearly saturated with moisture. It is to 
this cause that nearly all the rains of temperate 
climates are due. The passing of warm winds 
over cold surfaces, rarely produces more than 
mists or fogs, except in warm climates. When 
clouds, after being formed, begin to descend, in 
