April 2^, 1872] 



NATURE 



505 



THE TEMPERATURE OF THE SURFACE 

 OF THE SUN 



T T will be recollected that Messrs. M. E. Vicairc and 

 ^ Sainte-Claire Deville read some papers before the 

 Academy of Sciences at Paris last January, showing that 

 the temperature of the solar surface docs not exceed that 

 produced by the combustion of organic substances. 

 Their reasoning being based on the law of radiant heat 

 established by the investigations of Dulong and Petit. I 

 have in the meantime instituted a series of experiments 

 DH a comparatively large scale, in order to test the cor- 

 rectness of the said law. Accordingly, the dynamic 

 jnergy developed by the radiation of a mass of fused 

 iron weighing 7,000 pounds raised by ''overheating" in 

 the furnace to (a temperature of 3,000° F., has been care- 

 fully measured. 



Sir Isaac Newton assumed that the quantity of heat 

 lost or gained by a body in a given time is proportional 

 to the difference between its temperature and that of the 

 surrounding medium. Some eminent scientists, how- 

 ever, accepting Dulong's conclusions and formula, assert 

 positively that the stated assumption is incorrect. In so 

 doing they apparently overlook the conditions inseparable 

 from the Newtonian doctrine, namely, that the conducting 

 power of the radiating body should be perfect ; that at 

 every instant the temperatuie pervading the interior mass 

 should be transmitted to the surface.* It needs no demon- 

 stration to prove that if the conducting power of a body 

 be so perfect that the temperature of the centre is at all 

 times the same as that of the surface ; in other words, 

 that the fall of temperature at the centre, occasioned by 

 radiation, is as rapid as the fall of temperature at the sur- 

 face, the rate of cooling of such a body will be very 

 different from that observed by Dulong and Petit. The 

 investigation instituted by those experimentalists has in 

 reality established only the degree of conductivity of the 

 radiators employed, under certain conditions, but by no 

 means their true radiant energy at given temperatures. 

 M. E. Vicaire and Sainte-Claire Deville, therefore, com- 

 mit a serious mistake in assuming that the qiiaiiiity of 

 heat transmitted by the radiation of incandescent bodies 

 at high temperatures has been determined. It may be 

 observed that the relation between the time of cooling 

 and the quantity of heat transmitted by radiation which 

 Dulong and Petit established, aiso misled Pouillet re- 

 garding the temperature of the solar surface, which he 

 computed at 1,461'^ C, or at most 1,761° C. It will be 

 well to bear in mind that Pouillet had himself ascertained 

 with considerable accuracy the temperature produced by 

 solar radiation on the surface of the earth ; and also the 

 retardation suffered during the passage of the rays through 

 the terrestrial atmosphere. He was therefore able to de- 

 monstrate that the dynamic energy developed by solar 

 heat amounts to nearly 300,000 thermal units per minute 

 for each square foot of the surface of the sun. Consider- 

 ing the imperfect means employed by Pouillet, his 

 " pyrheliometre," the exactness of his determination of 

 solar energy is remarkable. The truth is, however, that the 

 near approach to exactness was somewhat fortuitous, the 

 eminent physicist having underrated the energy of radiant 

 heat on the surface of the earth, while proportionately 

 over-estimating the retarding influence of the terrestrial 

 atmosphere. The true dynamic energy developed by 



* The writer has just completed a set of experiments with a spherical 

 radiator, 2*75 in. in diameter, composed of very thin hammered copper, 

 charged with water kept in motion by a wheel applied within the sphere, re- 

 volving at a rate ofsotums per minute, the centrifugal action of which brings 

 the particles of the central portion of the fluid so rapidly in contact with the 

 thin spherical shell, that the apparently absurd condition of perfect conducti- 

 vity has been practically fulfilled. The result ol carefully conducted experi- 

 ments with this radiator, enclosed in an exhausted vessel kept at a constant 

 temperature, has established that Newton's law relating to radiant heat, up 

 to a differential temperature of 100° Fahr. (beyond which the investigation 

 has not extended), is rigorously correct. The subject will be fully discussed 

 in a future article. 



