328 Ttii: mechalMcal i UNcriuNS. 



vibrations, the greatest solidity is required, are the 

 densest parts of the skeleton ; and phosphate of 

 lime enters most largely into the composition of 

 these bones. The tympanic portions of the tem- 

 poral bone of the Whale and the Cachalot, where 

 the great size of the organ gives us advantages in 

 examining them, have the density and hardness of 

 a stone. The bony portions of the teeth, likewise, 

 afford instances of very hard calcareous formations ; 

 but the enamel, which consists almost wholly of 

 phosphate of lime, is harder still, and resembles the 

 siliceous stones, being, like flint, capable of striking- 

 fire with steel. It is scarcely necessary to point 

 out the obvious intentions which are fulfilled by 

 this peculiarity of structure, conferring extraordi- 

 nary hardness on a part of which the appropriate 

 office is that of breaking down hard bodies sub- 

 jected to its mechanical action. But this extreme 

 degree of crystalline hardness would be ill suited 

 to other parts of the frame. In ordinary bones, 

 absolute rigidity is not the quality which is alone 

 wanted ; for, in general, the hardest bodies are also 

 the most fragile. An excess of rigidity, therefore, 

 Avould have been attended with brittleness, and 

 been productive of the worst consequences to parts 

 exposed to sudden and violent concussions. It is 

 in order to guard against this evil that an elastic 

 animal matter is enq^loyed as the basis of the struc- 

 ture, acting as a strong cement interposed between 

 the calcareous particles. 



This composition of bone is rendered evident by 

 subjecting it to certain chemical processes. On 

 exposure to heat, we find it first becoming black, 

 from the developement of the charcoal, during the 



