The Purple-dye Shell (Purpura).

The Philanthropist, July 1, 1859
(Prison and Reformatory Gazette; a Record of Social Amelioration, and Journal of Charitable Institutions.)

Let any one look among the rocks at low water, and plenty of the purple-dye shells may be found tolerably close together. When a sufficient number are collected, they should be killed by placing them in fresh water, after the shell has been pierced or broken, as otherwise the animal shuts itself up so tightly that the water cannot gain admittance. When the creatures are quite dead, the colouring matter may be found in a yellowish-looking vessel, that derives its colour from the substance contained within. There is very little of this colouring matter in the vessel. Now, if this yellow substance be spread on white paper, and placed in the sunshine, a blue tinge enters the yellow, making it green. The blue gradually conquers the yellow, and the green soon becomes blue. Another colour, red, now makes its appearance in the blue, and turns it into purple. The red becomes gradually stronger, and in its torn almost vanquishes the blue, but does not quite succeed in doing so; for the blue, having taken so much pains to turn out the yellow, will not entirely vacate the premises, and, coalescing with the red, forms a deep purple, the red very much predominating. So we have here all the primary colours fighting for the dominion, and yellow, the most powerful of the three, forced to retire before its complementaries.

— The Common Objects of the Sea-shore, by the Rev. J. G. Wood.