said to have the same property; a purple fluid is poured out from beneath the mantle, but it is so fugitive that no application can prevent its speedily turning to a dull rusty colour.
Colonel Montagu mentions one of our marine shell-fish (Scalaria clathrus) as secreting a purple juice. "It may be collected either from the recent or dried animal, by opening the part behind the head; and as much can be procured from five individuals as is sufficient, when mixed with a few drops of spring-water, to cover half a sheet of paper. Neither volatile nor fixed alkali materially affects it; mineral acids turn it to a bluish green, or sea-green; sulphuric acid renders it a shade more inclining to blue; vegetable acids probably do not affect it, since cream of tartar did not in the least alter it. These colours, laid on paper, were very bright, and appeared for some months unchanged by the action of the air or the sun; but, being exposed for a whole summer to the solar rays, in a south window, they almost vanished. The application of alkali to the acidulated colour always restored it to its primitive shade, and it was as readily changed again by mineral acid."
- Test. Brit. Supp. p. 122.