Heat and Light from Heated Solid Bodies. 561 



indicated by the McLeod gauge, beyond a certain point makes 

 no difference to the amount of heat lost. 



It is not certain that the results would be quite the same 

 could all the mercury vapour, which must undoubtedly exist 

 in the vacuum-chamber, be removed ; and this is a question 

 which still requires investigation. Other condensable gases, 

 such as vapour of phosphorus (given off from the drying 

 apparatus) also need consideration. The last mentioned gas 

 I have, however, recently done away with, I hope com- 

 pletely, by using phosphorus pentoxide which I have specially 

 prepared for the purpose. 



I only desire at present to call attention to one particular 

 part of the general investigation. 



In 1887, together with Mr. Mortimer Evans, I pointed out 

 the marked difference in emissivity between a polished 

 metallic-like surface and a dull sooted surface. I think it is 

 now generally admitted* that such a difference does really 

 exist, but at the time my conclusions were controverted. For 

 example, in his book on ' Practical Electrical Measurement/ 

 Mr. Swinburne has the following passage : — 



(i Mr. Bottomley read a paper at the British Association in 1887 

 giving an account of some experiments as to colour and temperature. 

 His conclusions are opposed to those generally accepted by physicists, 

 and before they can be accepted a great deal more evidence must be 

 given. Mr. Bottomley, apparently, regards his paper as a mere 

 first notice, and intends to continue his important experiments. 

 At present it may be suggested that his arguments as to Mr. Evans's 

 experiments are open to criticism. The resistance of a lamp- 

 carbon varies with the temperature in accordance with no evident 

 law, and the colour of the light and radiation depend on the surface 

 temperature of the whole carbon. A carbon of high emissivity 

 would need a higher internal temperature to preserve a given 

 surface temperature than one of low emissivity. The same reason- 

 ing applies to two platinum wires, one of which is blackened. 

 Besides, Mr. Bottomley does not state whether the lampblacked 

 wire was heated to incandescence before the tube was sealed off 

 the pump t ; and if it were not, it would be coated with hydrocarbons, 



* [Note added July 31st, 1902.] In the discussion which followed this 

 paper at the meeting of the British Association, September 1901, 

 one of the speakers expressed the view that incandescent-lamp makers 

 do not find any difference between flashed and unflashed filaments. 

 I venture to think, however, that this is incorrect. Incandescent-lamp 

 makers are naturally reticent as to their experience, but I believe that all 

 first-class makers are now fully alive to a difference in economy between 

 a brightly flashed filament and a dull uuflashed filament. 



f The tube is never sealed off the pump. The pump is kept working 

 the whole time to remove occluded gases given off from the platinum. 



