SICT. XXV. EXPANSION. 223 



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 over- 

 come, 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 in- 

 creasing with the temperature ; and every substance ex- 

 pands according to a law of its own. Gases expand more 

 than liquids, and liquids more than solids. The expan- 

 sion 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 ae'riform condition, the equable dilatation 

 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 mejisuring 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 corres- 

 ponding 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. Tha solidification of some liquids, and es- 

 pecially their crystalization, 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 pow- 

 erful 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 eboule- 

 tnents of mountains in Switzerland. 



The dilatation of substances by heat, and their con- 

 traction by cold, occasion such irregularities in the rate 

 of clocks and watches as would render them unfit for 

 astronomical or nautical purposes, were it not for a very 



