INFLUENCE OF LIGHT ON ELECTRODES. 411 



In all the experiments I have made on the subject of this 

 paper the most marked effect upon the galvanometer is pro- 

 duced when the polarisation causes a small permanent deflec- 

 tion of from 5 to 10 ; when the polarisation of the plates is 

 extremely slight, the effect of light is very feeble ; and when 

 the polarisation is considerable, so as to deflect the galvano- 

 meter to 20 or 30, the increased force required to produce a 

 small increase of deflection is too great to afford notable 

 results. 



I have used the term polarisation, having no better word 

 to indicate the feeble currents which are always observed 

 when two platinum plates immersed in a liquid are connected 

 with a delicate galvanometer. The electrical currents which 

 would ensue if the plates were polarised by connecting them 

 with a voltaic battery and then detaching them, would be far 

 too powerful for the delicate indications which I have been 

 examining. There can be no doubt, at least to those who 

 adopt the chemical theory of the voltaic pile, that both these 

 classes of polarisation are due, when one homogeneous liquid 

 is employed, to slight deposits on the plates, either of films of 

 gas or of some substance which acts chemically on the liquids, 

 and the effect of light would seem simply to be an augmen- 

 tation of the chemical action taking place at the surface of the 

 electrodes, which is the locus where the chemical changes pro- 

 ducing 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 farther 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 com- 

 prehended among the various effects to which the term cata- 

 lysis 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 opaque porcelain, 

 and the other under a glass vessel of the same size, with cap- 

 sules of sulphuric acid by their sides ; I found that evaporation 

 took place much more rapidly in the one exposed to light, 



