242 THE WORLD OF THE SEA. 
caused him no anxiety; he had learnt by experience that the purple 
owed its existence to the sun, and that exposure to the bright 
Italian sky only deepened its rich tints. Duhamel, who has 
studied this property, thinks that it is due to the same power 
by which the sun paints the rosy cheeks of the apples and 
peaches. But this can hardly be; for in the one case the sun 
acts upon matter which is under the influence of a vital force, 
but in the other the liquid is acted upon when separated from 
the mollusk; hence it must possess this property within itself. 
The purple is thus entirely a photogenic substance. M. Lacaze- 
Duthiers has conducted some important experiments upon its 
sensitiveness, and the uses to which it might be put. He advises 
that the dyeing matter should be gathered with a flat, short 
brush; the secretive organ should be gently rubbed several times ; 
by this means the brush becomes laden with the viscous liquid. 
Then the fabric which it is wished to colour should be covered 
by frequently passing the brush over it; thus a layer of the 
liquid is spread, which is at first frothy, but soon the air-bubbles 
disappear, and it becomes uniform. To wholly cover the surface, 
this process may have to be repeated two or three times. To 
obtain the colour, the prepared substance must now be exposed 
to the sun-light. The rapidity with which the tint is developed 
varies with the actinic power in the rays. In Spain, exposure for 
two or three minutes will suffice. If the sky be overcast, three- 
quarters of an hour will be required. The process is quickened 
if the fabric be moistened with a little sea-water. When only 
a thin layer of the matter is applied, the deep shade cannot be 
obtained by any length of exposure. 
The mollusks which inhabit the “rock-shelis” are capable of 
furnishing matter which, thus manipulated, affords colours. Some 
give blue, some purple, some violet. But the subject will bear 
much further investigation, for M. Lacaze-Duthiers found that, 
although he used the same matter, and treated it apparently the 
same way, yet he could not assure the appearance of the same 
tint, such is the delicacy of this natural dye. 
