MATTER, AND ITS GENERAL PROPERTIES. 19 



Because the air or liquid contained in the pores becomes expanded bj heat, 

 and bursts the covering in which it is confined. 



7. "Why does light, poaors wood, like chestnut or pine, make more snapping in burn- 

 ing than any otuzb kind ? 



Because the porea are very large, and contain more air than wood of a closer 

 grain, like oak, etc. 



8. How is water, or any other liquid, made fube by filtering through paper, cloth, a 

 layer of sand, rock, etc. ? 



The process of filtration depends on the presence of pores in the substance 

 'used as a filter, of such magnitude as to allow the particles of hquid to pasa 

 freely, but not the particles of the matter contained in it, which we wish to 

 separate. 



9. Why is not the substance suitable for the filtration of oxe liquid equaDy adapted for 

 the filtration of all liquids ? 



Because the magnitude of the pores in different substances and of the im- 

 purities in hquids is diflferent ; and no substance can be separated from a 

 liquid by filtration, except one whose particles are larger than those of the 

 liquid- 



10. Gold and lead are metals of great density ; their pores are not visible. Is there any 

 FBOOF of their existence beside the fact that they can be compressed ? 



'U'ater can be forced mechanically through a plate of lead or gold without 

 rupturing any portion of the metal. Mercury, or quicksilver, confined in a 

 dish of lead or gold, will soak through the pores, and escape at the bottom. 



An interesting experiment was tried at Florence, Italy, nearly two centu- 

 ries ago, which furnished a striking illustration of the porosity of so dense a 

 substance as gold. A hoUow ball of this metal was filled with water, and the 

 aperture exactly and firmly closed. The globe was then submitted to a very 

 severe pressure, by which its figure was sUghtly changed. Xovr, it is proved 

 in geometry, that a globe has this peculiar property — that any change what- 

 ever in its figure necessarily diminishes its volume, or capacity. The result 

 was, that the water oozed through the pores, and covered the surface of tlio 

 globe, presenting the appearance of dew, or steam cooled by the metal This 

 experiment also proved that the pores of the gold are larger than the element- 

 ary particles of water, since the latter are capable of passing through them. 



11. When a casbiagd is in motion, drawn by nossES, why is the same exertion of power 

 in the horses required to stop it, as would be necessary to back it, if it were at rest ? 



Because, according to the laws of inertia, the /orce required to destroy mo- 

 tion in one direction is equal to that required to produce as much motion in the 

 opposite direction. 



12. If a carriage, railroad-car, or boat, moving with speed, be suddenly stopped or be- 

 tabdei), from any cause, why are the passengers, or the baggage carried, precipitated 

 from their places in the dibectios of the motion ? 



Because, by reason of their inertia, they persevere in the motion which they 

 shared in common with the body that transported them, and are not deprived 

 of that motion by the same cause. 



