MEMOIR XVII.] CHEMICAL BAYS AND RADIANT HEAT. 241 



tions ensue, by modifying the process slightly, and how 

 even on a sheet which is apparently washed uniformly 

 over, large blotches appear, which are either inordinately 

 sensitive or not sensitive at all. If, without altering 

 the chemical composition of a film on metallic silver, or 

 even its mode of aggregation, such striking changes re- 

 sult by difference of thickness, how much more may we 

 expect that the great changes in molecular condition, 

 which apparently trivial causes must bring about on sen- 

 sitive paper, should elevate or depress its capability of 

 being acted on by light. If I mistake not, it is upon these 

 principles that an explanation is to be given of the suc- 

 cessful modes of preparation which Talbot and Hunt have 

 described, and the action of the mordants of Herschel. 



I therefore infer 



6th. That the SENSITIVENESS of any given preparation 

 depends on its chemical nature and its optical qualities 

 conjointly, and that it is possible to exalt or diminish the 

 sensitiveness of a given compound by changing its optical 

 relations. 



Vth. That, as when radiant heat falls on the surface of 

 an opaque body, the number of rays reflected is the comple- 

 ment of those that are absorbed, so in the case of a sensitive 

 preparation, the number of chemical rays reflected from 

 the surface is the complement of those that are absorbed. 



This important proposition I prove 

 in the following way: I take a plate, 

 A Gr, Fig. 39, three inches by four, and 

 by partially screening its surface while 

 in the act of iodizing with a piece of 

 flat glass, I produce upon it five trans- 

 verse bands, b, c, d, e, f ; the fifth,/, 

 which has been longest exposed, is of 

 a pale lavender color, the fourth 

 bright blue, the third a red, the sec- tfig.39. 



Q 



