98 INTRODUCTION TO 



consists of various strata of complicated fibres, which 

 admit of being separated from each other. It is 

 from this skin that the hairs originate, and from which 

 they derive their nourishment. 



In regard to chemical composition, the external 

 covering approaches to the nature of horn, but it 

 differs from that substance by the admixture of a 

 peculiar substance, viz. chitine or entomoline. Por- 

 tions of phosphate of lime and magnesia are also 

 constituent ingredients, although they exist in small 

 quantities. One of the peculiar properties of chitine 

 is its insolubility in potass. " Exhibited separately, 

 which is very easy, by means of steeping horny parts 

 in a solution of potass, it appears as an almost colour- 

 less transparent substance, which becomes brown 

 in nitric acid, and in the dry distillation produces no 

 carbonate of ammonia, and therefore appears to 

 contain no azote ; it burns without previously melting, 

 but is soluble in boiling or heated sulphuric acid. 



" Besides the above, small portions of albumen, a 

 peculiar brown colouring matter, which dissolves in 

 caustic potass, but not in boiling alcohol, as well as 

 traces of phosphate of iron, have been found in the 

 horny integument of insects, upon different analyses. 

 The albumen belongs doubtlessly to the third tunic, 

 as does the brown colouring matter to the mucous 

 rete : to this, also, we attribute the chitine, where- 

 by the true horny skin, namely, the epidermis, will 



scales : yet even these scales, according to Cuvier, obtain their 

 colour from being, in the chrysalis, in a state of mucosity, 

 similar to that which is found under the skin of the caterpillar. 



