PHYSICS OF NINETEENTH CENT.-RADIATION. 513 



gradations of intensity in each class of rays throughout the spectrum. 

 It must not, however, be supposed that in the spectrum three physically 

 different spectra are superimposed. Modern theory regards light and 

 heat as physically identical, and if that theory be accepted, the ultra- 

 violet, chemical, actinic, or photographic rays must also be regarded 

 as physically continuous with the rest. They differ only as regards 

 rapidity of vibration. But while vibration-rates between certain limits 

 are capable of so acting on our organs as to produce visual impressions, 

 with colours depending on. the rate of vibration, so higher or lower 



FIG. 233. 



rates beyond these limits are incapable of giving visual impressions, 

 while they are nevertheless competent to produce thermic and 

 chemical effects. 



The history of the important researches which have been made upon 

 the chemical actions of light would require more detailed treatment 

 of the subject than is here deemed suitable. This subject derives 

 additional interest from its connection with the beautiful art of pho- 

 tography ; but in spite of the perfection to which that art has been 

 brought, and the existence of a large store of curious facts relating to 

 the chemical actions of light, no comprehensive theory has yet been 

 brought forward to satisfactorily explain the phenomena. 



There is, however, a chemical action due to the solar radiations, which 

 is every day taking place on the grand scale, and exercises the im- 

 portant function of restoring to the atmosphere the oxygen which is 

 constantly removed, combined with carbon, by the respiration of 

 animals, and other natural processes. Bonnet of Geneva, about the 

 middle of the eighteenth century, observed that leaves plunged in 

 water and exposed to sunlight gave off a gas from their surfaces. 

 In 1773 Priestley showed that the gas was vital air, or oxygen, and 

 that plants by this action prevented the vitiation, which would ulti- 

 mately result from the respiration of animals. Sennebier inferred 

 that the leaves of plants decompose carbonic acid under the influence 



33 



