This conclusion is much strengthened by the fact, that the _existence of caloric_ is well ascertained, _not as proceeding from the sun_, but in combination with all terrestrial substances; and also by the fact of the constant _analogy_ between the phenomena of light and heat. This analogy is so strong and striking that we are compelled to conclude, _if heat be the effect of a real substance, light must be also_. Indeed the analogy is so strong that it almost convinces us of the _identity_ of the matter of heat, light, electricity, and galvanism. Notwithstanding the amount of evidence is against this supposition at present, yet there is a strong tendency in recent philosophical experiments to confirm it; and I am inclined to believe that future discoveries will confirm this identity. Some of the most obvious evidences in favor of it may be introduced here. 1. Almost all the celebrated authors and experimenters have occasionally _suggested_ the probability of this identity. Mr. Turner, Elements of Chemistry, p. 67, says, in reference to heat and light: “It has been supposed that _they are modifications of the_ SAME AGENT; and though most persons regard them as independent principles, yet they are certainly allied in a way which at present is inexplicable.” Again, p. 71. “Mr. Leslie conceives that light when absorbed, _is converted into heat_.” Dr. Henry (Art. Light,) says, “A new fact has been lately ascertained by Dr. Delaroche, which seems to point out _a close connection_ between heat and light, _and a gradual passage of the one into the other_. The rays of _invisible_ heat pass through glass with difficulty at a temperature below that of boiling water; but they traverse it with a facility always increasing with the temperature, as it approaches the point at which bodies become luminous.” “The general facts, says Sir H. Davy, of the refraction and effects of the solar beam, offer an analogy to the agencies of electricity.” (_Ure, Chemical Dictionary, Article Light._) It is well known that this view pressed itself strongly on the attention of Sir Isaac Newton, during his philosophical investigation. _See Ure, Chem. Dic. Art. Light._ 2. This identity is strongly suggested by the constant and striking analogy between the laws of heat and light. _First:_ The color of surfaces has an influence on the passage of light and heat. _Secondly:_ The power of light, heat, and electricity diminishes as the squares of their distances. _Thirdly:_ The particles of heat, light, and electricity, are _idio-repulsive_. _Fourthly:_ The passage of the electric spark is generally attended with the production of light and heat. _Fifthly:_ Heat is emitted in all directions from the surface of an ignited body: so is light from the surface of a burning body.