September 22, 1904] 



NA TURE 



513 



the still shorter waves which affect a photographic plate 

 or a fluorescent >rreen, and will pass through certain 

 substances opaque to ordinary light. Here, for instance, 

 is a filter devised by Prof. Wood which stops visible rays, 

 but allows the shorter invisible waves to. pass and excite 

 the fluorescence of a platinocyanide screen. 



.\gain, beyond the red end are still longer waves, 

 which are present in very considerable amount, and can 

 he rendered evident by their heating effect. We can easily 

 filter out the visible rays and still leave these long waves 

 in the beam by passing it through a thin sheet of vulcanite. 

 .\ piece of phosphorus placed at the focus of these invisible 

 ravs is at once fired, or a thermometer quickly rises in 

 lemperBture. The waves which have been observed and 

 studied up to the present time range over some nine octaves, 

 from the long waves described to the section yesterday by 

 Prof. Rubens, waves of which there are only 400 in an inch, 

 down to the short waves found by Schumann in the radi- 

 ation given off by hydrogen under the influence of the 

 electric discharge, waves of which there are a quarter of 

 a million in an inch. No doubt the range will be e.\tended. 



Radiant energy consists of a mixture of any or all of 

 these wave-lengths, but the eye is only sensitive at the 

 most to a little more than one octave in the nine or more. 



This radiation is emitted not only by incandescent 

 bodies such as the sun, the electric arc, or flames. All 

 bodies are pouring out radiant energy, however hot or 

 cold they may be. In this room we see things by the radi- 

 ation which they reflect from the daylight. But besides 

 this borrowed radiation, every surface in the room is send- 

 ing out radiation of its own. Energy is pouring forth 

 from walls, ceiling, floor, rushing about with the speed of 

 light, striking against the opposite surfaces, and being 

 reflected, scattered, and absorbed. -And though this radi- 

 ation does not aff'ect our eyes, it is of the utmost importance 

 in keeping us warm. Could it be stopped, we should soon 

 be driven out by the intense cold, or remain to be frozen 

 to death. 



.As the temperature of a body is raised, the stream of 

 radiation it pours out increases in quantity. But it also 

 changes in quality. Probably the surface always sends out 

 waves of all lengths from the longest to the shortest, but 

 at first when it is cold the long waves alone are appreci- 

 able. As it gets hotter, though all the waves become more 

 intense, the shorter ones increase most in intensity, and 

 ultimately they become so prominent that they affect our 

 sense of sight, and then we say that the body is red or 

 white hot. 



The quality of the stream depends on the nature of the 

 surface, some surfaces sending out more than others at the 

 .same temperature. But the stream is the greatest from 

 a surface which is, when cold, quite black. Its blackness 

 means that it entirely absorbs whatever radiation falls upon 

 it, and such a surface, when heated, sends out radiation of 

 every kind, and for a given temperature each kind of radi- 

 ation is present to the full extent, that is, no surface sends 

 nut more of a given wave-length than a black surface at a 

 given temperature. 



\ very simple e.xperiment shows that a black surface is 

 a better radiator, or pours out more energy when hot, than 

 a surface which does not absorb fully, but reflects much of 

 the radiation which falls upon it. If a platinum foil with 

 some black marks on it be heated to redness, the marks, 

 black when cold, are much brighter than the surrounding 

 metal when hot ; they are, in fact, pouring out much more 

 visible radiation than the metal. 



It is with these black surfaces that I am concerned to-day. 

 But. inasmuch as it seems absurd to call them black when 

 they are white hot, I prefer to call them full radiators, 

 since they radiate more fully than any others. 



For a long time past experiments have been made to 

 seek a law connecting the radiation or energy flow from 

 a black or fully radiating surface with its temperature. 

 But it was only twenty-five years ago that a law was 

 suggested by Stefan which agrees at all satisfactorily with 

 experiment. This law is that the stream of energy is pro- 

 portional to the fourth power of the temperature, reckoned 

 from the absolute zero 273° below freezing point on the 

 centigrade scale. This suggestion of Stefan served as the 

 starting point of now and most fertile researches, both 

 theoretical and practical, and we are glad to welcome to 



NO. 182 I, VOL. 70] 



this meeting Profs. Wien, Lummer, and Rubens, who have 

 all done most brilliant work on the subject. 



.•\mong the researches on radiation recently carried out 

 is one by Kurlbaum in which he determined the actual 

 amount of energy issuing from the black or fully radiating 

 surface per second at 100° C, and therefore at any 

 temperature. 



Here is a table w'hich gives the amount at various 

 temperatures, as determined by Kurlbaum : — 



Raie of Flow of Energy from i sq. cm. of Fully Radiating 

 or " Black " Surface. 



As an illustration of the " fourth power law," let us see 

 what value it will give us for the temperature of the sun, 

 assuming that he is a full radiator, or that his surface, if 

 cooled down, would be quite black. 



We can measure approximately the stream of energy 

 which the sun is pouring out by intercepting the beam 

 falling on a surface exposed to full sunlight, measuring the 

 heat given to that surface per second, and then calculating 

 what fraction the beam is of the whole stream issuing from 

 the sun. 



This was first done by Pouillet, and his method will 

 serve to illustrate the principle of all other methods. 



In his apparatus the sunlight fell full on a box contain- 

 ing water, and the rate at which the water rose in tempera- 

 ture gave the energy in the stream of solar radiation fall- 

 ing on the box. 



Simple as the experiment appears, the determination is 

 beset with difficulties, the chief being the estimation of 

 the fraction of the energy intercepted by the atmosphere, 

 and we are still unable to give a very definite value. 

 Indeed, we cannot yet say whether the outflow of energy 

 is constant or whether it varies. In all probability, how- 

 ever, it does vary, and Prof. Langley, who has devoted 

 years of work to the subject, has recently obtained evidence 

 indicating quite considerable variation. 



We may, however, assume that we are not very far 

 from the true value if we say that the stream of radiation 

 from the sun falling perpendicularly on i sq. cm. outside 

 the earth's atmosphere will heat i gm. of water 1/24° C. 

 every second, or will give 1/24 calory per sec. 



Now the area of a sphere round the sun at the distance 

 of the earth is 46,000 times the area of the sun's surface. 

 The energy from i sq. cm. of the sun thus passes through 

 46,000 sq. cm. at the surface of the earth. It is therefore 

 46,000x1 '24 calories, or 1920 cal./sec. But from the table 

 already given, a black surface at 6250' absolute, say 

 6000° C, gives 1930 calories per second, or the temperature 

 of the sun's radiating surface is 6000° — if he is a full 

 radiator, and there is good reason to suppose that no great 

 error is made in taking him to be one. 



Let us now take another illustration of the fourth power 

 law. 



Imagine a little black body which is a good conductor 

 of heat placed in full sunlight at the distance of the earth. 

 Let it be i sq. cm. in cross section, so that it is receiving 

 1/24 calory per second. 



It will soon warm up to such a temperature that it gives 

 out just as much as it receives, and since it is so small, heat 

 will rapidly flow through it from side to side, so that it 

 will all be very nearly at the same temperature. A sphere 

 I sq. cm. in cross section has area 4 sq. cm., so that it 

 must be giving out from each sq. cm. of its surface 

 1/06 = 00104 calory each second. From the table above it 

 will be seen that this corresponds very nearlv indeed to a 

 temperature of 300° absolute or 27° C, say 70° F. 



It is to be noted that this only applies to a little round 

 body. .\ flat plate facing the sun would be about 60° C. 



