660 SCIENCE PROGRESS 



Draper (loc. cit.) also established the following facts concern- 

 ing the " normal period " in the case of mixtures of constant 

 composition : 



(i) When the radiant source is constant, the amount of 

 movement of the tithonometer is directly proportional to the 

 times of exposure. 



(2) The indications of the tithonometer are proportional to 

 the quantity of incident rays. 



In establishing the second of the above statements, it may 

 be mentioned that the same source of light was used, definite 

 fractions of the incident rays being cut out by obscuring sectors 

 of a lens through which the light passed. 



Both of the laws implied in these statements were after- 

 wards submitted to a thorough investigation by Bunsen and 

 Roscoe and established beyond doubt by accurate measure- 

 ments. 



The two laws amount to the statement that for a mixture 

 of the same composition and for light of the same quality the 

 chemical change is proportional to the energy absorbed. 



Several investigations have been undertaken with the object 

 of discovering a relation between the concentrations of the 

 interacting gases and the velocity of change. 1 The results 

 obtained in these researches, in the opinion of the writer, how- 

 ever, are vitiated by two circumstances : firstly, by the presence 

 in the gases of impurities the effect of which at different stages 

 of the interaction could not at the time be estimated ; secondly, 

 by the neglect to make an allowance for the decreasing amount 

 of energy absorbed by the system as the chlorine diminished in 

 concentration. 



Several attempts have been made to correlate the velocity of 

 chemical change with the refrangibility of the stimulating light. 

 Seebeck in 18 10 (Eder's Geschichte der Photochemie unci Photo- 

 graphic, 1 89 1, i. 93), by operating with coloured glasses, showed 

 that the action was limited to the blue end of the spectrum. 

 Berard {Ann. Chim. 1813, 85, 309) believed that the maximum 

 of action was in the violet. Draper {Phil. Mag. 1844, 25, 5) 

 experimented with the solar spectrum and states that in reality the 

 indigo is the active ray and that the effect gradually diminishes 

 towards each end of the spectrum from a maximum point which 



1 Of these researches that of M. Wildermann {Phil. Trans. 1902, 199, 337) 

 is the most extensive. 



