THE TYRIAN PURPLE. 75 



last waxes to a very deep purple-red. The light and air can 

 do no more ; but if the cloth is now washed in scalding 

 water and soap, it conies out from the lather of a fair bright 

 crimson, which no subsequent process can change or lessen. 

 While the cloth, wet with the dye, lies in the sun it exhales 

 a strong foetid smell, as if garlic and assafoetida were mixed 

 together,* nor am I aware how this was got rid of in ancient 

 practice, for I presume it is a quality inherent in the secre- 

 tion of all purpuriferous fish, notwithstanding that Pliny's 

 mode of expression would seem to imply that the *' stinking 

 savour " was peculiar to the less esteemed kinds. When 

 exposed to the light, the colour runs through all the above 

 changes in a few minutes ; and if the light or heat be very 

 strong, they succeed one another so quickly that the inter- 

 mediate hues cannot be observed. By moderating the light 

 the process is prolonged, and the whole series may be noted 

 with accuracy; and if the light is excluded entirely, no change 

 whatever takes place, but the dye remains of its native pale 

 yellow or cream colour, and will so remain for years, until 

 the admission of Hght revives its dormant energies. Dr. Ban- 

 croft kept pieces of linen stained with the liquor for nine 

 years between the leaves of a book without any visible 

 change, but which, at the expiration of that period, were 

 influenced by light in the same way as recently-stained 

 pieces, and as readily acquired the glowing purple. There 

 are two ways of explaining this curious series of changes. 

 We may suppose, with Berthollet, that they are owing to 

 the base gaining additional doses of oxygen from the atmo- 

 sphere and varying its hue accordingly ; but, though this 

 explanation has been received by many good chemists. Dr. 

 Bancroft appears to have satisfactorily proved that the very 

 opposite is the true theory ; — the base parting with a redun- 

 dant portion of oxj^gen " naturally combined for some 

 unknown purpose in the liquor of these shell-fish ; and in that 

 particular state which will not admit of its being separated 

 without the application and assistance of light ; as is also 

 the case of horned silver, rendered purple by the sun's rays ; 

 of vegetables, rendered green by the same cause, after they 

 had become white by growing in darkness ; of peaches, 

 purple grapes, and other fruit, which never acquire their 

 proper colours by any degrees of heat, but always remain 

 white or green, if shaded and secluded from the contact of 

 the sun's rays."-j- 



» Cole in Phil. Trans, xv. 1280-1 . 



t On Perm. Colours, i. 145. — This work contains the fullest aud best 

 account of the Tyrian purples of any I have had the opportunitv of readiny. 



