THE BONES. 27 



yielding a firm transparent jelly ; and it was a knowledge of this 

 property which suggested to Papin the invention of his digester, in 

 which, by boiling bruised bones under strong pressure, the jelly is 

 obtained, and a large quantity of strong soup is made from what 

 would otherwise be entirely worthless. The earthy part of the 

 bone is demonstrated in a different manner. If a bone is put into 

 a clear fire, and heated to redness, the animal part is entirely con- 

 sumed, and a w^hite friable earth is left behind. This earth consists 

 almost wholly of lime, in combination with phosphoric and carbonic 

 acids. It is on this account that phosphorus is obtained from cal- 

 cined bones. There are minute portions of other salts in the earthy 

 part of bones, but these it does not seem necessary to mention in 

 detail. When a bone has been thus treated, and is weighed, it will 

 be found about one-third lighter than at first, the loss consisting of 

 the gelatinous part. And as was remarked of the gelatinous part 

 when obtained by itself, so also the earthy part has exactly the 

 original shape of the bone, and the most minute bony threads are 

 still seen existing. 



In consequence of the entry of these two very different substances 

 into the formation of bone, it is both hard and elastic ; the earthy 

 part bestowing the density, the animal part the elasticity. The 

 hardness is indeed its most remarkable property, in consequence of 

 which it has that firmness and resistance by which it is so admirably 

 adapted for the offices it has to perform in the animal machine. 

 The hardness varies a little in different parts of the body, and 

 usually increases with age. Bones are somewhat flexible and 

 elastic, this elasticity varying however in different situations. We 

 find that the ribs may be much bent, and afterwards recover their 

 form perfectly. Hence boys have used a horse's rib for a bow. 

 This elasticity frequently saves them from fractures, and lessens the 

 shock which would otherwise be communicated to the delicate 

 structures which they defend. It was remarked above, that in old 

 age the hardness increases, and at the same time the elasticity 

 diminishes, and hence the reason why elderly people are much, 

 more liable to have their bones broken than young ones. A child 

 may often get a twist which will bend one of the long bones, but 

 instead of breaking, it recovers its proper form, if not immediately, 

 at least shortly after ; while in an old person, any such twist is 

 almost invariably followed by a fracture. The color of bone in the 

 living person, is a pale rose color, inclining in early life to red, and 

 in old age to a yellowish white. When bones are long macerated 



