INTRODUCTION 3 



hydroids consist of a horny substance known as chitin ; in 

 diatoms, in most radiolarians, and in many sponges, of 

 silica ; in the bones of vertebrates, chiefly of carbonate and 

 phosphate of lime ; in corals, echinoderms, mollusks and 

 many other animals and some plants, of carbonate of lime ; 

 in most plants, of woody or corky tissue : a larger or 

 smaller amount of organic matter is always combined with 

 the mineral. Of these substances, chitin is with difficulty 

 dissolved. Silica in its ordinary crystalline condition is 

 one of the most' stable of minerals, but when secreted by 

 an animal or plant it is glassy and isotropic (i.e. singly 

 refracting and without effect on polarised light), and is 

 dissolved with comparative ease, so that such skeletons 

 may be entirely removed by the action of percolating 

 water. In organisms with calcareous skeletons the car- 

 bonate of lime is readily dissolved by water containing 

 carbonic acid, but the degree of solubility varies according 

 to the condition in which the carbonate of lime is present. 

 In some animals it occurs as aragonite, in others as calcite. 

 Of these two minerals, aragonite is the harder and heavier, 

 its specific gravity being 2*93, whilst that of calcite is 

 only 2*72 ; aragonite crystallises in the rhombic system, 

 calcite in the hexagonal. Fossil calcite shells (e.g. Pecten 

 opercularis) are translucent, their surface is compact, but 

 their interior porous; on the other hand the aragonite 

 shells (e.g. Pectunculus glycimeris) are opaque, and have a 

 chalky appearance but a compact structure throughout. 

 If a shell of each kind be suspended in water containing 

 carbonic acid, it will be found that the one composed of 

 aragonite will lose, in the same time, a much greater pro- 

 portion of its weight than the other. Further, the calcite 

 shell remains firm longer than the aragonite, the latter 

 being soon reduced to the consistency of kaolin or china- 



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