no wise to be arrested; the physical last only as long as the external condition lasts; even the chemical colours have great mutability, they may be made to pass and repass from one side to the other by means of opposite re-agents, and may even be annihilated altogether.
PERMANENCE OF COLOUR.
714.
The chemical colours afford evidence of very great duration. Colours fixed in glass by fusion, and by nature in gems, defy all time and re-action.
715.
The art of dyeing again fixes colour very powerfully. The hues of pigments which might otherwise be easily rendered mutable by re-agents, may be communicated to substances in the greatest permanency by means of mordants.
PART V.
RELATION TO OTHER PURSUITS—RELATION TO PHILOSOPHY.
716.
The investigator of nature cannot be required to be a philosopher, but it is expected