[7] Page 55. _Crystallisation of Alum._--The solution must be
saturated--that is, it must contain as much alum as can possibly be
dissolved. In making the solution, it is best to add powdered alum to
hot water as long as it dissolves; and when no more is taken up, allow
the solution to stand a few minutes, and then pour it off from the dirt
and undissolved alum.
[8] Page 57. _Red Precipitate of Biniodide of Mercury._--A little care
is necessary to obtain this precipitate. The solution of potassium
should be added to the solution of perchloride of mercury (corrosive
sublimate) very gradually. The red precipitate which first falls is
redissolved when the liquid is stirred: when a little more of the
iodide of potassium is added, a pale, red precipitate is formed,
which, on the further addition of the iodide, changes into the
brilliant scarlet biniodide of mercury. If too much iodide of potassium
is added, the scarlet precipitate disappears, and a colourless solution
is left.
[9] Page 57. _Paper Coated with Scarlet Biniodide of Mercury._--In
order to fix the biniodide on paper, it must be mixed with a little
weak gum water, and then spread over the paper, which must be dried
without heat.
_Biniodide of Mercury_ is said to be _dimorphous_; that is, is able to
assume two different forms.
[10] Page 59. “_Prince Rupert’s Drops._”-These are made by pouring
drops of melted green glass into cold water. They were not, as is
commonly supposed, invented by Prince Rupert, but were first brought to
England by him, in 1660. They excited a great deal of curiosity, and
were considered “a kind of miracle in nature.”
[11] Page 60. _Thick Glass Vessels._--They are called _Proofs_ or
_Bologna phials_.
[12] Page 61. _Mica._--A silicate of alumina and magnesia. It has a
bright metallic lustre--hence its name, from _mico_, to shine.
[13] Page 62. _Common salt_, or chloride of sodium, crystallises in the
form of solid cubes, which, aggregated together, form a mass, which may
be broken up into the separate cubes.
[14] Page 62. _Iceland_ or _Calc Spar_.--Native carbonate of lime in
its primitive crystalline form.