Page 159 - The Wellington photographic handbook

" — — "
up or they attract moisture to such an extent as to liquefy. Pure
should be asked for. Solutions of potassium hydrate should not
be kept in stoppered bottles, as they make the stoppers stick.
Except for moisture, this substance keeps well in solid form and
in solution.
Potassium Iodide (otherwise known as iodide of potash).
This salt is sold in the form of opalescent white crystals, generally
more or less cubical in shape. It is not likely to be impure, is
readily soluble in cold water and keeps well (in a stoppered bottle),
both in the solid form and in solution.
Potassium Metabisulphite (sometimes called metabisulphite
of potash).—This salt should take the form of small, sharp-angled
crystals, generally with a slight coating of a white powder upon
them. This powdery layer should not go very far into the crystals,
and may be removed by washing, as described under the head of
sodium sulphite. The metabisulphite met with is generally suffi-
ciently pure. It is not very soluble, and should be dissolved in
cold or tepid water, as hot water drives off some of the sulphurous
acid, as can be smelt, which is the active part of the salt from the
photographer's point of view. It keeps fairly well in a properly
corked or stoppered bottle, both in crystals and in solution.
Pyro (also called pyrogallic acid, pyrogallol).—This is sold
in a fine, snow-like powder, also in very fine small crystals and in
larger crystals. The former dissolves most readily ; the two latter
are more easily weighed out. When bought it should be quite white.
Pyro keeps indefinitely in the dry form, and for a considerable
period in solution if sulphite or metabisulphite is present. Its
solution goes darker with age, and when black and thick is useless.
Silver Nitrate (nitrate of silver).—The quality known as
re-crystallized should be asked for, and should be in the form of
transparent flat-shaped crystals, with little or no white powder
amongst them. Silver nitrate keeps very well in solution or in the
solid form. When dissolved in ordinary water there is usually a
slight cloudiness formed, but this can be ignored.
Sodium Bicarbonate (otherwise bicarbonate of soda).
J-l 131