820 FOURTH REPORT — 1834. 



We are still far from a precise theory of absorption. When 

 such a theory shall have been established, there seems reason to 

 believe that it vrill bring with it also an insight into the internal 

 constitution of bodies even yet more close than that afforded by 

 the affections of polarized light ; and that the laws of molecular 

 action may perhaps, at some future day, be studied in the phe- 

 nomena of transmitted light. 



The properties of solar phosphori, which attracted so much 

 of the attention of experimental philosophers of the last century, 

 seem at first view to favour the account of absorption suggested 

 by the theory of emission, and to arise from the disengagement 

 of the light which had become united to the body. Canton ob- 

 served that light may remain in these bodies, as it were in a 

 latent state, for several months, until its re-emission is deter- 

 mined by the action of heat. But it must be observed, in the 

 first place, that the feeble light emitted from the phosphori 

 bears a very small proportion to that which they are supposed 

 to receive by absorption. Dessaignes has remarked that most 

 of these substances emit the same kind of light, whatever be 

 the species of light to which they have been exposed*. The 

 same fact has been observed by M. Grotthouss f and other sub- 

 sequent inquirers ; and in some of the diamonds possessing 

 the property of phosphorescence, the most efficacious exciting 

 light is of a different colour from that excited. These facts 

 seem to be inexplicable in the theory of emission. In the wave- 

 theory, on the other hand, the phenomenon is easily compre- 

 hended. As the vibrations of the air excite those of sounding 

 bodies, and communicate to them a motion which continues for 

 some time after the exciting cause has ceased to act ; so it must 

 also be with the undulations of the ether. When the body is 

 in unison with the incident light, their vibrations will continue 

 isochronous, and the undulations of the ether excited by the 

 body vpill be of the same length as those by which it is itself 

 excited. In the other case, the period of vibration, and conse- 

 quently the length of the wave, will be altered, and the excited 

 and exciting lights will be of different colours. The fact ob- 

 served by Canton is indeed not so easily explained. Young 

 supposed that the vibrations of the body may be abruptly sus- 

 pended by cold, and may proceed anew when released from this 



* Mem. Inst. torn. xi. 



t Schweigger's Journal, 1815. — The same observer discovered the curious 

 fact, that the electric current restored the property of phosphorescence, in 

 many cases where it appeared to have been destroyed by tlie action of violent 

 heat. 



