After premising some general remarks respecting the object of research, and enumerating the various authors who had written upon the subject, Dr Hope explained various terms which were to be used in the discourse. To the various coloured matters presented by the leaves and flowers of plants, De Candolle had applied the denomination of Chromule, which term he meant to adopt.

There resides in the same parts of plants, in addition to the chromule, some matter probably destitute of colour, which becomes red by the action of acids, and yellow or green by the action of alkalies. To it Mr Ellis gave the name of colourable matter, which the author changes to Chromogen, When an acid, added to a vegetable infusion, causes a red colour, and an alkali a yellow or green, it has been the universal opinion that both sets of agents act upon one and the same colourable principle.