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Show 145 ACTION OF liEAT. Hut we must just notice one or two of those ef fccts of beat which are not so obviously connected: with the display of light; for though ~e sh?uld continue to write, and read, and observe till the light of our own eyes were extingished, we should b~ no' more near the end of the beautiful subject of lightthan we are at this moment. . The· most general and most active property of heat is that of overcoming the cohesion of the parts: of substances ; and thus softening them, and ~x·· panding them into more bnlk or spll;ce. It acts with~ very different degrees both?~ rapidity and of energy_ in different substances; but It IS probable that there IS· no substance that could not be melted, and after that_ changed into air or vapour, by a sufficient degree of heat applied t:mder proper circumstances. Some of those substances which we call simple, because we· have not been able to find more than one ingredient in them, cannot be melted into liquids in the open air. The diamond is one of these; but though the· diamond cannot be melted, it can be burnt, or re duced wholly to vapour; an~ ther~ is no doubt t~at,. if sufficient heat were applied to It under sufficient pressure, it might be made as liquid as water. Ma!ble, or limestone, or chalk, or shells, when burnt m the open air, give out th~ very same kin~ of air into which the diamond is converted by burmng, and the lime (for it is lime in them all) remains and falls to powder when water is sprinkled on it. :But marble and chalk, fwen when i..'n powder, have been artificially melted by heat under pressure, and have been so completely melted that in cooling they for~ed into crystals ,of the very same figure as those whi~h the same compound of lime naturally assGmes In the rock. Nor is there any d0ubt that any substance whatever might be melted by a similar· mode of treatment. Heat is thus the grand instrument in. perhaps. all the operations of nature; for vnr not bemg sensible ACTION OF liEAT. 147 cf it. is no proof that it is not there, any more than our Ignorance o~ any other truth is a contradiction of that. The dJfferent susceptibilities of different substances to ~eat are the means by which almost ~very change IS performed, not only in nature but In .the arts i ar~d even when we cut wood with a ~mfe, or grmd Iron upon a stone, it is by no mean.s Improbable that we ~ffect our purpose chiefly,, ,if not whol1y, through the Instrumentality of heat. When we work hard, the tool gets heated in its whole substal?- ce ;. and .when a blacksmith has no other mea~s of l~ghtmg his forge, he has only to hammer a ie ~f uon on the anvil till it be red-hot and thr~st ci~ Into the coal~, aud he instantly has 'a fire E when we move our bodies the parts moved .beco~~ ~eat~d, nor can we get any instance in which mo bon ~s not accompanied by heat, and heat by motion~ a~fhi~t the;~ be ~nou.g~ of heat, there is light alon; WI I . . ometlmes mdeed we are sensible of th~ ~ne of these., and _not of tae other two ; and sometJmes we ai:e se.n~tble of any two of the three and onuort soef the .remamul<r one . but t hough I. 11 ' o . ' , n a cases uses are our evid~nce of that which the cl~ reven,l .to ~s, they can m no instance be evid~ooo of tha.t whwh they do not reveal. T~e general actiQn of heat, both in .nature :and -in .ar~ ih thus to separate the particles of s.imple bodie$ an e pa!ts of. campound ones ; and t:her-e can ·~ no separatiOn Wilthout motion whether that t" be suc. h as we .can d.l VI· de I· n su' ccession and bmyo thwant means observe, or not. ' In bodies .which are simple, or in com ounds th h:~~s ~f ';hwh ~re equally sensible to th~ action 0~ vapdur.ea B~e~~{ hsofie3s, melts, and conve~ts into int its action : there ~: on~e~o~~i ~Io~~~~h unthiformbly ~ ance becomes r "d d e su - jnto the state ~q~I ' an another at which it .passes three states it o air or vapour; an~ in each of its can, generally speakmg, bear a cer- |