1 64 OUR PHYSICAL WORLD 



fifteen quintillion molecules. There would be some three million 

 of them just along the line of its diameter. If such a bubble 

 were magnified to the size of the earth the molecules would be 

 somewhat over an inch in diameter (i.i). This is a magnifica- 

 tion of about twelve and a half billion diameters. The most 

 powerful microscopes now at our disposal magnify about ten 

 thousand diameters. 



These molecules are not standing still but, due to the radiant 

 energy imparted to them in the form of heat, they move in 

 straight lines at the rate, in the case of hydrogen, of a mile a 

 second, or in our magnified bubble at a rate over 12,000,000,000 

 miles a second. Oxygen gas with a molecule whose mass is 

 sixteen times as great travels only a quarter as fast. Such 

 molecules are, therefore, constantly bumping into each other and 

 against the sides of the container, and so must constantly be 

 shifting the direction of their movement. Hydrogen molecules 

 at ordinary conditions of temperature and pressure average 

 about 10,000,000,000 collisions every second. It is the constant 

 impact of the molecules of a gas against the walls of the contain- 

 ing vessel that makes the gas exert its pressure. 



The velocity of molecular movement increases with an 

 increase in temperature and diminishes with its decrease. It is 

 calculated that all molecular movement would cease at what is 

 called absolute zero, 271.3 C. below the freezing-point of water, 

 a temperature which has recently been nearly achieved in the 

 laboratory. The molecules move less rapidly and are closer 

 together in liquids than in gases and are still more closely spaced 

 and move still less freely in solids. When great quantities of 

 heat are absorbed without a rise in temperature, as occurs when 

 a solid is changed to a liquid, as in the melting of ice, or when a 

 liquid is changed to a gas, as in the change of water to steam, 

 the absorbed heat is used to impart the more vigorous motion to 

 the molecules, which necessitates their wider spacing and the 

 consequent increase in volume of the substance changed. When 

 the reverse process goes on, the latent heat again becomes sensible. 



