320 Sir William Siemens [April 27, 



lecturer's paper on the Electrical Eesistance Thermometer, which 

 formed the Bakerian Lecture of the Eoyal 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 uj) 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 



M being a coeflficient due to substance radiating ; an expression repre- 

 sented in the accompanying diagram (Fig. 3), in which the abscissas 

 represent energy dispersed and the ordinates the corresponding 

 temperatures. 



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 uV^h 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 Thom- 

 son's deductions to the lecturer's formula that the solar photosphere 

 does not exceed 2700° C, or, adding for absorption of energy between 

 us and the sun, about 2800° C, a temperature already arrived at 

 by the lecturer by a different method. 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, 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 surface temperature beyond that limit physically 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. Supposing interstellar space to 

 be filled with a highly attenuated compound vapour, it would clearly 

 be possible to effect its dissociation at any point where, by the concen- 

 tration of solar rays, a sufficient focal temperature could be estab- 

 lished ; 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 bo 

 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 tem- 

 perature. If, however, a single molecule of vapour not associated or 

 pressed upon by other molecules could be sent through the one focus 



