Water, from its extensive powers as a solvent, never occurs in a state
of absolute purity, although the nature and degree of its contamination
must necessarily vary according to circumstances and situation. It is
generally found holding earthy matter in a state of mechanical
suspension, or saline and other bodies in chemical solution. The usual
varieties of common water are classed and defined by Celsus; and modern
chemists have not found any reason to reject the arrangement. “_Aqua
levissima pluvialis est; deinde fontana, tum ex flumine, tum ex puteo;
posthæc ex nive, aut glacie; gravior his ex lacu; gravissima ex
palude._”
1. RAIN WATER. _Aqua Pluvialis_, when collected in the open fields, is
certainly the purest natural water, and consequently of the least
specific gravity; the bodies which it holds in solution are, carbonic
acid, a minute portion of carbonate of lime, with traces of muriate of
lime. DEW is said to be water saturated with air. Rain water ought,
however, to be boiled and strained whenever it is collected near large
towns; Hippocrates gives this advice, and M. Margraaf of Berlin has
shewn the wisdom of the precaution by a satisfactory series of
experiments.
2. SPRING WATER. _Aqua Fontana_, in addition to the substances detected
in rain water, generally contains a small portion of muriate of soda,
and frequently other salts; but the larger springs are purer than
smaller ones, and those which occur in primitive countries, and in
siliceous rocks, or beds of gravel, necessarily contain the least
impregnation. An important practical distinction has been founded upon
the fact, that the water of some springs dissolves soap, whilst that of
others decomposes, and curdles it; the former has been termed _soft_,
the latter _hard_ water; soft water is a more powerful solvent of all
vegetable matters, and is consequently to be preferred for domestic as
well as medicinal purposes; the brewer knows well from experience how
much more readily and copiously _soft_ water will dissolve the
extractive matter of his malt. Horses by an instinctive sagacity always
prefer soft water, and when by necessity or inattention they are
confined to that which is _hard_, their coats become rough and
ill-conditioned, and they are frequently attacked with the gripes.
Pigeons also refuse hard water when they have been accustomed to that
which is soft.[382]
3. RIVER WATER. _Aqua ex Flumine_, being derived from the conflux of
numerous springs and rain water, generally possesses considerable
purity; that the proportion of its saline ingredients should be small,
is easily explained by the precipitation which must necessarily take
place from the union of different solutions; it is, however, liable to
hold in suspension particles of earthy matter, which impair its
transparency, and sometimes its salubrity; this is particularly observed
of the Seine, the Ganges, and the Nile.[383]
4. WELL WATER. _Aqua ex Puteo_, is essentially the same as spring water,
being derived from the same source; it is, however, more liable to
impurity from its stagnation, or slow infiltration;[384] hence our old
wells furnish much purer water than those which are more recent, as the
soluble particles are gradually washed away. Mr. Dalton observes that
the more any spring is drawn from, the _softer_ the water becomes.
5. SNOW WATER. _Aqua ex Nive_, has been supposed[385] to be unwholesome,
and in particular to produce bronchocele, from the prevalence of that
disease in the Alps, but it does not appear upon what principle its
insalubrity can depend; the prejudice however is a very ancient one, for
Hippocrates observes that snow or ice water is unwholesome, in
consequence of its finer particles being evaporated and lost during its
solution: it appears to differ only from rain water in being destitute
of air, to which water is certainly indebted for its briskness, and
perhaps for many of its good effects upon animals and vegetables. The
same observations apply to _Ice Water_.
6. LAKE WATER. _Aqua ex Lacu_, is a collection of rain, spring, and
river waters, contaminated with various animal and vegetable bodies,
which from its stagnant nature have undergone putrefaction in it.
7. MARSH WATER. _Aqua ex Palude_ being the most stagnant is the most
impure of all water, and is generally loaded with decomposing vegetable
matter.
To what extent the impurities of water are capable of influencing its
salubrity, has been a subject of interesting inquiry from the age of
Hippocrates to the present day. To many of these natural contaminations,
too much importance has been certainly attached; it is an affected
refinement to suppose that the presence of minute portions of such
earthy and calcareous salts, as generally occur in solution, can impart
any noxious quality to water;[386] whilst on the contrary, animal and
vegetable impurities, or earthy bodies in a state of mechanical
suspension, cannot fail to prove injurious, and must be regarded as the
true “SCELERA AQUARUM.” Guided by false analogies many have supposed
that they recognised the origin of all calcareous diseases in the earthy
impurities of water; the researches however of chemistry have removed
this delusion, by demonstrating that the substances found in water never
enter into the composition of urinary calculi.[387] Metallic and other
accidental contaminations are necessarily highly injurious, and the
water in which their presence is suspected, should be submitted to the
most careful examination.
