we deal with in building up the structures on the earth are of strength—we use iron, stone, and other things of great strength; and only think that all those structures you have about you—think of the Great Eastern if you please, which is of such size and power as to be almost more than man can manage—are the result of this power of cohesion and attraction.
I have here a body in which I believe you will see a change taking place in its condition of cohesion at the moment it is made. It is at first yellow, it then becomes a fine crimson red. Just watch when I pour these two liquids together—both colourless as water. [The Lecturer here mixed together solutions of perchloride of mercury and iodide of potassium, when a yellow precipitate of biniodide of mercury fell down, which almost immediately became crimson red.] Now, there is a substance which is very beautiful, but see how it is changing colour. It was reddish-yellow at first, but it has now become red.[1] I have previously prepared a little of this red substance, which you see formed in the liquid, and have put some of it upon paper. [Exhibiting several sheets of paper coated with scarlet biniodide of mercury. [2]]
- ↑ 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.
- ↑ Page 43. 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.
