Chap. l.~\ Why Light does not always produce Heat. 1 77 



light by the darker colours is alfo found to have an 

 extraordinary effect in the production of heats but 

 the experiments to this effect have been already re- 

 lated *. 



That light (or more properly according to this 

 theory, the matter of fire) does not always produce 

 a&ual hear, is accounted for from the minutenefs 

 of the particles, and the extreme rarity of the fluid. 

 It was before ftated that light muft be exceedingly 

 concentrated to produce ignition. A plane 1 -mirror re- 

 flects the light in too difFufed a ftate ; but a concave 

 mirror collects and converges the particles to a point, 

 and is therefore capable of producing igninon $ and 

 yet we have feen that the light of the moon will not, 

 in the moft condenfed ftate, produce the lead degree 

 of heat, though the moft delicate thermometers mould 

 be employed. The light which is emitted by putre- 

 fcent fubftances, by the glow-worm, and fome other 

 infects, is analogous to the light of the moon in this 

 refpect; it is of too faint and rare a nature to produce 

 heat or ignition. In the fame manner, when the rays 

 of light pa& through a tranfparent medium, they fuo 

 ceed each other at an immenfe diftance f. I there- 

 fore, the rays concentrated by the moft powerful burn- 

 ing glafs are made to pafs through a phial containing 

 'fpirit of wine, or through any other tranfparent in- 

 flammable fubftance, the latter will not be fet on fire j 

 but if there is any opake body (as a fpoon or other 

 veffel) .placed under the fpirit or the tranfparent body, 

 which by intercepting may ferve to accumulate the 

 particles of light, the fpirit or inflammable matter 

 will be immediately inflamed. Conformably to thefe 



* See p. 76. f That of 1,000 nuks. 



VOL. I. N prin- 



