and on the Elementary Colours of the Solar Spectrum. 275 



I conclude, that the molecules of bodies, slightly heated, 

 vibrate slowly, and produce long and invisible waves in the 

 sethereal medium which surrounds them. As the temperature 

 rises the molecular vibrations chiefly augment in extent, pre- 

 serving the same isochronism ; but some among them increase 

 in frequency also. This increase, nevertheless, does not be- 

 come veiy distinct until near tlie point of incandescence. 

 Then a portion of the ponderable particles begin to vibrate 

 more swiftly than the rest, and produce in the aether shorter 

 undulations, which are consequently more refrangible; and 

 of which some become visible ; all contributing to increase 

 the energy and variety of the radiation, until at last a 

 great number of elements of luminous and obscure heat are 

 found united in the radiant flux from sources of high tem- 

 perature*. 



There are, however, certain bodies whose state of mole- 

 cular equilibrium is such that their particles possess a great 

 facility of vibration ; these particles acquiring long before the 

 period of incandescence, altogether or partly, that rapidity of 

 oscillation from which arises visible heat. These bodies con- 

 stitute the class of phosphorescent substances. 



When one body combines chemically with another, its 

 molecules acquire in an instant a very violent vibratory 

 motion, and then may subsequently assume vibrations that are 

 slower. This is what appears to take place in flames, which 

 originate in the combustion of bodies ; they commence almost 

 always by a blue or violet light, and then become white or 

 yellow. 



But to return to the case in which light and heat are deve- 

 loped by elevation of temperature only ; we discover that 

 aethereal undulations incapable of acting on the organ of vision 

 are not alone found in radiations coming from hot and dark 

 bodies, but also in those that arise from luminous sources. 

 These invisible rays are not homogeneous : they are of dif- 

 ferent kinds; and tiieir specific properties are altogether ana- 



* These views, being the direct consequences of the unduiatory theory, 

 are not only established in the memoir of Professor Draper here reviewed 

 by M. Mclloni, but also in an earlier paper inserted in the Philosophical 

 Magazine, Feb. 1847; of which a translation is given in \.he Bibliolhcque 

 UniverscUc, June 1847, respecting which M. De la Rive remarks in an in- 

 troductory note, " 11 nous scmble, en effet, d'une importance assez grande 

 par I'etude bienfiiite et I explication remarcjuable qu'il contient des phe- 

 nomenes si singuliers de Tantagonism iles rayons de lumi^re dans les effets 

 chimiques." It also appears from that paper, tliat he has held them ever 

 smcc 1842, and used them for the purpose ofcxplaining those remarkable 

 cases of chemical interference which Sir J. Ilerschel describes, on the 

 hypothesis that the ifithereal waves cause the particles of surfaces on which 

 they impinge to execute vibratory motions. — (Note of the Translator.) 



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