304 The Commercial Products of the Sea. 



When pulverized fine enough, this colour is used for 

 water-colour drawings ; but its hardness makes it necessary 

 to mix with it some foreign colour (sienna or the like), to 

 facilitate the operation of pulverizing. 



By the following method this laborious crushing, which 

 is always imperfect, is avoided, and the colour obtained 

 very pure. One hundred and fifty grammes of potash or 

 caustic soda are put into a capsule ; it is then placed on the 

 fire, and when the potash is dissolved in the water, 100 

 grammes of the dry matter are added gently, but keeping it 

 in motion until completely dissolved. It is then taken 

 from the fire, and after a few minutes a little water (the less 

 the better) is added, and so on until complete evaporation. 

 During this process an extremely strong ammoniacal odour 

 is given off, and the dry ink has become soluble in alkali, 

 but it is insoluble in water or in acid. 



There is great dispute as to the precise source of the 

 celebrated Tyrian purple dye, so much used for the 

 garments worn by kings and emperors of old. Some 

 authors attribute it to the rock lichens or orchella weed 

 of commerce of the present day, but the general and most 

 probable opinion is that it was obtained from some species 

 of Murex (M. brandaris and trunculus) and Purpura (P. 

 patula and P. persicd), the animals of which furnish a rich 

 colour. The small shells were bruised in mortars ; the 

 animals of the larger ones taken out. 



In Britain there are several kinds of mollusca which 

 furnish a dye of this sort. Helix Janthina, which occurs in 

 the Mediterranean, Atlantic, and South Seas, affords a 

 similar fluid. 



If the shell of Purpura lapillus is broken, there is seen 

 on the back of the animal, under the skin, a slender, 

 longitudinal, whitish vein, containing a yellowish liquor. 



