1886.] 



On the Radiation of Light and Heat. 



207 



February 18, 1886. 



Professor G. G. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The following Papers were read : — 



I. " Observations on the Radiation of Light and Heat from 

 Blight and Black Incandescent Surfaces." By Mortimer 

 Evans, M.Inst.C.E., F.R.A.S. Communicated by Lord 

 Rayleigh, M.A., D.C.L., F.R.S. Received February 3, 

 1886. 



In the course of an investigation into the nature of carbon filanients r 

 such as are ordinarily used in the construction of incandescent lamps r 

 my attention was arrested by certain variations in the amount of 

 light emitted from filaments which were, to the best of my belief, of 

 similar nature and composition, but which, when tested under pre- 

 cisely similar conditions, gave the most anomalous results. I was- 

 also struck with changes which occurred to a greater or less degree in 

 the light yielded by certain lamps when re-tested subsequent to a 

 shock of over- incandescence, or long continued hard running at a high 

 temperature ; the light yielded after this occurrence (indeed the light 

 yielded by any lamps that had been much used) I found to be 

 invariably lessened both in quantity and brightness, and to require a 

 considerable increase in the energy supplied to it to produce from 

 the same filament the light it originally gave. After seeking vainly 

 to account for these irregularities from structural differences in 

 the carbon filaments themselves, and after testing and re-testing many 

 carbons made in a variety of ways, both by myself and others, it 

 occurred to me that the composition or structure of the carbon itself, 

 of which the filaments were made, might have really little to do with 

 the discrepancies and changes I had noticed. 



All the carbons I had tried gave in turn the most irregular results, 

 and although some of these were porous, and some dense and com- 

 pact, the light emitted from any one of them per unit of surface for 

 each unit of electrical energy supplied to it was very varied and 

 uncertain, and did not appear to follow any condition of the porosity 

 or denseness of the filament itself. 



All the carbons in turn gave the same light per unit of surface 



