448 THE SCIENTIFIC PAPERS OF 



impossible, as proved by the lecturer's own experiments above 

 referred to. 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 Saiute-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. The diagram (Plate 37) 

 of the spectra alluded to shows clearly the gradual advance of 

 the luminous band, as marked by the letters A. to H. 



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 communication 

 to the Royal Society.* The experiment consisted in heating, by 

 means of an electric current, a platinum or iridio-platinum wire, a 

 metre long, and suspended between binding screws, as shown in 

 the sketch (Fig. 3, Plate 23) ; the energy of the current was 

 measured by two instruments an electro-dynamometer, giving it 

 in amperes, and a galvanometer of high resistance giving the 

 electro-motive 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 Royal Society in 1871, would show 

 that the varying electro-motive force in volts observed on the 

 galvanometer was a true index of the temperature of the wire 

 while being heated by the passage of the current. By combining 

 his former experiments on the dependence of resistance upon 

 temperature, with his recent one, a law of increase of radiation 

 with temperature was established experimentally up to the melting- 

 point of platinum ; this, when laid down in the form of a diagram, 

 gave very consistent results expressible by the simple formula 

 Rad tn = M P + $ t, M being a coefficient due to substance radiating ; 

 an expression represented in the diagram (Plate 36), in which the 



* Proceedings of the Royal Society, Vol. XXXV. p. 166, p. 434, ante. 



