the Theory of Luminous Flames. 375 



burning hydrogen which has been coloured yellow by the 

 presence of sodium chloride, is partly absorbed thereby, 

 and that the light which passes on shows a dark absorption- 

 band. So also sunlight is more or less absorbed when passed 

 through various glowing gases. As sunlight already shows 

 dark lines corresponding with light spectral lines of sodium, 

 barium, calcium, copper, &c, it is not to be expected that 

 flames coloured by these metals in the gaseous state should 

 cause absorption of any part of the sun's rays. But it is 

 otherwise with colourless luminous flames. Such flames ab- 

 sorb portions of the rays of almost every part of the solar 

 spectrum. These flames must therefore cause a shadow-like 

 appearance on the screen, unless the absorption be too trifling 

 to allow of our eye detecting the relatively dark spaces. 

 That the eye is unable to detect any dark spaces, I have proved 

 experimentally. I have not succeeded in obtaining any flame 

 owing its luminosity only to strongly heated gases which is 

 capable of throwing an appreciable shadow on a white screen 

 when viewed in sunlight. That the flames of carbon monoxide, 

 sulphur, selenium, sulphuretted hydrogen, and carbon disul- 

 phide should cause no shadow (although affording continuous 

 spectra) may perhaps be chiefly due to the small quantity of 

 light emitted by them. But it was also found that the exceed- 

 ingly luminous flames of arsenic, phosphorus, and phosphu- 

 retted hydrogen burning in oxygen, as also of oxygen and 

 nitric oxide in carbon disulphide, produced no appearance of a 

 shadow on the white screen. The absorption caused by these 

 flames upon the sunlight was therefore too small to admit of 

 detection by the unaided eye. On account of their volatility, 

 arsenious and phosphoric oxides must be present in the gaseous 

 condition in the flames produced by burning arsenic and phos- 

 phorus in oxygen. These flames are therefore transparent : 

 it is only at some distance above the flames that the products 

 of combustion assume the solid form ; the white smoke so 

 produced casts a deep shadow on the screen. 



From an extended series of observations, we conclude that 

 luminous flames consisting only of gases and vapours are in- 

 capable of producing an appreciable shadow when viewed in 

 sunlight ; lighter and darker streaks, due to the varying den- 

 sities of the vapours, of course appear in the images of these 

 flames thrown on the screen. 



The appearance of a shadoiv is therefore proof of the presence 

 of suspended solid matter in the flame ccMsing the shadow. 



It follows from this that the luminous flames of hydrocar- 

 bons contain solid matter, inasmuch as they produce very sen- 

 sible shadows. It is evident that this solid matter can be 



