228 MOLLUSCA. 
blue, and purple, and at last settles in a fine unchangeable 
crimson. Neither acids nor alkalis affect its colour, and it 
may be conveniently employed in marking linen, where an 
indelible ink is desirable. The Scalaria clathrus (Turbo 
clathrus of Linnzus) also furnishes a purple liquor of con- 
siderable beauty, but it is destructible by acids, and gradu- 
ally vanishes by the action of light. The Planorbis cor- 
neus likewise yields a scarlet dye, but of still less perman- 
ency than the scalaria, as all attempts to fix it have hitherto 
proved ineffectual. 
We cannot conclude this chapter without remarking, that 
the study of molluscous animals rises in importance as we 
perceive its utility. When we are told, that searching for 
shell-fish, and conveying them to the market, give employ- 
ment to a British population of upwards of 10,000; that 
these animals furnish nourishing feod to innumerable fami- 
lies, and in years of scarcity prevent the horrors of famine ; 
we will be disposed to regard with a favourable eye the la- 
bours of that naturalist who examines the structure and 
economy of those animals, that, from a knowledge of their 
nature, he may render them still more subservient to our 
purposes. 
Even when considered as objects of amusement, mollus- 
cous animals are not devoid of interest. In the preceding 
division of our subject, we have considered them as appli- 
cable to various useful purposes, and expressed our regret, 
at the same time, that no one qualified for the task had ever 
bestowed on economical conchology an attentive examin- 
ation. We cannot therefore consider the present condition 
of the science as the result of the labours of its practical 
admirers. The lovers of this study, as an agreeable amuse- 
ment, have at all times been numerous, from the days of 
