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absorb most readily — that is to say, that molecular condition 

 ^vhich is more or less favourable for imparting to adjacent matter 

 the wave-motion of heat is in the same degree more or less favour- 

 able to its reception ; and the same holds good with respect to the 

 selective absorption of heat — namely, that any substance absorbs 

 more freely the special kind of heat which it radiates. Thus, 

 while a plate of rock-salt absorbs little more than 3 percent, of 

 tlie heat radiated by heated black platinum, it absorbs 30 per cent, 

 of the heat radiated by a piece of its own substance heated to 

 the same temperature. Precisely the same phenomena are 

 observed with respect to light : for example, the scoriae floating 

 on the surface of a pot of molten metal glow more brightly than 

 the clean surface of the metal ; and if an encaustic tile with a 

 pattern on it— say of black and white — be heated red hot, and 

 placed in a dark room, the black portion will be observed to 

 glow much more brightly than the white. In these instances 

 the molecular conditions that facilitate absorption equally 

 facilitate emission ; and the case is the same with regard to 

 selective absorption. Thus, a piece of red glass, when heated, 

 emits a greenish light — that is, the absorbed correspond with 

 the emitted rays. And a still more striking instance has been 

 observed by Kirchhoff — namely, that a tourmaline, heated to 

 incandescence, emits light polarised in a plane perpendicular to 

 that which it transmits. Here the structure, that enables the 

 crystal to take up wave-motion in one direction only, compels it 

 to impart motion exclusively in the same direction. If, then, it 

 be admitted that the molecules of all kinds of matter are sus- 

 ceptible of thermic energy, how can it be denied that they are 

 equally susceptible of the energy of light, when the varied 

 phenomena of light and heat are shown to be in all cases 

 precisely analogous. 



42. All substances in the state of incandescent vapour are 

 found to originate or emit rays of definite refrangibility, and to 

 form an interrupted spectrum, consisting of bright lines only ; 

 moreover, the vapour of every substance is capable of absorbing 

 the rays that itself emits when incandescent — that is to say, of 

 responding to and appropriating those special vibrations of 

 which it is most susceptible. This is readily demonstrated by 

 means of sodium. If burnt in a spirit-lamp it emits only the 

 double D line in the spectrum, and if interposed in a state of 

 vapour, it absorbs the vibrations of the same period, and cuts 

 out the same line from a continuous spectrum. A similar 

 reciprocity of emission and absorption exists in sonorous 

 vibrations. If two harps tuned exactly in unison be placed at 

 the opposite sides of a room, a note struck on one will excite 

 vibrations in the corresponding string, and in that only, of 



VOL. VII. K 



