4 o6 EXPERIMENTAL INVESTIGATIONS. 



breathing on them, and gave a strong impression of the glass 

 having suffered a superficial disintegration or decomposition, 

 but I could not by the microscope or by polarised light 

 detect a difference in its structure. I hope, however, to do 

 so by a more prolonged and varied examination. 



It may be well to state that counter-experiments were 

 made, such as allowing the plates to remain with the letters 

 between them, but without electrisation, in which case no 

 effect was produced ; but it is probable, by analogy with 

 Moser's class of phenomena, that a very long period of con- 

 tact would produce some effect. 



ON THE INFLUENCE OF LIGHT ON THE POLARISED 

 ELECTRODE.* 



Phil. Mag., December 1858. 



SOON after the experiments of Daguerre were published it 

 occurred to me that the galvanometer might be used as a test 

 for the chemical effects of light ; and I succeeded in obtaining 

 a deflection of the needle by allowing a beam of light sud- 

 denly to impinge on a daguerreotype plate in a trough of 

 water the plate being connected with one extremity of a 

 galvanometer, and a gridiron of silver wire placed in front of 

 the plate with the other. This experiment I showed at a 

 lecture at the London Institution in 1843 J an ^ it was subse- 

 quently used as an illustration of the convertibility of force, in 

 my essay on the ' Correlation of Physical Forces/ 



I tried some farther experiments at the time, without ob- 

 taining results of any importance, but, as galvanometers at 

 that period had not reached the degree of delicacy they have 

 since attained in the hands of M. Ruhmkorff, I determined 

 this summer to resume the enquiry ; and the results which I 

 have obtained I now proceed to describe. The galvanometer 

 used in the following experiments is by Ruhmkorff, formed 

 of 544 feet of fine copper wire, and though not as delicate as 



* Read at the meeting of the British Association, Leeds, Sept. 1858. 



