an insoluble salt. It is essential
that the carbonate of ammonia be neutral.

This test was first pointed out by Dr. Wollaston.

The presence of oxygen gas loosely combined in water may readily be
discovered in the following manner.

EXPERIMENT.

Fill a vial with water, and add to it a small quantity of green sulphate
of iron. If the water be entirely free of oxygen, and if the vessel be
well stopped and completely filled, the solution is transparent; but if
otherwise, it soon becomes slightly turbid, from the oxide of iron
attracting the oxygen, and a small portion of it, in this more highly
oxidated state, leaving the acid and being precipitated. Or, according
to a method pointed out by Driessen, the water is to be boiled for two
hours in a flask filled with it, and immersed in a vessel of water kept
boiling, with the mouth of the flask under the surface of the water: it
is to be inverted in quicksilver, taking care that no air-bubble adheres
to the side of the flask, and being tinged with infusion of litmus, a
little nitrous gas is to be introduced: if the oxygen gas has been
sufficiently expelled from the water, the purple colour of the litmus
does not change; while, if oxygen be present, it immediately becomes
red.[16]

If we examine the different waters which are used for the ordinary
purposes of life, and judge of them by the above tests, we shall find
them to differ considerably from each other. Some contain a large
quantity of saline and earthy matters, whilst others are nearly pure.
The differences are produced by the great solvent power which water
exercises upon most substances. Wells should never be lined with bricks,
which render soft water hard; or, if bricks be employed, they should be
bedded in and covered with cement.

METHOD OF ASCERTAINING THE RELATIVE QUANTITY OF EACH OF THE DIFFERENT
SUBSTANCES USUALLY CONTAINED IN COMMON WATER.

To ascertain the quantity of earthy and saline matter contained in
water, the following is the most simple and easy method.

EXPERIMENT.

Put any measured quantity of the water into a platina, or silver
evaporating basin, the weight of which is known, and evaporate the water
upon a steam bath, at a temperature of about 180 deg., nearly to dryness;
and, lastly, remove the basin to a sand bath, and let the mass be
evaporated to perfect dryness. The weight of the platina basin being
already known, we have only to weigh it carefully. When the solid saline
con