the Theory of Luminous Flames. 107 



The fact that soot is deposited on a cold object held in a 

 luminous name is no proof of decreased temperature within 

 the flame, inasmuch as it has been shown that decreased tem- 

 perature causes a diminution in the quantity, or even total 

 suppression, of the separated carbon. 



Soot is also deposited on heated surfaces, but is quickly 

 burned on admission of air. Inasmuch as admission of air 

 cannot be altogether prevented, less soot is accumulated on 

 hot than on cold objects. The surface which collects the soot 

 may be compared to a redoubt which stops the progress of 

 the shots fired against it. 



Burners constructed of iron or other material possessed of 

 high conductivity for heat, cause a greater diminution in the 

 luminosity of the lower part of the flame than burners of stea- 

 tite. The difference between the luminosities of the two flames 

 is rendered more apparent by artificially cooling the burner. 



The top of the burner and the entering cold gas both exert 

 a cooling action upon the lower part of the flame, and are the 

 cause of the space noticeable between the burner and the 

 flame-mantle. By heating the burner, and simultaneously 

 the stream of gas, a more luminous flame is obtained without 

 an increased consumption of gas, the increase in luminosity 

 being greater the smaller is the consumption of gas. 



The change which is hereby caused in the chemical com- 

 position of the gas is without appreciable effect upon the 

 luminosity, inasmuch as no diminution in luminosity is 

 occasioned by again cooling the stream of gas. 



The heat communicated by increasing the temperature of 

 the burner-tube acts in two ways : it increases the intensity 

 of light of the flame-mantle, and simultaneously enlarges the 

 flame itself. Carbon is also sooner separated in the flame, 

 and separated at a higher temperature. 



The eye is able to distinguish between these two actions ; 

 the photometer registers only the total effect. 



It is very essential to distinguish between the " light-effect" 

 of the whole flame, and the " intensity of light " (that is, the 

 quantity of light emitted by the various constituent parts of 

 the flame). For practical purposes (comparisons of different 

 flames, &c.) it would be well to determine the total light-effect, 

 and also the maximum quantity of light obtained by allowing 

 the rays to pass through a small, accurately measured, open- 

 ing in a shade placed between the flame and the diaphragm of 

 the photometer. The numbers so obtained might be regarded 

 as approximative values of the relative " intensities of light " 

 of the most brilliant parts of the various flames. 

 [To be continued.] 



