On Light and Heat. M 



tUl the thermometer corrects our judgment. Thus 

 the climate of Philadelphia, to a Greenlander, feels 

 hot ; to a West- Indian, cold. The heat of our land 

 air, in summer, would be intolerable to fishes and 

 aquatic animals. Nor can the rein-deer, which de- 

 lights in the snows of Lapland, subsist in the tempe- 

 rate climate of Great-Britain. 



Combustion consists in the chemical union of air 

 with the inflammable principles of bodies, and flame 

 is only ignited vapour. As soon as the air is ex- 

 cluded, or its oxygen consumed, the combustion im- 

 mediately ceases. It is not a litde singular, that a 

 red-hot iron inflames gunpoM^der, but is quenched in 

 ardent spirit ; on the other hand, that flame kindles 

 ardent spirit, but not gunpowder. When any liquor 

 is brought to its boiling point, it can receive no ad- 

 ditional heat, in an open vessel, because it then be- 

 gins to fly ofi" in steam, which absorbs and carries 

 with it a great quantity of heat, in a latent state : 

 hence the steam is never hotter than the boiling li- 

 quor from which it exhales. Yet water may be ren- 

 dered red-hot, in a close vessel, well secured, as has 

 been performed in Papin's digester. 



Cold is not a principle in bodies, like heat ; it is 

 only a negative quality, denoting the absence of a pro- 

 portionate quantity of heat : yet the sudden reduc- 

 tion of temperature, from hot to cold, sensibly aflccts 

 the human frame, and its effects on all other bodies 

 are, of course, directly opposite to those of heat. 

 Thus melting snow, by absorbing a large portion 



