270 RAIN COMBUSTION. SECT. XXVII. 



between the tension of the vapour which is forming, and that 

 which is already in the atmosphere. In calm weather vapour 

 accumulates in the stratum of air immediately above the evapo- 

 rating surface, and retards the formation of more ; whereas a 

 strong wind accelerates the process by carrying off the vapour as 

 soon as it rises, and making way for a succeeding portion of dry 

 air. 



Rain is formed by the mixing of two masses of air of different 

 temperatures ; the colder part, by abstracting from the other 

 the heat which holds it in solution, occasions the particles to 

 approach each other and form drops of water, which, becoming 

 too heavy to be sustained by the atmosphere, sink to the earth 

 by gravitation in the form of rain. The contact of two strata 

 of air of different temperatures, moving rapidly in opposite 

 directions, occasions an abundant precipitation of rain. When 

 the masses of air differ very much in temperature, and meet 

 suddenly, hail is formed. This happens frequently in hot plains 

 near a ridge of mountains, as in the south of France, from the 

 sudden descent of an intensely cold current of wind into a mass 

 of air nearly saturated with vapour. Such also is the cause of 

 the severe hail-storms which occasionally take place on extensive 

 plains within the tropics. 



An accumulation of heat invariably produces light : with the 

 exception of the gases, all bodies which can endure the requisite 

 degree of heat without decomposition begin to emit light at the 

 same temperature ; but, when the quantity of heat is so great 

 as to render the affinity of their component particles less than 

 their affinity for the oxygen of the atmosphere, a chemical com- 

 bination takes place with the oxygen, light and heat are evolved, 

 and fire is produced. Combustion so essential for our comfort, 

 and even existence takes place very easily from the small 

 affinity between the component parts of atmospheric air, the 

 oxygen being nearly in a free state ; but, as the cohesive force 

 of the particles of different substances is very variable, different 

 degrees of heat are requisite to produce their combustion. The 

 tendency of heat to a state of equal diffusion or equilibrium, 

 either by radiation or contact, makes it necessary that the 

 chemical combination which occasions combustion should take 

 place instantaneously ; for, if the heat were developed pro- 

 gressively, it would be dissipated by degrees, and would never 



