light, and of light unaccompanied by any sensible degree of heat, on the one 

 hand, and of an extensive and complicated group of properties, in which light 

 and heat agree in their physical characters, on the other, have given rise to two 

 distinct hypotheses respecting the nature of these principles. By the one they 

 are regarded as distinct physical agents, which enjoy some common properties, 

 while on the other they are assumed to be the same principle manifesting itself 

 in different ways, according to the property which, under different circumstan- 

 ces, acts with the greatest degree of energy. Our object at present shall 

 be confined to the statement of the principal effects upon which one or the 

 other theory must be founded, and which any theory must explain before its 

 validity can be admitted. 



If heat be communicated to solid bodies which are difficult of fusion, it is 

 observed that after having absorbed a certain quantity, they begin to become 

 luminous. If the process be conducted in a dark chamber, the body will 

 gradually begin to be visible by emitting a dull red light. This luminous 

 quality gradually increases as the body absorbs heat, and at length it emits 

 sufficient light to render the surrounding objects visible ; and the color of the 

 light changes from an obscure, dusky red, gradually to the color of bright red. 

 The body is then said, in common language, to be red-hot. If the communica- 

 tion of heat be still continued, the color of the light will change to an orange, 

 and subsequently will become yellow. If the application of heat be still fur- 

 ther continued, it will at length emit a clear white light, the color of sunlight ; 

 the body is then said to be white-hot. 



The state in which a heated body naturally incapable cf emitting light be- 

 comes luminous, is called a state of incandescence. The term ignition is some- 

 times applied to this state ; but the former term is preferable, since ignition is 

 sometimes used to express the commencement of inflammation or combustion, 

 which is a process of a totally different nature. 



The temperature at which a body becomes incandescent is extremely diffi- 

 cult to be ascertained with exactness, being beyond the reach of the mercurial 

 thermometer. The uncertainty of the indications of pyrometers, and other 

 means by which fierce temperatures are measured, has been before noticed. 

 There are, however, some circumstances which render it probable that bodies 

 in general, which have been rendered incandescent by increase of temperature, 

 have attained that state at nearly the same temperature. Mr. Wedgwood placed 

 some gilding on a piece of porcelain, and exposed both to the heat of an in- 

 tense furnace, until the porcelain became red-hot : no difference could be ob- 

 served in the time of the porcelain and the gilding upon it becoming luminous, 

 yet these substances are of so very different a nature that it might be expected 

 that a difference in their incandescence would be observable. 



The point of fusion seems to have no relation whatever to the point of in- 

 candescence. While yet solid, some bodies attain a clear white heat without 

 fusion. Others again, such as silver and lead, fuse before they become lumin- 

 ous. If the boiling point of a body be below its point of incandescence, it can- 

 not attain the latter state unless its vaporization be resisted by pressure. It is 

 supposed that liquids submitted to a pressure which will resist their vaporiza- 

 tion, are capable of attaining a state of inc idescence. Thus, in some experi- 

 ments of Perkins, water is said to have been rendered red hot without being 

 permitted to expand into vap,>r. 



The determination of the temperature at which bodies become incandescent 

 has occupied the attention of several distinguished philosophers. Newton 

 fixed it at the temperature of 635 ; but there is no doubt that this is consider- 

 ably below the true temperature. Newton possessed very imperfect means 

 of determining the temperature, and measured it by observing the rate at which 



