548 Drs. J. T. Bottomley and J. C. Beattie on 



In one respect, the determinations, an account of which is 

 given in the present paper, are not perfectly satisfactory. 

 The temperature of the circumambient air in all these experi- 

 ments was from 16° to 18° Cent. ; but we have not been able 

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

 envelope enclosing the vacuous space in which the radiating 

 wires were placed. 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 arrange- 

 ments 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 

 experiments, and the heating was unequal in the cases of the 

 bright wire and the sooted wire. It has already been pointed 

 out* that the proportion in which the radiations of longer 

 period and shorter period are present in the total radiation 

 depends on the radiating surface, other things being the same. 

 In the case of the sooted wire, the quantity of long-period 

 radiation is, in proportion, far in excess of that proceeding 

 from a bright metallic polished surface. Consequently, with 

 the same total electric energy supplied to both wires, the glass 

 tube containing the sooted wire became much hotter than the 

 tube containing the bright wire. 



It has also been pointed out f that with a substance like 

 glass, conducting badly and somewhat diathermanous, it is 

 impossible to tell how much heat is returned to the radiating 

 wire from the interior skin of the tube, which no doubt rises 

 to a high temperature during the experiment. To a certain 

 extent, therefore, the results which we have obtained must be 

 considered as not affording results strictly comparable with 

 those formerly obtained in which a metallic envelope cooled 

 with water was used. 



The absolute value of the radiation observed ought to be 

 somewhat lower in amount than would have been found had 

 the enclosing envelope been of metal and properly kept cool, 

 and the disturbance from this cause must have been relatively 

 greater in the case of the dull, than in the case of the bright, 

 wire. 



Experiments were made with platinum wires from three 

 separate hanks. A pair of wires of equal length was taken in 

 each case. One of these was left with its surface exactly as 



* Phil. Trans. 1887, A, p. 450. 

 f Loc. cit. p. 444. 



