ON GASTEROPODA. 365 
deep purple red. These changes, though very rapid, are 
quicker or slower in proportion to the heat of the sun. 
On washing the cloth with scalding water and soap, and 
again exposing it to the sun, the colour changes to a beautiful 
crimson, and then no further alteration takes place from sun 
or air, or any of the agents usually employed to try colours. 
The linen marked with the white juice, while drying, always 
yielded, for the first time, a strong fetid smell, resembling a 
mixture of garlic and assafoetida. A similar scent was attached 
to the purple of the ancients. 
Duhamel informs us, with regard to the purpura, that its 
juice neither receives nor communicates colour without ex- 
posure to the sun, and that this colour is evolved, not by the 
heat, but by the light of the sun’s rays; for when the silk or 
linen which is stained, is covered with thin opake bodies, 
which transmit heat without light, no colour is produced, 
which is not the case when transparent bodies are employed. 
Also the light of a fire, though concentrated by convex 
glasses, and concave mirrors, has no effect upon it. 
Duhamel gives the colours which it assumed, on exposure 
to the sun, in the following order ; 1. a pale green, or yellow ; 
2. an emerald green ; 3. a dark blueish-green ; 4. a blue, with 
an incipient redness; 5. a purple. These colours appeared 
in less than five minutes; but it only became green when it 
was not exposed to the light. ‘This succession of colours is 
best observed when the sun is low; in the light of noon in 
summer, they come on so quickly as not to be easily dis- 
tinguished. 
Observers, however, are far from being perfectly agreed re- 
specting the circumstances which cause the colouring matter 
to pass from the pale-green, or greenish-white to the purple- 
red. Respecting the permanence of this tincture, however, 
there is no dispute. Cole, Reaumur, Templeman, and par- 
ticularly Duhamel, have proved, that when the stuff has been 
