233 
absorb most readily — that is to say, that molecular condition 
which 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 per cent, of 
the 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, ^hen 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. R 
