A Se EE 
perature. Thus, the heated air is cooled by 
evaporation from the earth, without which all 
tropical regions would be uninhabitable by man, 
and even our temperate latitudes so hot in sum- 
mer as to cause death. When the body is over- 
heated, nature has provided a flow of aqueous 
perspiration, which appears upon the skin, and 
by its evaporation keeps the body at a proper 
temperature. Anything impeding this perspira- 
tion causes disease and death. All our supplies 
of fresh water also come from the atmosphere as 
a result of this general evaporation, which is a 
modifier of heat, and the source of the freshness 
and fertility that make the surface of the earth 
so beautiful to look upon, when teeming with 
luxuriant verdure, and with grain and fruit for 
the use of living creatures. When the air be- 
comes surcharged with moisture, or when a 
change of temperature or a change of electrical 
action takes place, the vapour is condensed and 
falls to the earth in refreshing showers, or in the 
form of snow or hail. Snow is the vapour frozen 
ere it has had time to condense into rain-drops; 
hail is formed by the freezing of the drops them- 
selves; and all three have their several uses in 
the economy of nature. The water which thus 
falls, charged with some matters contained in 
the atmosphere, has several functions to perform. 
The greater portion of it sinks into the earth, 
forming springs, sources of rivers, or remaining 
hidden in great reservoirs till called forth by 
the energy of man; another portion is immedi- 
ately evaporated back again into the atmosphere; 
and a third portion is decomposed by the plants 
upon which it falls, and which, assimilating its 
hydrogen for their nourishment, give out the 
oxygen to the air,—and this last explains why 
the proximity of vegetation is so refreshing im- 
mediately after a heavy shower, the portion of 
oxygen being increased without having yet had 
time to be diffused. See the articles Evapora- 
TION, ATMOSPHERE, CioupD, and Rain. 
Rain water, though freer from foreign substances 
than any water which has flowed in contact with 
the solid earth, contains atmospheric air, carbo- 
nic acid, ammonia, and sometimes other gases ; all 
ef which it absorbed during its formation in the 
clouds or during its descent thence to the earth; 
and all the ammonia—but very particularly in the 
first rain which falls after a drought—is distinct- 
ly charged with the offensive odour of animal 
perspiration, excrements, and putrefaction. See 
the article Ammonia. Though in London rain- 
water is seldom applied to domestic purposes, as 
the Thames supplies the wants of the laundress, 
and no contrivances are generally used for its 
preservation, yet in many provincial cities and 
towns, and in the rural districts, it is carefully 
preserved for the purposes of the wash-house ; 
and where no other soft water is to be obtained, 
rain-water forms a very important article of do- 
mestic use and consumption. But it is unfit to 
drink; though in some countries, where none 
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other is to be had, it is preserved in tanks, and 
applied to the purposes for which hard water 
is preferable. Rain-water, however carefully 
filtered, is mawkish and unpleasant to the taste, 
and, if not actually unwholesome, is very inferior, 
as a solvent in the stomach and for the supply of 
the human frame, to water from springs and 
fountains, or even to that from rivers and lakes. 
Filtering does not divest it of any matter held in 
chemical solution; and those who drink rain- 
water must take it with the unpleasant accom- 
paniments which are combined with its am- 
monia. 
Water derived from rivers and lakes is some- 
times as soft as rain-water, and sometimes as 
strongly impregnated with lime and other com- 
pounds as water from hard springs, or from those 
termed ‘mineral springs.’ What it carries with 
it depends entirely upon the soil through which 
it runs. But it requires a number of years be- 
fore the water from heaven can, by contact with 
the soil, extract an alkaline salt from it. This 
is the reason why rivers and lakes, especially 
when fed by heavy rains, or periodical meltings 
of snow and ice, have their waters soft ; and these 
‘waters are sometimes freed from the ammonia 
derived from the atmosphere, sometimes strongly 
impregnated with it. When at hand, they serve 
for the same domestic purposes as rain, and are 
used besides as a beverage for men and cattle. 
But such waters, especially in rich valleys, are 
frequently charged with matters which render 
them unwholesome unless carefully filtered. They 
carry with them, in mechanical mixture, not 
only the alluvium which renders them turbid, 
but much putrid animal and vegetable matter. 
In addition to this, in large cities, especially in 
those where manufactures exist, and through or 
near which a river flows, this latter receives, in 
addition, various very unwholesome compounds, 
besides the worst filth and soil of the place, from 
which no filtering will free the water. No at- 
tention is ever paid to keep such rivers from 
pollution, and the inhabitants are not .unfre- 
quently compelled to draw water for their tea 
and other purposes from a river into which run 
the drains and sewers of the town. The soft 
water of rivers, springs, and brooks, not far from 
the sea, generally holds in solution a minute 
quantity of common salt. Unless it run over a 
rocky bed, the water of rivers requires filtering 
before it is fit for human drinking. Many of the 
mountain torrents in Switzerland are said to be 
impregnated with salts which, when the water 
holding them in solution is applied to domestic 
use, produce that deformity in the throat known 
by the name of goitres,—a complaint common 
in mountainous districts,even in England. The 
soft water of rivers, lakes, ponds, or fountains is 
generally preferred for the preparation of vege- 
table infusions and decoctions, and of vegetable 
extracts. It is stated to be preferable likewise 
for the brewing of ale, porter, and beer; though 
