274 



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



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 experi- 

 ment. To a certain extent, therefore, the results which we have 

 obtained must be considered as not affording results strictly com- 

 parable 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 it was on being taken 

 from the hank ; the other was sooted. The two wires were then fixed 



in the glass tubes. The wires are designated Ptj,Pt2, Pt3,Pt4, Pt5,Pt6. 

 The first of each pair is the bright wire ; the second is the sooted 

 wire. The diameters of the wires are as follows : — Pt^ and Pto, 

 0-0542 cm., Pts and Pt4, 0-025 cm. ; Pto and Pte, 0-015 cm. 



In Tables I and II specimens are given of the results obtained, in 

 the manner described above, by observation and calculation. The 

 remainder of the results are embodied in the curves appended, which 

 it is hoped will be found self-explanatory. At the head of each table 

 the particulars as to the wires referred to in the table are given. 



In the following tables. III, IV, V, the loss of heat per square centi- 

 metre of surface for the several pairs of wires, bright and sooted, at 

 various temperatures, is compared ; and the ratio between the radiation 

 from the sooted wire and the radiation from the bright wire is calculated. 

 It will be seen that the numbers are in fair agreement. What may be 

 the causes of divergence from exact agreement it is impossible to say 

 at the present stage of the inquiry ; but it may be conjectured that 

 part of it at least is due to the difiiculty, or impossibility, of keeping 

 the vacuum which surrounds the wires in these experiments unchanged. 

 When the pressure is very low, the accession of the smallest quantity 

 of gas to the surrounding space causes an enormous change in the 

 rate of loss of heat, as has been shown in a previous part of this 

 research; and as the temperature rises it is always found that the 

 vacuum becomes deteriorated, owing to the expulsion of gas from the 

 body of the wire itself. This gas must be removed by a fresh appli- 

 cation of the pump, and, in fact, during the experiments the pump must 

 be kept always at work. Thus the vacuum is incessantly changing • 

 and, moreover, as the indications of the McLeod gauge lag very much 

 behind, it is not even possible to know the exact state of the pressure 

 at the instant when it is desired to make an observation as to current 



