UNIVALVES. 135 
remains in the body of the shell-fish, and while the 
creature is in health; but no sooner is it exposed to 
the sun, than it begins to change, and in less than 
five minutes passes through several gradations of 
pale, yellowish, and vivid emerald-green ; after which 
it becomes of a duskier hue, then blue and red, and 
finally, of a deep and very beautiful purple. Here 
the action of the sunbeams terminates, but by wash- 
ing the linen in hot water, and afterwards drying it, 
the colour ripens to a bright and brilliant crimson. 
It is worthy of remark, that if a piece of linen be 
rubbed with this juice, and partially exposed to 
the action of the sunbeams; that part of it only will 
turn red, which is so exposed; the other retaining 
the natural tint, without any alteration ; and that if a 
needle, or any other opake body, be laid upon the 
linen while it is yet green, the spot on which it is 
laid will remain unaltered, while a rapid change has 
taken place in every other part. 
A plate of glass, though it be three inches thick, 
will not prevent the colour from changing to purple by 
being laid over it, but the thinnest metal effectually 
precludes any alteration; the one being opake, and 
the other pellucid, are evidently the occasion of this 
difference. But if the coloured linen be successively 
covered by three pieces of paper, the one blackened 
with ink, the other in its natural state, and the third 
