LIGHT. ioi 



events in some cases, regard the action on photographic com- 

 pounds as resulting from a function of light. So viewing it, 

 we get light as an initiating force, capable of producing, 

 mediately or immediately, the other modes of force. Thus, 

 it immediately produces chemical action ; and having this, 

 we at once acquire a means of producing the others. At my 

 Lectures in 1843, I showed an experiment by which the pro- 

 duction of all the other modes of force by light is exhibited : 

 I may here shortly describe it. A prepared daguerreotype 

 plate is enclosed in a box filled with water, having a glass 

 front with a shutter over it. Between this glass and the plate 

 is a gridiron of silver wire ; the plate is connected metallically 

 with one extremity of a galvanometer coil, and the gridiron of 

 wire with one extremity of a Breguet's helix an elegant instru- 

 ment formed by a coil of two metals, the unequal expansion of 

 which indicates slight changes in temperature the other 

 extremities of the galvanometer and helix are connected by 

 a wire, and the needles brought to zero. As soon as a beam 

 of either daylight or the oxyhydrogen light is, by raising the 

 shutter, permitted to impinge upon the plate, the needles are 

 deflected. Thus, light being the initiating force, we get chemical 

 action on the plate, electricity circulating through the wires, 

 magnetism in the coil, heat in the helix, and motion in the 

 needles. 



If two plates of platinum be placed in acidulated water, 

 and 'connected with a delicate galvanometer, the needle of this 

 is always deflected, a result due to films of gas or other 

 matter on the surface of the platinum, which no cleaning can 

 remove. If, after the needle has returned to zero, which will 

 not be the case for some hours or even days, one of the plati- 

 num surfaces be exposed to light, a fresh deflection of the 

 needle takes place, due, as far as I have been able to resolve 

 it, to an augmentation of the chemical action which had occa- 

 sioned the original deflection, for the deviation is in the same 

 direction. If, instead of white light, coloured light be per- 

 mitted to impinge on the plate, the deviation is greater with 

 blue than with red or yellow light, showing, in addition to 

 other tests, that the effect is not due to the heat of the sun's 

 rays, as the calorific effects of light are greater with red 



