16 
« Dear Sir,—I beg to return you my best thanks for the very 
interesting specimens of land-shells from the Philippine Islands, 
which you have been so kind as tosendme. The disappearance of 
the white pattern by immersion in water or any other evaporable 
fluid, and its subsequent reappearance when the shell is dry, are 
phenomena perfectly analogous to those of hydrophanous opal, taba- 
sheer, and other porous substances. 
«« The phenomenon in the land-shells is still more beautiful when 
we examine them by ¢ransmitted light. ‘The pattern which is white 
by reflected light, is dark by transmitted light, and vice versd. This is 
particularly beautiful in the Helix pulcherrima, where the ground of 
the white pattern is almost black by reflected light, and of a light 
reddish colour by transmitted light, the pattern which is white by 
reflection, having a dark red colour by transmitted light. 
«< In all these shells, the difference of structure by which the pat- 
tern is produced, does not exist in the shell, but in the epidermis, 
and hence the pattern may be wholly obliterated by removing the 
epidermis. It appears to me, from very careful observations, that 
the epidermis consists of two layers, and that it is only the upper 
layer which is porous wherever the pattern is white. These white 
or porous portions of the epidermis differ from the other parts of the 
upper layer only in having been deprived of, or in never having 
possessed, the element which gives transparency to the mem- 
brane, in the same manner as hydrophanous opal has become white, 
from the expulsion of its water of crystallization. 
«« When the shell is immersed in water or any other fluid, the fluid 
enters the pores of the white epidermis, and having néarly the same 
refractive power as the epidermis, no light is reflected at the sepa- 
rating surface of the water and the pores which contain it, so that 
the light passes through the membrane, which thus loses its white 
appearance. When the water escapes from the pores by evapora- 
tion, or is driven from them by heat, the membrane again reflects 
white light from the numerous surfaces of its pores. 
«« As the colouring matter resides in the shell itself, its peculiar 
colour is seen through the epidermis as distinctly where it is porous, 
as where it is not porous, when the porous portion has been ren- 
dered transparent by the absorption of a fluid. 
«< If we apply oil or varnish to the white pattern, we may oblite- 
rate it permanently, or we may change it into a pattern entirely dif- 
ferent from the original one. 
‘If these observations appear to you to have any interest, you 
are welcome to make any use of them you please. 
“I am, dear Sir, ever most faithfully yours, 
« D. Brewster.” 
It will be observed, that Sir David Brewster points out how the 
application of oil or varnish to the white pattern may obliterate it 
permanently ; such a case has already happened more than once: 
persons who have become possessed of some of the species whose 
patterns are lost on immersion, not content with their natural 
beauties, and unaware of their peculiarity, have had recourse to art, 
