the
stomach. A single drop may cause death. It dissolves readily in water,
its solution being commonly called prussic acid.

The salts of hydrocyanic acid are called _cyanides_, the cyanides of
sodium and potassium being the best known. These are white solids and
are extremely poisonous.

~Solutions of potassium cyanide are alkaline.~ A solution of potassium
cyanide turns red litmus blue, and must therefore contain hydroxyl ions.
The presence of these ions is accounted for in the following way.

Although water is so little dissociated into its ions H^{+} and OH^{-}
that for most purposes we may neglect the dissociation, it is
nevertheless measurably dissociated. Hydrocyanic acid is one of the
weakest of acids, and dissociates to an extremely slight extent. When a
cyanide such as potassium cyanide dissolves it freely dissociates, and
the CN^{-} ions must come to an equilibrium with the H^{+} ions derived
from the water:

H^{+} + CN^{-} <--> HCN.

The result of this equilibrium is that quite a number of H^{+} ions from
the water are converted into undissociated HCN molecules. But for every
H^{+} ion so removed an OH^{-} ion remains free, and this will give the
solution alkaline properties.

EXERCISES

1. How can you prove that the composition of the different allotropic
forms of carbon is the same?

2. Are lampblack and bone black allotropic forms of carbon? Will equal
amounts of heat be liberated in the combustion of 1 g. of each?

3. How could you judge of the relative purity of different forms of
carbon?

4. Apart from its color, why should carbon be useful in the preparation
of inks and paints?

5. Could asbestos fibers be used to replace the wire in a safety lamp?

6. Why do most acids decompose carbonates?

7. What effect would doubling the pressure have upon the solubility of
carbon dioxide in water?

8. What compound would be formed by passing carbon dioxide into a
solution of ammonium hydroxide? Write the equation.

9. Write equations for the preparation of K_{2}CO_{3}; of BaCO_{3}; of
MgCO_{3}.

10. In what respects are carbonic and sulphurous acids similar?

11. Give three reasons why the reaction which takes place when a
solution of calcium acid carbonate is heated, completes itself.

12. How could you distinguish between carbonates and sulphites?

13. How could you distinguish between oxygen, hydrogen, nitrogen,
nitrous oxide, and carbon dioxide?

14. Could a solution of sodi