APPLICATION OF THE DYE. 61 



I placed in a basin of fresh-water, proved to be quite 

 uninjured when I broke the shells eighteen hours 

 afterwards, as was seen by their forcible contraction 

 when divested of their shelly covering. Doubtless 

 this power of resisting the action of fresh-water con- 

 sists in the close-fitting operculum, which is forcibly 

 drawn in under the stimulus, so as to keep the water 

 perfectly out. . 



When the animals appear dead, examine them for 

 a vessel of yellow or cream-coloured matter, that runs 

 diagonally across the body, behind that projecting 

 veil under which the tentacles retire when contracted. 

 It is sufficiently conspicuous, flat, somewhat wrinkled, 

 as if not quite full, with one margin blackish. Insert 

 into the membrane, which is very tender, the point of 

 a sharp pair of scissors, or a needle, and open the vein, 

 which you will find filled with a substance exactly 

 resembling in colour and consistence the pus or mat- 

 ter formed in a boil. You will not find much ; that 

 of a large Purpura I managed to spread over a space 

 of calico as large as a shilling. From its viscid con- 

 sistence it is difficult to use with a pen, and I do not 

 know how it may be uniformly diluted ; but with a 

 small camel's-hair pencil I have used it with much 

 more facility. 



As soon as the matter is applied to the linen, its 

 hue is a rich " King's yellow," but becomes in a few 

 minutes a delicate pea-green. In about an hour, if the 

 weather be cloudy, it has become a yellow grass-green, 

 from which it slowly and imperceptibly turns to a blue 

 green, thence to indigo, and thence to blue. A red 

 tinge now becomes apparent, generally in parts, cans- 



h 



