PHYSICAL PROPERTIES. 15 



By indestructibility is meant that, according to the 

 present laws of nature, matter never ceases to eJr- 

 ist. It may change its form, but it cannot be de- 

 stroyed. Thus we see water and volatile substan- 

 ces dissipated by heat ; wood and coal are consu- 

 med in the fire and disappear. But, in such cases, 

 not a particle of matter is annihilated. The body 

 appears, indeed, to have been destroyed, but it has 

 only undergone a change of form ; a part has been 

 driven off in vapour, a part in gas, while a still 

 larger portion remains behind in the form of ashes 

 or cinders. This fact, as connected with changes 

 upon the surface of the earth, was known to the 

 ancients, for we read in Ovid* " Nothing perishes in 

 this world ; but things merely vary and change their 

 form. To be born means simply that a thing be- 

 gins to be something different from what it was be- 

 fore ; and dying is ceasing to be the same thing. 

 Yet, although nothing retains long the same image, 

 the sum of the whole remains constant." Some 

 of the illustrations which are adduced are as follows : 

 " Solid land has been converted into sea. Sea has 

 been changed into land. Marine-shells lie far dis- 

 tant from the deep, and the anchor has been found 

 on the summit of hills. Valleys have been exca- 

 vated by running water, and floods have washed 

 down hills into the sea. Marshes have become dry 

 ground. Dry lands have been changed into stag- 

 nant pools. During earthquakes, some springs have 



are not elastic. Thus the operation of the air-gun depends on 

 the elasticity of the atmospheric air, which is increased in pro- 

 portion to the pressure. Mica, or isinglass, is highly elastic 

 among minerals ; and elastic bitumen, or mineral caoutchouc, 

 is still more so. Talc, which closely resembles mica, is highly 

 flexible., but not elastic ; and slabs of limestone in some parts of 

 Massachusetts possess the same property. Among minerals, 

 steel, or iron combined with carbon, is the most elastic sub- 

 stance known. The other metals possess this property in a 

 much inferior degree. 

 * Ovid's Metamor., lib. 15. 



