Page:Various Forces of Matter.djvu/188

176 and, in chemical language, is called carbonate of lime. When sulphuric acid is added to it, the carbonic acid is set free, and the sulphuric acid unites with the lime to form sulphate of lime.

Carbonic acid, under ordinary circumstances, is a colourless invisible gas, about half as heavy again as air. Dr. Faraday first showed that under great pressure it could be obtained in a liquid state. Thilorier, a French chemist, afterwards found that it could be solidified.

LECTURE II.

($7$) Page 41. 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 43. Red precipitate of biniodide of mercury. A little care is necessary to obtain this precipitate. The solution of iodide 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 stained: 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 43. Paper coated with scarlet biniodide of mercury. In order to fix the biniodide on paper, it must 