20 



NATURE 



[May 3, 1883 



composed " ; but within only seven years of that time Messrs. 

 Bunsen and Kirchhoff published their famous research showing 

 that, by connecting the dark Fraunhofer lines of the solar 

 spectrum w ith the bright lines observed in the spectra of various 

 metals, it was possible to prove the existence of those substances 

 in the solar photosphere, thus laying the foundation of spectrum 

 analysis, the greatest achievement of modern science. Dr. 

 Huggins and others, applying this mode of research to other 

 heavenly bodies, including the distant nebula;, had extended our 

 chemical knowledge of them in a measure truly marvellous. 



Solar observation had thus led to an analytical method by 

 which chemistry had been revolutionised, and it would be, in the 

 lecturer's opinion, through solar observation that we should attain 

 to a much more perfect conception of the nature and effect of 

 radiant energy, in its three forms of heat, light, and actinism, than 

 we could as yet boast of. The imperfection of our knowledge 

 in this re-pect was proved by the circumstance that whereas some 

 astronomers and physicists, including Waterston, Secchi, and 

 Ericsson, had, in following Sir Isaac Newton's hypothesis, 

 attributed to the sun a temperature of several millions of degrees 

 Centigrade, others, including Pouillet and Vicaire, in fol- 

 lowing Dulong and Pelit, had fixed it below 1800° C. ; between 

 these two extremes other determinations based upon different 

 assumptions had placed the solar temperature at between 60,000" 

 and 20,000°. 



The lecturer, having conceived a process by which solar energy 

 may be thought self-sustaining, had felt much interested for 

 some years in the question of solar temperature. If the tem- 

 perature of the solar photosphere should exceed 3000° C, com- 

 bustion of hydrogen would be prevented by the law of dissocia- 

 tion, as enunciated by Bunsen and Sainte-Claire Deville, and his 

 speculative views regarding thermal maintenance must fall to 

 the ground. To test the question he in the first place mounted 

 a parabolic reflector on a heliostat, with a view of concentrating 

 solar rays within its focus, which, barring comparatively small 

 losses by absorption in the atmosphere and in the metallic sub- 

 stance of the reflector should reproduce approximately the solar 

 temperature. By introducing a rod of carbon through a hole at 

 the apex of the reflector until it reached the focus, its tip became 

 vividly luminous, | reducing a light comparable to electric light. 

 When a gas burner was arranged in such a way that the gas 

 flame played across the focal area, combustion appeared to be 

 retarded but was not arrested, showing that the utmost tempera- 

 ture attained in the focus did not exceed materially that pro 

 ducible in a Deville oxyhydrogen furnace or in the lecturer's 

 regenerative gas fur race, in which the limit of dUsocialion is 

 also leached. 



