LECTURE II.
[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.