ON THE SOURCES AND EFFECTS OF HEAT. 643 



that "the substance has become heated, and that a blow, or the agitation 

 produced vvhen they are made to sound by the friction of the bow of a vioHn, 

 may sometimes be observed to cause them to assume the state of equilibrium 

 with greater rapidity. Brass expands about one hundred thousandth of its 

 length for each degree of Fahrenheit, copper and gold a little less ; silver 

 somewhat more; glass and platina less than half as much; iron and steel about 

 two thirds as much; tin one third more, and lead and zinc about half as 

 much more. Wood and earthenware are the least expansible of all known 

 solids. The diminution of the elasticity of iron and steel by the elevation of 

 their temperature amounts to about -j-bW of the whole for each degree: but 

 probably various substances are variously aiFected in this respect. 



/ 

 The liquefaction of solids, and their conversion into fluids by the operation 

 of heat, is liable to fewer irregularities than any other of its effects; the 

 change depending only on the temperature, and not being accelerated or retard- 

 ed by any accidental circumstances. When the temperature is too low, or 

 the pressure too small, for the existence of the substance in a liquid form, it 

 may still be ccnverted into vapour, either mixed with air, or in a separate 

 state: thus ice loses weiglit when it is exposed to a dry frosty wind; and 

 camphor, benzoin, and ammonia are sublimed by heat without being melted, 

 although it is probable, that a pressure sufficiently strong might enable them 

 to e>ist as liquids in elevated temperatures. In all changes from solidity to 

 liquidity or to elastic fluidity, a certain quantity of heat disappears, except 

 some cases in which a chemical decomposition has accompanied the change; 

 thus, in the detonation of gunpowder, a large quantity of gas acquires 

 the state of elasticity, but at the same time a great degree of heat is pro- 

 duced. 



The effects of the expansion of bodies by heat, and of their contraction by 

 cold, are observed in the frequent accidents which happen to glass and to 

 porcelain from a sudden change of temperature. Glass conducts heat so 

 slowly, that one side of a vessel may become much heated, and consequently 

 expanded, while the other remains much colder, and if the vessel canno>t 

 readily accommodate its form to this change of proportions, it will most com- 

 monly crack, the colder parts dividing, in consequence of their being too 

 much stretched by the adjoining hotter parts. Hence the thinner the ^lass is, 



