10 



A YEAR IN SCIENCE 



intermingling or gradual mixing of two substances. 

 Thus if two liquids of different density or weight are 

 placed in contact with each other with the heavier 

 liquid on the bottom and left undis- 

 turbed, it will be found in a few days 

 that the heavier solution is gradually 

 rising into the lighter substance. This 

 phenomenon could not occur unless 

 the substances were composed of 

 moving particles with spaces between. 

 Solids will diffuse also if left in con- 

 tact with each other, but at a very 

 much slower rate, because of the much 

 slower movement of their molecules. 

 If a brick of lead and a brick of gold 

 with planed surfaces are placed to- 

 gether, after a few months small 

 particles of each will be found in the 

 other. 



Molecules in gases. In gases the 

 molecules are much farther apart 

 than in solids and liquids, and consequently they move 

 very much more rapidly than in either of the latter. 



If two bottles of gas of different density are placed 

 mouth to mouth, with the bottle of heavier gas on the 

 bottom, and left in that position for a few minutes, a 

 large portion of the heavier gas will be found in the 

 upper bottle. Again, if a bottle of perfumery is opened 

 in a corner of a room in which the air seems perfectly 



Fig. 5. If two 

 liquids of different 

 densities are placed 

 in contact, they will 

 gradually mix. 



