136 CHEMICAL CONSTITUTION OF CUTICLE. [BOOK i. 



fourteen days. 2nd. Common nitric acid with four atoms of 

 water dissolves immediately in the cold about an equal 

 volume of insect integument ; while it leaves to vegetable 

 epidermis for more than a month its structure and outward 

 form. 3rd. Hydrochloric acid of twenty-one degrees, or with 

 six atoms of water, penetrates in a few minutes the covering 

 of insects, destroys and dissolves it, but acts very slowly on 

 the epidermis of plants. 4th. All these solutions of animal 

 structures, when neutralised by a soluble base, give an 

 abundant precipitate with tannic acid; this precipitate, if 

 washed and dried, gives ammoniacal vapours on calcination ; 

 none of these phenomena occur under the same circumstances 

 with the membrane of plants. 5th. When an almost satu- 

 rated solution of powdered chloride of lime, prepared in the 

 cold, is brought into contact with each of these substances, 

 and then boiled for a few seconds, it destroys and consumes 

 rapidly the tissue of insects, whilst it acts on the epidermis 

 of the Cereus peruvianus but slowly, and the cuticle remains 

 longer unaffected than the subjacent cellular substance. 

 An elementary analysis gave the following results : 



Cuticle of the shell of the crab . . . 8-935 per cent, of nitrogen. 



Skin of the silkworm 9-050 



Epidermis of potato 2-531 ,, 



Epidermis of Cereus peruvianus (one year old) 2-059 ; 



, .(two years old) -906 



Cuticle of ditto . 2-551 



J? 



The greater preponderance of nitrogen in the animal 

 kingdom is distinctly shown by these valuable researches. 