radiation at the surface of the sun, exclusive of the ab- 

 sorption of the solar atmosphere — no doubt exceedingly 

 small — determined by the solar calorimeter mentioned in 

 a pre\ious article, is 312,500 thermal units per minute 

 upon an area of one square foot. It will be proper to 

 notice that this amount is not a mean result of a number 

 of observations, but the greatest energy developed at any 

 time during observations continued upwards of three 

 years, namely February 2S, 1S71. It will be proper to 

 add that this' result has been withheld from publication 

 until it could be verified by a second observation indicating 

 an equal energy. Fortunately the sky at noon, March 7, 

 1872, proved to be as clear as on the previous occasion 

 referred to, the indicated energy differing only a few 

 hundred units from that developed February 28, 1871. 



Temperature being a true index of molecular and 

 mechanical energy, conclusively established by the exact 

 relation between the degree of heat and the expansive force 

 of permanent gases under constant volume, it is surprising 

 that Pouillet did not perceive that an intensity of 1,461" C. 

 or 1,761° C, could not possibly develop on a single square 

 foot of surface the enormous energy represented by 

 300,000 thermal units per minute. M. Vicaire, adopting 

 like Pouillet Dulong's formula, states in the paper pre- 

 sented to the French Academy that " an increase of 600° is 

 sufficient to increase the radiation a hundred fold ;" and 

 that Pouillet has verified Dulong's law to more than 1,000°. 

 '"Supposing," he observes, "that beyond this temperature 

 the law ceases to be true, it cannot be absolutely remote 

 from the truth for the temperatures of from 1,400° to 1,500° 

 which we deduce by adopting the law." Sainte-Claire 

 Deville concludes his essay on solar temperature thus : — 

 " In accordance with my first estimate I believe that this 

 temperature will not be found far removed from 2.500° to 

 2,800", the numbers which result from the experiments of 

 M. Bunsen, and those published long ago by M. Debray 

 and myself." The French savans then agree that the 

 temperature of the surface of the sun does not exceed the 

 intensity produced by the combustion of organic sub- 

 stances, their grounds for this assumption being, as we 

 have seen, Dulong's formula relating to the velocity of 

 cooling at high temperatures. But Dulong and Petit did 

 not carry their investigations practically beyond the tem- 

 perature of boiling mercury ; hence their formula relating 

 to high temperatures is mere theory, the soundness of 

 which we have now been enabled to test most effectually 

 by measuring the radiant power of a mass of fused metal 

 raised to a temperature of 3,000' F., 30 inches in depth, 

 presenting an area of 900 square inches. 



Before describing the means which have been employed 

 in measuring its radiant power, let us briefly consider the 

 condition of the fused mass during the experiments. In 

 the first place, the temperature has been sufficiently high 

 to produce an intense white light, luminous rays of great 

 brilliancy being emitted by the radiant surface during the 

 trial ; (2) the bulk of the fused mass being adequate, the 

 intensity of radiation has been sustained without appreci- 

 able diminution during the time required for observation ; 

 (3) the temperature being higher than that which the 

 French investigations assign to the surface of the sun, 

 while the bulk, as stated, is sufficient to maintain the tem- 

 perature of the fused mass, it may be reasonably asked, 

 why an area of one square foot of our experimental rad a- 

 tor should not emit as much heat in a given time as an 

 equal area on the solar surface, if its temperature be that 

 assumed by Pouillet ? It may be positively asserted, more- 

 over, that an increase of the dimensions of our radiator to 

 any extent, laterally or vertically, could not augment the 

 intensity or the dynamic energy developed by a given 

 area. Again, Dulong's formula, as applied by scientists, 

 shows that the emissive power of a tnetallic radiator raised 

 to a temperature of 3,000°, reaches the enormous solar 

 emission computed by Pouillet, 



Let us now briefly examine the calorimeter constructerl 



