648 APPENDICES 



III. SOME ELEMENTARY FACTS OF PHYSICS. 



Diffusion. 



The molecules of a gas are in continual movement, and if two 

 different gases be brought into contact the molecules of the two gases 

 freely intermingle till a homogeneous mixture results. The molecules of a 

 liquid are also in continual motion, the only difference from the gaseous 

 state being that, owing to the greater concentration of the molecules, the 

 paths are more restricted. When two miscible fluids, e.g. alcohol and 

 water, are brought into contact, intermingling of molecules occurs as in 

 the case of the gases with a resulting homogeneous mixture. This 

 phenomenon whereby two gases or two miscible fluids in contact become 

 uniformly mixed is known as diffusion- It may occur through a 

 membrane permeable to the molecules. 



In solids, molecules are also in motion, but the paths are so restricted 

 and the mutual attraction of the molecules so great owing to their 

 proximity to each other, that difi'usion between solids in contact is 

 extremely difficult, and only proceeds at a very slow rate. That it can 

 occur, however, has been shown by the fact that if gold and lead be kept 

 in contact for several years some gold is found to be deposited on the lead. 



Kinetic Energy of Molecules. 



As molecules have a definite mass, their movement endows them with 

 kinetic energy which is in proportion to their velocity. When the 

 temperature of a substance is raised the velocity of the molecules is 

 increased, and as a result their kinetic energy is increased. Consequently 

 when a gas is heated the molecules impinge on the containing vessel with 

 gieater force, tending to distend it. In other words, the pressure of the 

 gas is increased. 



When liquids are heated the kinetic energy of the molecules is 

 increased so that they may break loose from the surface of the fluid and 

 become free, forming gas, as occurs when water reaches 100° C. at ordinary 

 atmospheric pressure. In the same way, under the influence of heat, the 

 molecules of a solid may increase their movement and the solid become a 

 liquid. Conversely, reducing the temperature decreases the kinetic 

 energy and the velocity of the molecules, and consequently a gas may 

 become a liquid, and, on still further cooling, a solid. 



The heat of a substance is thus identical with its molecular energy. 

 As heat is added the velocity and consequently the energy of the mole- 

 cules are increased ; as heat is withdrawn the velocity is decreased. 



Solutions. 



If a piece of solid cane sugar be put in water some of the molecules of 

 the sugar break loose from the surface of the solid and move among the 



