710 Mr. F. B. Pidduck on the Absorption of 



We further may note that the steps can be reversed ; so 

 that, if we assume the experimental fact of the dependence 

 of induction currents on relative motion only, we may 

 deduce the equation 



curl E = T - 



dt 



from the expression for the electromagnetic force, or, in other 



words, from the law \ for the magnetic force due to 



an element of a current together with the law of action and 

 reaction. 



In other words, again, the two circuital relations 

 curl H = 47r C, 



curlE a=— -j- 

 at 



are deducible from one another on the electron theory solely 

 by the help of the experimental facts of relative motion with 

 respect to induction currents. 



LXV. Note on the Absorption of Ultra- Violet Light by Dilute 

 Solutions. By F. B. Pidduck, B.A., Fellow of Queen s 

 College, Oxford *. 



1. r|\BE experiments here described originated in the 

 JL observation of a great decrease of the electrical 

 action of ultra-violet light caused by transmission through 

 ordinary clear tap-water as compared with the effect after 

 passage through the same thickness of distilled water. In the 

 attempt to elucidate the effect, a few very dilute solutions of 

 known strength were made up and the relation of absorption 

 to concentration and thickness traversed, as well as the result 

 of mixture, was investigated. 



The electrical connexions were made in the usual way. 

 The ultra-violet light was produced by a spark between zinc, 

 electrodes in a Leyden-jar discharge circuit. The light 

 passed through a wire grating which formed the positive 

 plate of a parallel-plate condenser, the negative plate being 

 of zinc and being attached to the insulated pair of quadrants 

 of a Dolezalek electrometer. A force large enough to 

 produce saturation was applied from a battery of lead cells, 

 the negative pole of which, as well as the other pair of 

 quadrants, was earthed. It was found that the electrometer 

 readings were steadier when the zinc plate was allowed to 

 remain some little time after cleaning, although the deflexions 

 were thereby reduced. 



* Communicated by Prof. J. S. Townsend, F.R.S. 