For the purification and preservation of water numerous methods have
been adopted; the mechanical impurities may be removed by filtration,
which is performed through porous stones, or alternate layers of sand or
charcoal; muddy water may be also cleared by adding a few grains of alum
to each pint,[388] and in that proportion, the water is not rendered in
the least disagreeable: when water has contracted a putrid smell, it may
be rendered sweet by agitating it with a small portion of magnesia, or
with black oxide of manganese, in the proportion of 1½ parts to 250
parts of water. Dr. Black observes that nitrate of silver, which is one
of the most antiseptic substances known, will preserve water from
putrefaction for ever, and that it may at any time be separated
therefrom in a few minutes by adding a small lump of common salt; this
fact in itself is curious, but the experiment is too hazardous to be
recommended. Dr. Alston prefers lime, as a preservative of the water,
and proposes to remove it by the addition of a carbonate of magnesia;
Dr. Henry has however found that it is more economically precipitated by
the introduction of a current of carbonic acid into the cask. As that
peculiar property of water which is termed _hardness_, generally depends
upon the presence of _sulphate of lime_, the addition of an alkaline
carbonate twenty-four hours previous to its being used, will be found to
restore it, or if it should depend upon _supercarbonate of lime_, long
ebullition without any addition will be found sufficient for its cure.
Water when kept for a long time in casks, especially on long voyages, is
partially decomposed, and a volume of carburetted hydrogen is
evolved,[389] imparting to such water the peculiar smell and taste which
characterise it; this decomposition may in a great degree be obviated by
charring the interior of the water casks; it is, however, prevented in
the Navy by substituting iron tanks for wooden vessels. In Pharmacy it
ought to be remembered that whenever common water is employed it should
not be _hard_; filtered rainwater maybe recommended as the most eligible
on such occasions.
AQUA DESTILLATA. L.E.D. Distilled Water.
QUALITIES. _Taste_, vapid from the absence of air, and slightly
empyreumatic in consequence probably of the presence of a small quantity
of extractive matter which has undergone partial decomposition; a
fluid-ounce weighs 454½ grains. MEDICINAL USE. In extemporaneous
prescriptions, distilled water should be always ordered whenever the
formula contains any of the following substances:—_Acidum Sulphuricum_;
_Acidum Citricum_; _Antimonium Tartarizatum_; _Argenti Nitras_; _Cuprum
Ammoniatum_; _Ferrum Tartarizatum_; _Hydrargyri Oxy-murias_; _Liquor
Ammoniæ_; _Liquor Plumbi Sub-Acetatis_; _Liquor Potassæ_; _Plumbi
Acetas_; _Solutio Muriatis Barytæ_; _Vinum Ferri_; _Zinci Sulphas_;
_Ferri Sulphas_. Distilled water ought also to be employed in
preparations where much water is evaporated, as in the formation of
extracts, since the residual matter of common water will remain mixed
with the product of the process, and uselessly add to its bulk, or even
in some cases produce in it chemical changes; unless however under such
circumstances, common water purified by filtration should be ordered, as
the air which it contains imparts to it a pleasant and sprightly
flavour. In making infusions or decoctions, it is very important that
the water should be free from those impurities which impart to it
_hardness_, and which render it a far less powerful solvent of vegetable
matter, nor indeed can resinous substances be mixed with such water,
even when assisted by a mucilaginous medium. On which account, in
prescribing emulsions, it may perhaps be prudent to direct the
employment of distilled water. TESTS OF ITS PURITY. Its transparency
ought not to be disturbed by the addition of nitrate of silver, or
muriate of baryta.
AQUA MARINA. Sea Water.
Until the late able researches of Dr. Murray, we possessed but an
imperfect knowledge of the composition of sea water; it is not therefore
surprising that the analysis performed by different chemists should be
found to be so materially at variance; the true cause of such
discordance is now easily understood, for it appears, that in the
examination of a mineral water or any compound saline solution, the
substances obtained from it are not necessarily the original
ingredients, but frequently the products of new combinations established
by the operation of analysis, and that consequently the nature of the
result obtained may vary according to the modes in which such analysis
has been conducted, or even according to the degree of dilution in which
the saline substances exist.[390] The elements of the salts contained in
a pint of sea water are _Lime_ 2·9, _Magnesia_ 14·8, _Soda_ 96·3,
_Sulphuric Acid_ 14·4, _Muriatic Acid_ 97·7, total 226·1 grains, and
supposing these elements to be combined in the modes which Dr. Murray’s
views appear to establish, the saline contents of a pint of sea water
may be expressed as follows, _Muriate of Soda_ 159·3, _Muriate of
Magnesia_ 35·5, _Muriate of Lime_ 5·7, _Sulphate of Soda_ 25·6 grains,
total 226·1 grains; besides such saline contents, it is contaminated
with various animal and vegetable bodies, in consequence of which it
becomes, when long kept, highly offensive; it ought also to be stated
that Dr. Wollaston has discovered the presence of a minute proportion of
potass in sea water; and Dr. Marcet has more lately detected ammonia in
combination with muriatic acid. MEDICINAL USE. As a cathartic, a pint is
the ordinary quantity, which should be taken in the morning, at two
doses, with an interval of half an hour between each; this quantity
contains half an ounce of purgative salt, of which about three-fourths
are muriate of soda, but it is much more active than a similar portion
of any artificial combination. In procuring sea water for medicinal
purposes, there is a precaution, the importance of which experience has
suggested to me, that it be not hastily drank on the beach, before the
particles of sand, with which under such circumstances it is generally
mixed, are allowed to subside; from the neglect of this precaution I
have witnessed serious consequences. The most important advantages of
sea water are derived from its external use as a bath.