18 THE ATMOSPHERE. 



58. Another cause of the penetration of water is the force 

 which draws fluids of different densities through a par 

 tition of thin skin or film placed between them, and 

 makes them mix. This is called Osmotic Action. 



We can easily conceive how this action takes place. 

 Water spreads itself continually, and enters into what 

 ever is in contact with it more readily than any other 

 fluid. Thus it moistens and gets through a film more 

 rapidly than the fluid on the other side, which also 

 penetrates, but less readily. Both of them continue to 

 move on, but the water always more rapidly. 



59. Oxygen combines with the particles of metals and 

 turns them into rusts or oxides; and, aided by moisture 

 and warmth, it unites with the elements of wood and all 

 other things made of carbon and hydrogen, and causes 

 them to decay. 



60. Do not the heavy gases, like carbonic acid, sink 

 to the bottom of the atmosphere, and the light ones, like 

 hydrogen and carburetted hydrogen, rise to the top ? 



No. Each gas spreads or diffuses itself throughout 

 all the atmosphere. As much carbonic acid is found at 

 the top of a mountain as in the bottom of a valley. If a 

 plant has an attraction for ammonia, it draws to itself 

 the ammonia near it, and combines with it; but the 

 ammonia at a distance rushes in, comes near, and is 

 attracted and combined also, and streams of it keep 

 coming in from all quarters. 



61. Heat, too, spreads itself, unceasingly, in every 

 direction, and that in two ways. If it spreads from par 

 ticle to particle, as it docs in a piece of iron, or any other 

 solid, or as it docs in the earth, it is said to be conducted, 

 or to spread by conduction. If it darts out, as it docs, in 

 straight lines, from all things surrounded by air or open 



