CHAP. VIII.] EPITHELIAL TISSUES, &C. 309 



Tyrian Purple 1 . 



This colouring matter, which was employed in remote antiquity 

 to dye the robes of royalty, and which even in the luxurious days of 

 Imperial Rome retained its position as the dye of greatest beauty 

 and value, is derived from the secretion of a glandular organ which is 

 situated at the lower part of the mantle, between the gill and the 

 rectum, of various species of Murex and Purpura. The secretion 

 when first poured out is colourless or yellowish, but when exposed to 

 the light, especially if it be first diluted with water, it assumes first a 

 bluish-green, then a red and lastly a purple-violet colour, at the 

 same time emitting a strong alliaceous smell. This change occurs 

 spontaneously in the case of Murex trunculus even though the juice 

 be kept in the dark, in sunlight it occurs in a few minutes. In 

 Murex brandaris the colour is produced only in the light and more 

 slowly. The dried juice, when powdered, appears red ; it is insoluble 

 in water, alcohol and ether, in dilute acids and cold alkaline leys. 



Punicin tlie Schunck 2 , to whose investigations we owe so much of our 



colouring knowledge of certain animal colouring matters, has examined 



matter ob- t j ie b r ig}jt purple colouring matter obtained by exposing to 



action oHight light the secretion of the purpurogenous gland of Purpura 

 from the lapillus. This colouring matter is insoluble in water, 



Chromogen of alcohol, and ether ; it is slightly soluble in boiling benzol, 

 'urpura an( j j n boiling glacial acetic acid. It dissolves entirely and 



with comparative ease in boiling aniline. The solution is at 

 first green, but as it approaches saturation, it becomes purplish-blue ; on 

 cooling, it again becomes green, depositing at the same time small granular 

 masses of colouring matter, and retains at last only a faint greenish tinge. 

 The solution at its darkest stage, while still warm, shews a broad but 

 well-defined band, beginning near the line C of the spectrum, and extending 

 beyond D, but as the solution cools, depositing the substance contained in it, 

 the band becomes gradually narrower, until it occupies the space midway 

 between C and 2), and it then disappears. The masses of colouring matter 

 deposited from the solution in aniline, are seen under the microscope to 

 consist of star-shaped groups of irregular crystalline needles. Punicin, 

 when cautiously heated, furnishes a crystalline sublimate. 



Chlorophylloid Colouring Matters. 



The consideration of these will be postponed to Book ill. (Respira- 

 tion). 



1 The reader interested in the subject of this paragraph is referred to the fine memoir 

 of Lacaze-Duthiers (see foot-note 2, p. 307) , and to an interesting article by Dr Schunck, 

 entitled " Note on the Purple of the Ancients." Journal of the Chemical Society, 1879, 

 p. 589. 



2 Edward Schunck, " Note on the Purple of the Ancients." Journal of the Chemical 

 Society, No. 202 (1879), p. 589. 



