Light on the Polarized Electrode. 431 



where tlae cliemical changes producing or produced by voltaic 

 currents are always observable. 



With more sensitive galvanometers^ and with a greater variety 

 of solutions^ this class of experiments may, I venture to hope, 

 be found important in further investigating the effects of light 

 on chemical actions ; and the pure coloured rays of the spectrum 

 may be employed. 



There can, I think, from analogy, be little doubt that light 

 would influence those actions of surface which are comprehended 

 among the various effects to which the term catalysis is applied. 

 In an experiment I made in the month of September 1851, two 

 similar glass tubes, containing each 15 grains of water, were 

 placed, the one under an opake porcelain, and the other under a 

 glass vessel of the same size, with capsules of sulphuric acid by 

 their sides : I found that evaporation took place much more 

 rapidly in the one exposed to light, though it was in a room 

 with a northern aspect, on which the sun never shone. In twelve 

 days the water under the glass vessel had lost 6"3 grains, that 

 under the porcelain 5 '4, showing a difference of nearly a sixth 

 pai't in the evaporation in favour of the tube exposed to light. 

 I mention this experiment here as showing a probability that 

 the liberation of vapour or gas may be accelerated by light, as 

 M. Donny's remarkable experiments seem to show that evapo- 

 ration is a surface action ; and the effect of light on polarized 

 plates may be somewhat of the same nature. I will not, how- 

 ever, enter into theories, which must be necessarily vague on 

 such a novel subject, but conclude by giving a Table of some 

 of the most trustworthy results which I have obtained when the 

 sunlight has been most steady, in order to show the extent of de- 

 flections and differences with coloured light : they were all made 

 between the hours of 10 a.m. and 12, and on the finest days I 

 could select. My first experiments were made in London, in the 

 months of June and July. I was then absent for some time on 

 circuit ; and I have resumed and continued them during my vaca- 

 tion in the months of i^ugust and September. 



June 21, apparatus having been arranged on June 12. Solution, 

 water with a few drops of Sulphuric Acid. 



Deflection by sunlight 8°, or, from 1 2°, the deflection by polar- 

 ization, to 20°. 



Exposed platinum positive. 



July 4. Same solution, platinum having been changed on 

 the 24th. 



Deflection by sunlight of 9°, or from 1° to 10°. 

 Exposed platinum positive. 



