•18!§&] Rev. Mr. Emmett on Combustion, 4^3 



appears, that when a solid which evolves much light during 

 combustion is volatilized, the vapour (if it may be so called) 

 evolves light, most probably in proportion to its quantity. 

 When this is diminished, the quantity of hght is reduced, until 

 arriving at a state of purity in oxygen and hydrogen gases, the 

 light and heat approach so nearly to the radiant state (owing to 

 the want of a sufficient quantity of matter in the flame) that 

 Tery little light is evolved. Hence we may infer, that the quan- 

 tity of the light depends upon the quantity of solid matter in 

 the flame ; for if there be no soHd matter, heat sufficient to melt 

 platina, and to produce most vivid light, may pass through a 

 transparent medium without producing any sensible effect ; yet 

 if solid matter be introduced, it will be most vividly ignited. 

 Hence a platina wire may slowly consume a mixture of oxygen 

 and inflammable gas. The combustion is too slow to produce 

 flame, yet it is sufficient to ignite the wire : this is analogous to 

 an experiment of Wedgwood, and also to the ignition of spongy 

 platina by a stream of hydrogen, which seems to be owing to 

 the large surface of action and the great density of the metal. 

 Although the quantity of light emitted by gases of the same 

 nature be nearly in proportion to the quantity of oxygen con- 

 sumed, the law does not obtain with regard to different inflam- 

 mables ; for hydrogen, which consumes the greatest quantity, 

 evolves the least light : the light is in proportion to the quantity 

 of oxygen consumed, the intensity of the heat, the quantity of 

 unconsumed solid raided in vapour, its power of emitting light 

 during combustion directly, and the quantity which escapes 

 combustion in the flame, together with its power of obstructing 

 light, inversely. 



From the phenomena of combustion is derived a powerful 

 argument in favour of the materiality of heat or caloric. The 

 smallest spark will set fire to phosphorus, and some other sub- 

 stances : from this combustion other combustibles may be 

 inflamed without limit. If then heat be only a vibratory motion 

 excited among the particles, since the cause is equal to the 

 effect ; or since the quantity of motion in the exciting body is 

 equal to the whole elapsed ; the quantity of motion in the 

 smallest spark is equal to that in the greatest conflagration. 

 Therefore a body of given magnitude can communicate its own 

 force, quantity of force and velocity, to any assignable quantity 

 of matter, which is absurd. 



New Series, vol. xii. 2 f 



