Heat and Light from Heated Solid Bodies. 567 



strong feeling, though I am not quite able to prove the 

 assertion, that the platinum wires become excessively brittle, 

 or as it is sometimes called with respect to brass wire, 

 " rotten," during the long process of heating and pumping 

 which is necessary, as a preliminary to the experiments, 

 in order to remove from the platinum the gases occluded 

 in it*. 



In a paper which I published in 1900 along with Prof. 

 J. C. Beattie (Royal Society Proceedings, March 1900) t, 

 the radiation from polished and blackened platinum wires 

 was determined in a somewhat different way. The apparatus 

 used was practically the same as that which I have just now 

 described, but instead of comparing the energy lost by the 

 wires when they presented the same appearance to the eye 

 as to light-giving quality, the comparison was made between 

 pairs of wires at the same temperature, the temperature being 

 known by the resistance of the wires. Three pairs of wires 

 having different diameters were used in these experiments, 

 and it was shown that the thermal energy lost by the sooted 

 wire was from four to five times as great as that lost by the 

 polished wire, the two being, as 1 have said, at the same 

 temperature. 



In one respect the determinations, an account of which is 

 given in the present paper, and in the paper by Dr. Beattie 

 and myself, are not perfectly satisfactory. We have not 

 been able to take account in a proper way of the temperature 

 of the enclosing envelope. In order to be able to see the 

 condition of the wires, and in particular to observe their 

 appearance when they became luminous, glass envelopes 

 were used in these experiments ; and owing to the nature of 

 the arrangements and the method of experimenting it was 

 not found possible to immerse the glass envelopes in a cooling 

 bath. Consequently, the glass became more or less heated 

 during the experiment, and the heating was unequal in the 

 cases of the bright wire and the sooted wire. It has already 

 been pointed out (Phil. Trans. A, 1887, p. 450) that the 



* [Footnote added Oct 18th, 1902.] hi a paper by Dr. Hartley (Phil. 

 Mag. J uly 1902) it is mentioned that the presence of carbon and phosphorus 

 render platinum brittle. It is quite possible that some traces of phos- 

 phorus vapour may still find their way into my tubes, although I have 

 made the greatest effort, by preparing the phosphorus pentoxide which I 

 use with every care and every precaution I can think of, to avoid this 

 contamination, of which I have always had a fear. My phosphorus 

 pentoxide, the best I could buy, was most carefully redistilled, by myself, 

 in a current of oxygen. The distillation was carried on in special tubes of 

 hard glass, constructed bv Mr. Evans, following the method of Shenstone. 



t Phil. Mag. June 1900, pp. 543-559. 



