356 Mr. G. M. Whipple on the Determination of the [May 2, 



thus electrical variations may be produced, resembling the effects 

 observed during the action of light on the eye. 



Certain modifications taking place in the chlorous acid by exposure 

 to light increase its sensibility, and as a general result it is found that 

 the fluid through these alterations increases in resistance. We have 

 thus an anomalous kind of battery where the available electromotive 

 force increases with the resistance. The addition of neutral substances 

 which increase the resistance without producing new decompositions 

 improves the action of the cell. 



Care has to be taken in these experiments to use the same apparatus 

 in a series of comparative experiments, as infinitesimal differences in 

 the contact of the active pole render it difficult to make two instru- 

 ments giving exactly the same results. Cells have been constructed 

 with two, three, and four poles, and their individual and combined 

 action examined. Quartz surfaces have also been employed instead 

 of glass, thus enabling the chemical opacity of different substances to 

 be determined. 



The electrical currents derived through the action of light on definite 

 salts are strong in the case of ferro- and ferri-cyanide of potassium, 

 but remarkably so in the case of nitroprusside of sodium. 



Of organic acids the tartrate of uranium is one of the most active. 

 A mixture of selenious acid and sulphurous acid in presence of 

 hydrochloric acid yields strong currents when subjected to light in 

 the form of cell described. The list of substances that may be proved 

 to undergo chemical decomposition by the action of light is very ex- 

 tensive ; full details will be found in the completed paper. 



IV. " On the Determination of the Scale Value of a Thomson's 

 Quadrant Electrometer used for Registering the Variations 

 in Atmospheric Electricity at the Kew Observatory." By 

 G. M. Whipple, B.Sc, Superintendent of the Kew Observa- 

 tory. Communicated by .Robert H. Scott, M.A., F.R.S. 

 Received April 3, 1878. 



The Meteorological Council, being desirous of discussing the pho- 

 tographic traces produced by their electrograph at the Kew Obser- 

 vatory some time since, requested the Kew Committee to institute a 

 series of experiments, with the view of determining the scale value of 

 the instrument, in order to prepare a suitable scale for measuring the 

 curves. 



The Kew Committee, at their meeting in November, entrusted the 

 matter to me, and accordingly, having obtained the loan of a battery 

 of 300 Bunsen cells, some preliminary experiments were made, which 



