Gustav Kirchhoff

"Look here, I have succeeded at last in fetching some gold from the sun."

"Heat rays have the same nature as light rays... The invisible heat rays are distinguished from light rays only by the period of or the wave length."

"All heat rays follow the same laws in their propagation, which are known for light rays."

"Of the heat rays... sent... to a body by its surroundings a part are absorbed, the others are... varied by reflection and . The rays refracted and reflected... pass off... with those sent out by it, without... mutual disturbance..."

"Through the radiations... a body sends out, the quantity of ... it contains will... sustain a loss... equivalent to the ' of those rays, and through the heat rays... it absorbs, a gain... equivalent to the vis viva of the absorbed rays."

"[I]n certain cases an exception to this rule may occur... [when] absorption and the radiation produce other changes in the body.., for example in bodies... chemically changed by light... [etc.]"

"Such cases should be excluded on the assumption that neither by means of the rays which it radiates or absorbs, nor by... other influences... does the body... change, if its is kept constant by the addition or the subtraction of heat. Under these conditions... the... heat... transferred to a body in a given time to prevent cooling... in consequence of its radiation, is equivalent to the vis viva of the emitted rays; and the amount of heat... withdrawn... to counterbalance the heating from absorption of radiations, is equivalent to the vis viva of the absorbed rays."

"Let a body which satisfies these conditions be surrounded by an enclosure, having the same temperature [and kept constant], through which no heat rays can penetrate... The body sends out heat rays and is encountered by... heat rays... in part... from the enclosure, in part... thrown back... by reflection from it, absorbing a part of them. Its temperature must thus remain the same, unless heat is withdrawn from it or communicated to it as follows on the principle from which Carnot's law results. For this reason the vis viva of the rays, which it sends out in a certain time, must equal the vis viva of the rays which it absorbs in the same time."

"This investigation will be... simplified if we imagine the enclosure... composed... of bodies which, for infinitely small thickness, completely absorb all rays which fall upon them. I... call such bodies perfectly black, or more briefly, black."

"A '... must have the same as the medium... then there will be no reflection at its surface, and all incident rays... wholly absorbed."

"[R]adiation in empty space will be investigated..⇒ the [associated] black bodies must have a refracted index which differs infinitely little from 1."

"[I]t will be assumed that perfectly diathermanous bodies are conceivable, that is, such which will absorb none of the incident heat rays of whatever nature these may be, and finally, that a perfect mirror is conceivable, i.e., a body which reflects completely all heat rays."

"A perfect mirror, like every diathermanous body, can itself send out no rays; for if it did (confined in an enclosure of like temperature) it would warm this enclosure... and cool itself more and more."

"Before a body... imagine two screens, S1 and S2 placed in which are two openings 1 and 2, whose dimensions are infinitely small with respect to their distance apart, and each of which has a center."

"Through these openings passes a pencil of rays sent out by the body... consider the part [of the pencil], whose lies between \lambda and \lambda + \partial \lambda, and let this be divided into two polarized components, whose planes of polarization are the [perpendicular] planes a and b... passing through the axis of the ray pencil."

"Let E \partial \lambda be the intensity of the component polarized in a, or... [equivalently] the increase, which the vis viva of the ether beyond the screen S2 experiences through this component in the unit of time."

"The quantity E is called the emissive power of the body."

"Conversely, upon the body there falls through the openings 2 and 1 a pencil of rays having the wave length \lambda, polarized in the plane a; of this, the body absorbs a part while it reflects or transmits the remainder; let the ratio of the intensity of the absorbed rays to the incident rays be A and let this be called the absorptive power of the body... The quantities E and A depend upon the nature of the condition of the body.., also upon the form and position of the openings.., the wave length \lambda and the direction of the plane a."

"Under these conditions the following law holds : The ratio between the emissive and the absorptive power is the same for all bodies at the same temperature."

"This law will be proven, first, for the case where only black bodies are compared with each other, that is, those whose absorptic power = 1; i.e., it will be shown that the radiating power of all black bodies is the same at the same temperature."

"The proof of this special law is similar to that of the general law, but simpler; it will therefore facilitate the understanding of the latter. Moreover, conclusions which are drawn from the special law will be used in the proof of the common law."

"Another result of this law... When a space is surrounded by bodies of the same temperature, and no rays can penetrate through these bodies, every pencil in the interior of the space is so constituted, with respect to its quality and intensity, as if it proceeded from a perfectly black body of the same temperature, and is therefore independent of the nature and form of the bodies, and only determined by the temperature. The truth... is evident if we consider that a pencil of rays, which has the same form, but the reverse direction to that chosen, is completely absorbed by the infinite number of reflections which it successively experiences at the assumed bodies. In the interior of an opaque glowing hollow body of given temperature there is, consequently, always the same brightness whatever its nature may be in other respects."

"Why do you want to come into physics? All is done and understood."

"We owe to Kirchhoff..., the first rigorous proof of the celebrated law (...Kirchhoff's law) of the emission and absorption of light and heat, and the application of the same by both Kirchhoff and Bunsen to Spectrum Analysis. The radiation of solids and liquids and gases follows the law exactly when the conditions upon which he founded it are rigorously fulfilled, namely, the complete transformation from one to the other of radiant energy and their intrinsic ."

"[M]ost radiations from gases are not exclusively thermal... [T]he substances, cited by Kirchhoff and Bunsen, also give off... chemical.., electrical and fluorescent radiations which Kirchhoff excluded in the proof of his law."

"[N]one of the gases giving line spectra at temperatures heretofore used, do so by simple , but essentially by luminescent actions (chemical, electrical, and photogenic), so... we cannot, in general, apply the law of Kirchhoff of the proportionality between radiation and absorption to either terrestrial or celestial substances. In these cases the principle of usually holds, since in luminescence the radiation of line spectra is accompanied by selective absorption of the same spectral lines, so that the law may be used qualitatively, which is... the way Kirchhoff and Bunsen... attempted to confirm it."

"The formulation of the complete law for radiations of a is only given in part by Kirchhoff. The formula of Wien, and more particularly the most recent one of Planck, deduced on theoretical grounds, approximates closely the latest observations on a black body at different temperatures and over different wave lengths."

"By 1845 he had investigated electric currents, and established the two so-called Kirchhoff's laws for current conduction."

"In 1854 he... became associated with Bunsen. ...[H]e ...for twenty years ...in connection with Bunsen achieved some of the most important discoveries in the history of physical science."

"In 1875 he accepted... the chair of at Berlin where he became associated with his former colleague von Helmholtz."

"His contributions extend over optics, heat, fluid, motion, electricity, elasticity, etc., and all bear the imprint of the great genius..."

"His papers and lectures... form one of the enduring monuments in physical science."