208 



Mr. M. Evans. Observations on the [Feb. 18, 



when raised to the same incandescence, but the energy required to 

 produce this light, or raise the filament to this incandescence, varied 

 sometimes in a remarkable way. At times a filament was found 

 which, with 2 watts or volt amperes passing through it, would yield 

 the light of the standard candle. And again, with other filaments it 

 sometimes occurred that no less than 5 volt amperes were required to 

 produce this light. 



On collating a number of these observations, and comparing the 

 filaments themselves with their various testings, I noticed, I thought, 

 some difference in the outward appearance of those filaments which 

 had tested well and those that had required any large amount of 

 energy to give a satisfactory light, and, following np this idea, I soon 

 became convinced that it was to this surface appearance or condition 

 that the whole question of economical light giving or otherwise might 

 be traced. All the filaments, it appeared, whose surfaces were of a 

 dull black required the larger amounts of energy to yield the usual 

 unit of light, while from those filaments with even moderately bright 

 surfaces the light of the standard candle could be obtained for an 

 expenditure of energy surprisingly less. To ascertain with greater 

 certainty if this idea were correct, I prepared a number of carbons 

 made from a vegetable fibre which, though yielding a somewhat 

 porous carbon, was strong and uniform in texture. 



Having selected two filaments as like each other as the eye could 

 determine, and having ascertained by careful measurement that they 

 were both of exactly the same length and diameter, and therefore of 

 equal surface, I subjected each carbon in turn to the action of an 

 electrical current in a hydrocarbon atmosphere, so regulating the 

 current as to maintain the carbon filament at a white heat in the 

 vapour until a sufficient deposit of carbon upon its surface was 

 obtained. 



To provide for the deposit of carbon upon the one filament being as 

 dull a black as possible, I used for the depositing medium an atmo- 

 sphere of ordinary coal-gas drawn from a domestic burner. A large 

 glass jar was filled with this, and a constant current of the cold gas 

 kept circulating through it during the deposit, and the resulting 

 surface was all that could be desired. It had all the appearance of 

 being coated with lampblack, but the coating was quite permanent, 

 and did not brush off, or even soil the fingers in handling. 



On the other of these two filaments I now deposited carbon of a 

 bright silvery appearance in marked contrast with the dull black of 

 that just described, and this deposit I found I could readily effect by 

 using as the depositing atmosphere the very hot vapour of almost any 

 hydrocarbon having a high boiling point, though from the porous 

 nature of the carbons I was using I did not get the surface as brilliant 

 as I subsequently obtained it from smoother carbons. 



