340 M. A. J. Angstrom's Optical Researches. 



possibility of exciting in two bodies coincident molecular vibra- 

 tions, where under ordinary circumstances such do not exist. 



Tbe chemical activity belongs, nevertheless, to a certain class 

 of rays. Becquerel states, " que chaque substance sensible voie le 

 rayonnement a sa maniere," and this is capable of very extensive 

 application. Thus hydrogen, in contact with spongy platinum, 

 is thrown into a condition in which it can enter into combination 

 with oxygen ; and the blue colour of sunlight excites in chlo- 

 rine the power of entering into union with hydrogen. These 

 blue rays act also most powerfully on the solid constituents of 

 plants, while the yellow rays have most to do in producing 

 the green colour, and so on. Hence there are no rays in the 

 solar spectrum which do not possess a chemical power, although 

 the violet end of the spectrum, on account of the part acted by 

 oxygen in most chemical changes, shows itself here most influ- 

 ential. Further, the combustion of most bodies and their com- 

 bination with oxygen produces a blue flame; zinc has a great 

 affinity for Oxygen, and nitrogen a feeble one ; and all this seems 

 to speak in favour of the above coincidence in the motions of 

 vibration, which I have stated to be a condition of chemical 

 combination, and proves at the same time that the oscillations of 

 oxygen belong more particularly to the blue and violet portions 

 of the spectrum. 



That isomeric bodies possess different chemical properties 

 seems also natural, as a change of position of the molecules must 

 of necessity be accompanied by other vibrations. 



21. It is more difficult to explain how, in some cases, a mo- 

 derate heating calls chemical action into play ; for the electric 

 relations of the medium cannot thereby have suffered changes so 

 considerable as to produce new series of oscillation ; or, in other 

 words, how it is that one and the same kind of oscillation, simply 

 by increasing its intensity, is capable of producing quite differ- 

 ent effects. This phenomenon seems to be explicable in the 

 following manner : 



Let £, rj, f be the coordinates of a molecule with reference to 

 its position of equilibrium, and let them be supposed to be so 

 small, that, in the differential equations for the motion, their 

 higher powers may be neglected. For a certain kind of motion 



n = — -, we may then set 



g=a . cos (nt + \) 



rj = b . cos (nt + X) 



f — c . cos {nt + X) . 



Let the amplitudes be now supposed to increase, so that 

 finally £ 2 , if-, ^ become sensible ; in this case I have found that 



