236 THE MECHANICAL FUNCTIONS. 



The difference between the textures of these 

 two kinds of shell is further illustrated by the 

 impression made upon them by fire. Porcel- 

 laneous shells, when exposed to a red heat, give 

 out neither smell nor smoke : they lose indeed 

 their colour, but retain their figure unaltered. 

 Membranous shells, on the contrary, emit a 

 strong fetid odour, and become black ; after 

 which the plates separate, and the structure falls 

 to pieces. 



This variety in the composition and structure 

 of different kinds of shell is accompanied by 

 corresponding modifications of their mechanical 

 properties. The toughness of the fibrous basis 

 of membranous shells imparts to them greater 

 strength than is possessed by the porcellaneous 

 shells, which, in consequence of the tenuity and 

 uniform intermixture of the animal cement with 

 the calcareous particles, present a harder and 

 more transparent, but at the same time more 

 brittle compound. It is these qualities, together 

 with their smooth enamelled surface, often beau- 

 tifully variegated with brilliant colours, and pre- 

 senting altogether a close resemblance to porce- 

 lain, that have procured them the name they bear. 

 When the transparency and brittleness of these 

 shells are very great, they have been considered 

 as forming another class, and they have been 

 termed Vitreous shells, from their making a nearer 

 approach to glass. Some shells present interme- 



