the Theory of Luminous Flames. 879 



have long known, however, that the oxide does not exist in 

 the solid form in the flame. 



I have noticed a faint shadow cast by the flame of phos- 

 phorus burning in air ; but as this shadow disappears on 

 substituting oxygen for air, I conclude that in the former 

 combustion the temperature is scarcely high enough to ensure 

 the complete conversion of solid into gaseous matter. 



Him farther says that the flame of a petroleum-lamp throws 

 no shadow on a screen when viewed in sunlight, but that 

 when the flame is rendered smoky the smoke causes a most 

 marked shadow. From the detailed description of Hirn's 

 experiment, it is evident to me that he placed the screen at 

 too great a distance from the flame. He also obtained no 

 distinct shadow when the sunlight was sent, by means of a 

 looking-glass, through a series of eight petroleum-flames. 

 On account of the interference between the flames, and also 

 on account of their distance from the screen, this result is not 

 to be wondered at*. 



The petroleum-flame employed by Hirn was surrounded by 

 a glass cylinder, which seriously interfered with the distinct- 

 ness of the shadow produced. 



It is not possible, by removing the screen further and 

 further from the flame, to obtain a clearly defined shadow 

 much exceeding in size the dimensions of the flame itself. If, 

 however, the sunlight be collected by a convex lens, and be 

 thrown on the flame, a large and well-defined shadow may be 

 obtained ; the eye is also less fatigued by the glare of the 

 white screen. 



Fig. 7 represents the shadow thrown by a non-smoky coal- 

 gas flame, burning at the orifice of a tube 7 millims. in width : 

 fig. 8 represents the appearance on the screen after removing 

 the luminosity of this flame by admission of carbon dioxide. 

 If the flame was rendered wholly or partially non-luminous 

 by the introduction of a piece of metal, the shadow wholly or 

 partially disappeared : emission of light and production of 

 shadow are therefore closely related to one another. 



The flat flame of a very small bat's-wing burner produces 

 no shadow ; but if the flame be turned edgeways towards the 

 screen, a small but distinctly perceptible shadow is produced. 

 In the first instance the layer of luminous flame is too thin to 

 cause the production of a shadow ; if an ordinary-sized burner 



* Hirn remarks that the smoke arising from a petroleum-flame ap- 

 peared white when illuminated in a dark room. This is, I think, due to 

 reflection of light from the smoke, chiefly from the particles of water, 

 tarry matter, &c, the black smoke being itself nearly invisible against 

 the dark backo-round. 



