562 Dr. J. T. Bottomley on Radiation of 



which would spoil the vacuum in that tuhe. It is, no doubt, un- 

 likely that one so careful as Mr. Bottomley would make such 

 mistakes ; but, on the other hand, his results are so opposed to 

 those of others that they may be disregarded for the present, and 

 the law that efficiency is a test of surface-temperature, and colour 

 is a test of both, may be taken as true." 



I have always desired to examine further the question here 

 referred to, and to come to definite conclusions on the subject, 

 but it is only recently that I have been able to arrange for a 

 set of perfectly conclusive experiments. 



The apparatus which I use is shown in PL V., which will 

 serve to explain the details. 



The radiating body which I have used recently has been a 

 thin platinum strip heated by an electric current. The strip 

 was rolled for the purpose by Messrs. Johnson and Matthey, 

 to whom I beg to express my thanks for the great trouble 

 they have taken in order to provide me with perfectly uniform 

 material. The way in which it is mounted is shown in the 

 figures. The strip AB is held stretched between two spiral 

 springs in a glass tube: the outer ends of the spiral springs 

 terminate in loops, and two pieces of copper rod are passed 

 into the tubes CC and C'C so that the springs pull on these 

 rods. These rods pass down through long narrow side tubes 

 into mercury cups, and by means of these the electric current 

 which heats the platinum strip is passed into it. 



At the points EE (see elevation) of the platinum strip fine 

 platinum wires are attached by welding, and they are brought 

 out through the sides of the glass envelope. These serve as 

 potential electrodes ; and it is to keep the platinum strip AB 

 in the middle of the tube, and to avoid pulling unduly on the 

 potential electrodes that the two spiral springs, one at each 

 end of the tube, are introduced. 



Two exactly similar tubes are employed, as shown in 

 PI. V. (plan). They are connected together at each end, as 

 shown ; and by means of a branch tube, attached to one of 

 the two end tubes and connected to a Sprengel-pump, the 

 air is withdrawn from both tubes at the same time. By this 

 arrangement it is provided that the vacuum in the two 

 experimental tubes shall be at all times precisely the same. 



Jn one of the tubes the platinum strip is brightly polished 

 and perfectly smooth, just as it came from the makers' hands. 

 The other tube contains a platinum strip of exactly the same 

 length between the points EE, and cut from the same hank, 

 but with the surface covered with an exceedingly fine coating 

 of lampblack. 



The arrangements for drying the vacuum consist in (1) a 



