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Show 254 JIEAT ONLY A SENSATION. gives out, in the same manner that the oil in a lamp is consumed by the burning of the lamp," are a?surd. The light of the lamp does not consume the ml, and as little does the light of a fire consume the fuel. The light is, in these cases, only one . <?f the appearances attendant upon th~ d~composttlon of matter, and if it measure any thmg 1t me?~ures only the rapidity with which the decomposition takes place. Further, as light is reflected f~o~ ~urfaces, and reflected from them though it be mvisible previous to the reflection, the light of the sun may be a reflection from the sun, or the sun's atmosphere, which comes invisibly to that luminary from some far distant source. The co~o.urs which we observe direetly in nature ~re not VISible, and therefore. do not exist at any pomt between the coloured object and the eye ; and reflecte~ colours, such as those of the face in a mirror, extst nowhere between the face which is reflected and the eye that sees the reflection. So also, in the casP- of beat, we can .never observe it as any thing else than an ac~ompamm~nt of .some action, and intense in proportiOn to the mtens1~y of that action "\i..,rom what we can observe and JUdge of the oth~r appearances attending col~ and heat, even heat up to the most intense combustiOn, all that we can say is, that absolute cold appears to be but another name for absolute rest; and absolute heat, absolute motion and conflict. . To speak of " parts" of light and heat, Ill H;nY sense of the word, is, therefore, t_? speak that whtch has no meaninf! even if we consider those parts as the mere result~ of mechanical division. To .speak of the half, or the quarter, or any other.f~actw~ of light, is absurd ; and one of those absurdities which, having crept into common language, te~ds to ~on fuse our understanding of things, even 111 theu very simplest elements. Air, water, and solid matte~ w~ can c:oncm~e of as divisible into parts, mech~mcally, or mto ptecea HEAT ACTING ON COMPOUNDS. 255 In all ~heir various forms; and, chymically, into their constituent parts. or elements, in all kinds of matter e~cept those whteh we consider them, and we constder the~. simple, just because we have not been able to d1v1de them chymically. Our mea~s and methods of decomposition have b~en much tmproved since the time when fire, and atr, and water, and solid matter un,der the general name of earth, were considered as the four elements of . aU created th~~gs. And as we find in every case of decomposition that the constituent or elementary parts have al~ .different qualities, and that the compound has qualities of which we could have had no knowledge or even suspicion if we had known the elements only in their sep~rate states we are enab~ed !o say to :what the properties which we, observe m d1tferent _km?s ~f matter are owing. But, as .every new combmatwn IS attended with new properties, we have strong grounds for believing that every property of matt<:r, and every change in the appearat~ce ?f any portiOn of matter, is the result of. c?mbm~twn: that the property which we find ongmally l!l a~y su~tance is the result, or effect, of ~ combmatwn whtch took place before we exammed that substance.; and that every change which we find ~ ta~e p~ace m any substance is the result of a combn~atH;m Immediately preceding that change. The c?mbma~wn may take place in two ways, becaus~ It may, m the case of the individual substance be mther an .a~ding to it or a taking away from it i and the. addttwn or th~ subtraction may either be that wh1~h we can obtam and examine in a separate state, or It may ~ot. It may happen, also, that those t~vo !!lodes of cnange are combined, and the combmabons of them may be varied,-it may consist of any two of them, or of any three or of all the four. ' Removing a spot of tar is a familiar instance of th~t. Common soap will not dissolve the tar and OOlthe~t tar nor grease will dissolve in water ~ but |