THEORY OF LIGHT AND COLOURS. 175 



fufpencled by the intervention of cold, whether as contracting 

 the bulk of the fubftance or otherwife ; and again, after the 

 reftraint is removed, to proceed in their motion, as a fpring 

 would do which had been held faft for a time in an interme- 

 diate ftage of its vibration ; nor is it impoilible that heat itfelf 

 may, in fome circumftances, become in a fimilar manner la- 

 tent. (Nicholfon's Journal, Vol. II. p. 399.) But the af- 

 fections of heat may perhaps hereafter be rendered more in- 

 telligible to us ; at prefent, itfeems highly probable that light 

 differs t from heat only in the frequency of its undulations or Light and heat 

 vibrations; thofe undulations which are within certain limits, fj ! ^^y £ C e 

 with refpect to frequency, being capable of affecting the optic undulation or 

 nerve, and constituting light ; and thofe which are flower, Vlbrat,0n > 

 and probably ftronger, conflicting heat only ; that light and 

 heat occur to us, each in two predicaments, the vibratory or 

 permanent, and the undulatory or tranfient ftate j vibratory 

 light being the minute motion of ignited bodies, or of folar 

 phofphori, and undulatory or radiant light the motion of the 

 ethereal medium excited by thefe vibrations ; vibratory heat 

 being a motion to which all material fubftances are liable, and 

 which is more or lefs permanent ; and undulatory heat that 

 motion of the fame ethereal medium, which has been mown capable in both 

 by Mr. King, (Morfels of Criticifm, 1786, p. 99,) and M. ££* jJjjJS^. 

 Picket, {Ejfais de Plu/fique, 1790, alfo in Saulfure's Voyage tions. 

 dans les Alpes, 1786.) to be as capable of reflection as light, 

 and by Dr. Herfchel to be capable of feparate refraction. 

 (Phil. Tranf. for 1800, p. 284,) How much more readily 

 heat is communicated by the free accefs of colder fubflances, 

 than either by radiation or by tranfmiflion through a quiefcent 

 medium, has been mown by the valuable experiments of 

 Count Rumford. It is eafy to conceive that fome fubflances, 

 permeable to light, may be unfit for the tranfmiflion of heat, 

 in the fame manner as particular fubflances may tranfmit fome 

 kinds of light, while they are opaque with refpect to others. 



On the whole it appears, that the few optical phenomena This theory ex- 



which admit of explanation by the corpufcular fyftem, are P lains , ail that 1S 



.,,., i • f t • i i° !vec| b y the 



equally confident with this theory ; that many others, which corpufcular fyf- 



have long been known, but never underflood, become by tem » and much 



thefe means perfectly intelligible ; and that feveral new facts 



are found to be thus only reducible to a perfect analogy with 



other facts, and to the fimple principles of the undulatory 



fyftem. 



