EXERCISES 



101 



of gas inside the cup and tube. Hence some of it es^ 

 capes through the tube. 



This experiment shows that gases have the ability to 

 mix when placed together. We call this mixing diffusion, 

 and we explain it by saying that the energy of the molecules 

 (cf. 61) causes the molecules of the one gas to move 

 rapidly into the spaces between the molecules of the 

 other gas, until the mixing is complete. 



If we place a bottle of hydrogen, with its mouth downward (Fig. 

 83), over a bottle of air, the hydrogen diffuses downward, and the air 

 upward, in spite of the fact that air is 14.4 times as dense 

 as hydrogen. When the gases are separated by a wall 

 through which they can pass, as in the porous cup ex- 

 periment, they still diffuse into each other; but we can 

 see that they move at different rates: the lighter gas 

 diffuses more rapidly than the heavier one. 



Liquids and solutions diffuse, in spite of gravity, just 

 as gases do. Thus, if some blue vitriol (cf. 95) is put 

 into the bottom of a tall jar, under a deep layer of water, 

 the blue vitriol solution that is formed at the bottom 

 rises gradually to the top, coloring all the water. Also, 

 if some alcohol is put upon water, it descends into the 

 water, even though the water is the heavier; and water 

 ascends into the alcohol, until the mixture is complete. 



The diffusion of oxygen inward, through the walls of 

 the blood-vessels, into the blood, and of carbon dioxide 

 outward causes the exchange of gases necessary for the 

 life of the higher animals (cf. 52). 



107. Exercises. 



Hydrogen 



FIG. 83. 

 Hydrogen 

 passes down- 

 ward into 

 the air, and 

 air upward 

 into the hy- 

 drogen.. 



1. Are the following changes physical or chemical? Souring of 

 milk, freezing of water, decomposition of mercury oxide by heat, dis- 

 solving of sugar in water, incandescence ("getting white hot") of an 

 electric light, burning of candy on a stove, heating a penny by striking 



