SJJt WILLIAM SIEMENS, F.R.S. 449 



in which the abscissa) represent energy disperse 1 and ihe ordinates 

 tin- 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 ^yth part of the energy emitted from the same 

 area of the solar photosphere, and taking the temperature of the 

 incandescent carbon at 1800 0. (the melting-point of platinum, 

 which can just be heated to the same point), it follows in apply- 

 ing Sir William Thomson'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 con- 

 clusion that at a point exceeding 3000 C. radiation would become, 

 as it were, explosive in its character, rendering a surface tempera- 

 ture 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 inter- 

 stellar 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 concentration of solar rays, a sufficient 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 or period of 

 vibration as numerous rays associated in a focus, it seemed reason- 

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