B84 Pi-of. Draper on the Ckeniictil Action 0/ LigJit. 



silver, by exposing it in an atmosphere of hydrogen. These 

 were all failnres, for the reason that is now obvious. 



The observations here made apply only to the case in which 

 the substance mixed lea\'es the original body unaffected. Thus, 

 if a solution of oxalic acid be mixed with peroxalate of iron, it 

 unites at once therewith, an entirely new body of a different 

 colour and of a different absorptive power arises, and the expe- 

 riments are no lon£;er comparable.- ' ^ "' '^"'•'" 



— !"ii - <i ' b nf^ 



I now pass to the second point of inqiiii^y . 



2. To what extent, and in what manner, is the decomposition of 

 a compound body affected hy the frequency of vibration of a ray ? 



From the beginning of optical chemistry, investigations have 

 been made for the purpose of determining the action of rays of 

 different refrangibilities. Wollaston, Ritter and Scheele, recog- 

 nised the action of the more refrangible rays, even those which 

 are extra-spectral. To these the designation of deoxidizing rays 

 was given. ;\^lmost a hundred years ago it had been shown, in 

 special cases, that there is an antagonism between the opposite 

 ends of the spectrum ; thus the phosphorescence excited in Can- 

 ton's preparation by the violet end of the spectrum is extin- 

 guished by the red. As respects coloured compounds, Grotthuss 

 showed that the active ray is very commonly of the complemen- 

 tary tint. ,^, J 



This branch of the subject has more recently teen examined 

 to a great extent, and the behaviour of all kinds of substances 

 in the solar spectrum made kno^^^l. From these inquiries it 

 would appear, that in general the rays of higher rcfrangibility, 

 and therefore of more frequent vibration, betray the greatest 

 activity ; though the cases are quite coram9n in which all th^e 

 other rays exhibit an effect. 



We have already seen that amplitude of vibration has nothing 

 to do with the chemical effect of a ray, beyond determining the 

 rapidity of its action ; but it is very different with frequency. 

 On this, the number of impulses it can communicate in a given 

 period of time, depends its power of destroying the constitution 

 of any group of atoms. We might readily suppose that a com- 

 pound which could resist a slow recurring motion may be unable 

 to maintain itself when the impulses increase in frequency ; and 

 in this we ])robably have a general explanation of the greater 

 efficiency of the more refrangible rays ; as it is said that there 

 have been singers who, by gradually raising theii* voice from a 

 low note, could cause vessels of glass to fly to pieces. 



If the decomposition of bodies arises from their molecules 

 being throAvn into \'ibration by the impulses of the ray, so that 



