246 On a New Proposition in the Theory of Heat. 
the rays present in solar ight. The power of emission (E) of the 
platinum wire is therefore equal to 0 for the red rays at all tem- 
peratures lower than that at which the wire begins to glow; for 
yellow rays it ceases to be equal to O at a rather higher tempera- 
ture; for green at a still higher temperature, and soon. Accord- 
ing to equation (1), the emission-power (e) of a completely black 
body must cease to be equal to 0 for red, yellow, green, &c. rays at 
the same temperatures at which the platinum wire began to emit 
red, yellow, green, &c. rays. Let us now consider the case of 
any other body which is gradually heated. According to equa- 
tion (2), this body must begin to give off red, yellow, and green 
rays at the same temperatures as the platinum wire. All bodies 
must therefore begin to glow at the same temperature, or at the 
same tempcrature begin to give off red, and at the same tempera- 
ture yellow rays, &c. This is the theoretical explanation of an ex- 
perimental conclusion obtained by Draper thirteen years ago. The 
intensity of the rays of given colour which a body radiates at a 
given temperature may, however, be very different,—according 
to equation (2) it is proportional to the power of absorption (A). 
The more transparent a body is, the less luminous it appears. 
This is the reason why gases, in order to glow visibly, need a tem- 
perature so much higher than solid or liquid bodies. 
A second deduction which I will mention brings me back to 
my special subject. The spectra of all opake glowing bodies are 
continuous; they contain neither bright nor dark lines. Hence 
we can conclude that the spectrum of a glowing black body (the 
term being used in the sense already defined) must also be a con- 
tinuous one. ‘The spectrum of an incandescent gas consists, at 
any rate most frequently, of a series of bright lmes separated 
from each other by perfectly dark spaces. If the power of emis- 
: > Sa 
sion of such a gas be represented by H, the relation = has an 
appreciable value for those rays which correspond to the bright 
lines of the spectrum of the gas, but it has an inappreciable value 
for all other rays. According to equation (3), however, this 
relation is equal to the absorptive power of the incandescent gas.. 
Hence it follows that the spectrum of an incandescent gas will be 
the converse of this, as I express it, when it is placed before a 
source of light of sufficient intensity, which gives a continuous 
spectrum ; 2. e. the lines of the gas-spectrum, which before were 
bright, will be seen as dark lines on a bright ground. A remark- 
able deduction from my proposition which I will nfention is, 
that, if the more remote source of light is an incandescent solid 
body, the temperature of this body must be higher than that of 
the mcandescent gas in order that such a conversion of the spec- 
trum may occur. 
