222 RAIN COMBUSTION. SECT. XXV. 



Ram 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, occa- 

 sions the particles to approach each other and form 

 drops of water, v which, becoming too heavy to be sus- 

 tained by the atmosphere, sink to_ the earth by gravita- 

 tion 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 tempera- 

 ture, and meet suddenly, hail is formed. This happens 

 frequently in hot plains near a ridge of mountains, as in 

 the south of France ; but no explanation has hitherto 

 been given of the cause of the severe hail-storms which 

 occasionally take place on extensive plains within the 

 tropics. 



An accumulation of caloric invariably produces light : 

 with the ^exception of the gases, all bodies which can 

 endure the requisite degree of heat without decompo- 

 sition begin to emit light at the same temperature ; but 

 when the quantity of caloric is so great as to render the 

 affinity of their component particles less than their 

 affinity for the oxygen of the atmosphere, a chemical 

 combination 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, differ- 

 ent degrees of heat are requisite to produce their com- 

 bustion. 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 progressively, it would 

 be dissipated by degrees, and would never accumulate 

 sufficiently to produce a temperature high enough for 

 the evolution of flame. 



It is a general law that all bodies expand by heat and 

 contract by cold. The expansive force of tfaloric has a 

 constant tendency to overcome the attraction of cohesion, 