Having thus far satisfied himself, his next step was to ascer- 

 tain whether terrestrial sources of radiant energy were capable 

 of imitating solar action in effecting the decomposition of car- 

 bonic acid and aqueous vapour in the leaf-cells of plants, which 

 led him to undertake a series of researches on electro-horticul- 

 ture extending over three years, a subject which he had brought 

 before the Royal Society and the Royal Institution two years 

 ago. By these researches he had proved that the electric arc 

 possessed not only all the rays necessary to plant-life, but that a 

 portion of its rays (the ultra-violet) exceeded in intensity the 

 effective limit, and had to be absorbed by filtration through 

 clear glass, which, as Prof. Stokes had shown, produced this 

 effect without interference with the yellow and other luminous 

 and intense heat-rays. He next endeavoured to e-timate the 

 solar temperature by instituting a comparison between the spectra 

 due to different known luminous intensities. Starting with the 

 researches of Prof. Tyndall on radiant energy, supplementing 

 them try experiments of his own on electric arcs of great power, 

 and calling to his aid Prof. Langley of the Alleghany Observa- 

 tory to produce for him a complete spectrum of an Argand 

 burner, he concluded that with the temperature of a radiant 

 source the proportion of luminous rays increased in a certain 

 ratio : v hereas in an Argand oil-burner only 2 J per cent, of the 

 rays emitted were luminous, and mostly red and yellow, a bright 

 gas flame emitted 5 per cent., the carbon thread of an incan- 

 descent electric light between 5 and 6 per cent , a small electric 

 arc 10 per cent., and in a powerful 5000-candle electric arc as 

 much as 25 per cent, of the total radiation was of the luminous 

 kind. Prof. Langley, in taking his photometer and bolometer 

 up the Whitley Mountain, 18,000 feet high, had proved that 

 of the solar energy not more than 25 per cent, was of the 

 luminous kind, and that the loss of solar energy sustained 

 between our atmosphere and the sun was chiefly of the ultra- 



violet kind, which rays, if they penetrated our atmosphere, 

 would render vegetation impossible. It was thus shown that 

 the temperature of the solar photosphere could not materially 

 exceed that of a powerful electric arc or indeed of the furnaces 

 previously alluded to, leading him to the conclusion already 

 foreshadowed by Sainte-Claire Deville and accepted by Sir 

 William Thomson, that the solar temperature could not exceed 

 3000° C. The energy emitted from a source much exceeding 

 this limit would no longer be luminous, but consist mainly of 

 ultra-violet rays, rendering the sun invisible, but scorching and 

 destructive of all life. 



Not satisfied with these inferential proofs, the lecturer had 

 endeavoured to establish a definite ratio between temperature 

 and radiation, which formed the subject of a very recent com- 

 munication to the Royal Society. It consisted simply in heating 

 a platinum or iridio-platinum wire, a metre long and suspended 

 between binding screws, by means of an electric current, the 

 energy of which was measured by two instruments, an electro- 

 dynamometer giving the current in amperes, and a galvanometer 

 of high resistance giving the electromotive force between the 

 same points in volts. The product of the two readings gave the 

 volt-amperes or watts of energy communicated to the wire, and 

 dispersed from it by radiation and convection. A reference to 

 the lecturer's paper on the Electrical Resistance Thermometer, 

 which formed the Bakerian Lecture of the Koyal Society in 

 1 87 1, would show that the varying electromotive force in 

 virlts observed on the galvanometer was a true iirdex of the tem- 

 perature of the wire, while being heated by the passage of the 

 current ; a law of increase of radiation with temperature was 

 thus established experimentally up to the melting-point of 

 iridio-pla inum, which when laid down in the form of a diagram 

 gave very consistent results expressible by the simple formula — 



Radiation = Mi- + <f>/, 



At being a coefficient due to substance radiating. 



Sir William Thomson had lately shown that the total radiating 

 energy from a unit of surface of the carbon of the incandescent 

 lamp amounted to 1/67 part of the energy emitted from the 

 same area of the solar photosphere, and taking the temperature 

 of the incandescent carbon at 1800° C. (the melting-point of 

 platinum which can just be heated to the same point), it follows 

 in applying Sir William Thorns >n's deductions to the lecturer's 

 formula that the solar photosphere does not exceed 2700° C. , or, 

 adding for absm-ption of energy between us and the sun, about 

 2Soo° C. — a temperature already arrived at by different methods. 

 The character of the curve was that of a parabola slightly tipped 

 forward, and if the ratio given by that curve held good absolutely 

 beyond the melting-point of platinum iridium, it would lead to 

 the conclusion that at a point exceeding 3000° C. radiation would 

 become as it were explosive in its character, rendering a rise of 

 temperature beyond that limit difficult to conceive. 



Clausius had proved that the temperature obtainable in a focus 

 could never exceed that of the radiating surface, and Sainte- 

 Claire Deville that the point of dissociation of compound 

 vapours rises with the density of the vapour atmosphere. Sup- 

 posing interstellar space to be filled with a highly attenuated 

 compound vapour, it would clearly be possible to effect its disso- 

 ciation at any point, where, by the concentration of solar rays, a 

 focal temperature could be established, but it was argued that 

 the higher temperature observable in a focal sphere was the 

 result only of a greater abundance of those solar vibrations 

 called rays within a limited area, the intensity of each vibration 

 being the outcome of the source whence it emanated : thus, in the 

 focal field of a large reflector, the end of a poker could be 

 heated to the welding point, whereas in that of a small reflector 

 the end of a very thin piece of wire only could be raised to the 

 same temperature. If, however, a single molecule of vapour 

 not associated or pressed upon by other molecules could be sent 

 through the one focus or the other, dissociation in obedience to 

 Deville's law must take place irrespective of the focal area ; but 

 inasmuch as the single solar ray represented the same potential 

 of energy as numerous rays associated in a focus, it seemed 

 reasonable that it should be as capable of dealing with the 

 isolated molecule as a mere accumulation of the same within a 

 limited space, and must therefore 1 ossess the same dissociating 

 influence. Proceeding on these premises, the lecturer had pro- 

 cured tubes filled with highly attenuated vapours, and had ob- 

 served that an exposure of the tubes to the direct solar rays or 

 to the arc of a powerful electric light affected its partial or 

 entire dissociation ; the quantity of matter contained w ithin such 



