256 COMBUSTION. EXPANSION. [SECT. xxv. 



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 progress- 

 ively, 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 con- 

 tract by cold. The expansive force of caloric has a constant 

 tendency to overcome the attraction of cohesion, and to sepa- 

 rate the constituent particles of solids and fluids ; by this 

 separation the attraction of aggregation is more and more 

 weakened, till at last it is entirely overcome, or even changed 

 into repulsion. By the continual addition of caloric, solids 

 may be made to pass into liquids, and from liquids to the 

 aeriform state, the dilatation increasing with the temperature ; 

 and every substance expands according to a law of its own. 

 Gases expand more than liquids, and liquids more than solids. 

 The expansion of air is more than eight times that of water, 

 and the increase in the bulk of water is at least forty -five 

 times greater than that of iron. Metals dilate uniformly 

 from the freezing to the boiling points of the thermometer ; 

 the uniform expansion of the gases extends between still 

 wider limits ; but, as liquidity is a state of transition from the 

 solid to the aeriform condition, the equable dilatation of liquids 

 has not so extensive a range. This change of bulk, corre- 

 sponding to the variation of heat, is one of the most important 

 of its effects, since it furnishes the means of measuring relative 

 temperature by the thermometer and pyrometer. The rate 



