310 Royal Society : — 



mention only one other. Amongst the chemical reactions celebrated 

 for the production of dazzling light, there are few which surpass the 

 active combustion of phosphorus in oxygen. Now phosphoric an- 

 hydride, the product of this combustion, is volatile at a red heat ; 

 and it is therefore manifestly impossible that this substance should 

 exist in the solid form at the temperature of the phosphorus-flame, 

 which far transcends the melting-point of platinum. For these 

 reasons, and for others stated in the lectures above quoted, I consider 

 that incandescent particles of carbon are not the source of light in 

 gas- and candle-flames, but that the luminosity of these flames is due 

 to radiations from dense but transparent hydrocarbon vapours. As 

 a further generalization from the experiment above mentioned, I was 

 led to the conclusion that dense gases and vapours become luminous 

 at much lower temperatures than aeriform fluids of comparatively 

 low specific gravity, and that this result is to a great extent, if not 

 altogether, independent of the nature of the gas or vapour, inas- 

 much as I found that gases of low density, which are not luminous 

 at a given temperature when burnt under common atmospheric pres- 

 sure, become so when they are simultaneously compressed. Thus 

 mixtures of hydrogen and carbonic oxide with oxygen emit but little 

 light when they are burnt or exploded in free air, but exhibit intense 

 luminosity when exploded in closed glass vessels, so as to prevent 

 their expansion at the moment of combustion. 



I have recently extended these experiments to the combustion of 

 jets of hydrogen and carbonic oxide in oxygen under a pressure 

 gradually increasing to twenty atmospheres. These experiments 

 were conducted in a strong iron vessel, furnished with a thick plate 

 of glass of sufficient size to permit of the optical examination of the 

 flame. The results are so remarkable that, although still far from 

 being complete, I venture to communicate them to the Kcyal Society 

 before the close of the Session. The appearance of a jet of hydrogen 

 burning in oxygen under the ordinary atmospheric pressure is too 

 well known to need description. On increasing the pressure to two 

 atmospheres, the previously feeble luminosity is very visibly aug- 

 mented, whilst at ten atmospheres' pressure the light emitted by 

 a jet about 1 inch long is amply sufficient to enable the observer 

 to read a newspaper at a distance of 2 feet from the flame, and this 

 without any reflecting surface behind the flame. Examined by the 

 spectroscope, the spectrum of this flame is bright and 'perfectly con- 

 tinuous from red to violet. 



With a higher initial luminosity, the flame of carbonic oxide in 

 oxygen becomes much more luminous at a pressure of ten atmo- 

 spheres than a flame of hydrogen of the same size and burning under 

 the same pressure. The spectrum of carbonic oxide burning in air 

 is well known to be continuous ; burnt in oxygen under a pressure 

 of fourteen atmospheres, the spectrum of the flame is very brilliant, 

 and perfectly continuous. 



If it be true that dense gases emit more light than rare ones 

 when ignited, the passage of the electric spark through different 

 gases ought to produce an amount of light varying with the density 



