TURPLE SECRETIONS. 243 



almost vanished. The application of alkali to the acidu- 

 lated colour always restored it to its primitive state, and it 

 was as readily changed again by mineral acid : in this 

 particular it differs materially from the succus of Buccinum 

 lapillus, which, as we have before remarked, is unalterable."* 

 It is from the difference pointed out, in the latter part of 

 the sentence just quoted, between the fluid of Scalaria (and, 

 I may add, of Ianthina and Aplysia) and the Buccinum or 

 Purpuriferae, and because it is from the first of a purple 

 colour, that I cannot agree with Plancus,f Colonel Montagu, 

 and many other naturalists of eminence, in their opinion, of 

 its having formed any part of the Tyrian dye; for un- 

 changeableness was one of the characters that enhanced the 

 value of the latter ; and Aristotle and Pliny state expressly 

 that the colour of that fluid, on its first discharge from the 

 animal, was white. Such a coloured liquor can be procured, 

 as these authors say it was procured, from several univalves 

 belonging to the genera Murex and Purpura ; % and Colonel 

 Montagu furnishes us with a good account of it in the Pur- 

 pura lapillus : — " The part containing the colouring matter 

 is a slender longitudinal vein, just under the skin on the 

 back, behind the head, appearing whiter than the rest of the 

 animal. The fluid itself is of the colour and consistence of 

 thick cream. As soon as it is exposed to the air, it becomes 

 of a bright yellow, speedily turns to a pale green, and con- 

 tinues to change imperceptibly, until it assumes a bluish 

 cast, and then a purplish red. Without the influence of 

 the solar rays, it will go through all these changes in the 

 course of two or three hours ; but the process is much acce- 

 lerated by exposure to the sun. A portion of the fluid, 

 mixed with diluted vitriolic acid, did not at first appear to 

 have been sensibly affected ; but, by more intimately mixing 

 it in the sun, it became of a pale purple, or purplish red, 

 without any of the intermediate changes. Several marks 

 were now made on fine calico, in order to try if it was pos- 

 sible to discharge the colour by such chemical means as were 

 at hand ; and it was found that, after the colour was fixed at 

 its last natural change, nitrous no more than vitriolic acid 

 had any other effect than that of rather brightening it : aqua 

 regia, with or without solution of tin, and marine acid, 

 produced no change; nor had fixed or volatile alkali any 

 sensible effect. It does not in the least give out its colour 



* Test. Brit. Supp. 122. t Dc Conchis minus notis, 28. 



X Of Tritonium nodiferum Philippi says, " Animal in siceura positum 

 paullo ante mortem saniem pulclicrrime ccelestem exspuit." — Moll. Sial. 

 li. 184. 



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