SECT. XXVII. EXPANSION. 271 



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 heat has a constant 

 tendency to overcome the attraction of cohesion, and to separate 

 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 heat, 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 transi- 

 tion from the solid to the aeriform condition, the equable dilata- 

 tion of liquids has not so extensive a range. This change of 

 bulk, corresponding 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 of expansion of solids varies at their transition to liquidity, 

 and that of liquidity is no longer equable near their change to 

 an aeriform state. There are exceptions, however, to the general 

 laws of expansion ; some liquids have a maximum density cor- 

 responding to a certain temperature, and dilate whether that 

 temperature be increased or diminished. For example water 

 expands whether it be heated above or cooled below 40. The 

 solidification of some liquids, and especially their crystallization, 

 is always accompanied by an increase of bulk. Water dilates 

 rapidly when converted into ice, and with a force sufficient to 

 split the hardest substances. The formation of ice is therefore 

 a powerful agent in the disintegration and decomposition of 

 rocks, operating as one of the most efficient causes of local 

 changes in the structure of the crust of the earth ; of which we 

 have experience in the tremendous eboulemens of mountains in 

 Switzerland. But Professor W. Thomson has proved experi- 

 mentally that it requires a lower temperature to freeze water 

 under pressure than when free. 



