LIGHT. 117 



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 plat- 

 inum 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 than 

 with blue light, while the chemical effects are the inverse. 



There are other apparently more direct agencies of light 

 in producing electricity and magnetism, such as those ob- 

 served by Morichini and others, as well as its effects upon 

 crystallization ; but these results have hitherto been of so in- 

 definite a character, that they can only be regarded as pre- 

 senting fields for experiment, and not as proving the relations 

 of light to the other forces. 



Light would seem directly to produce heat in the phenom- 

 ena of what is termed absorption of light : in these we find 

 that heat is developed in some proportion to the disappear- 

 ance of light. To take the old experiment of placing a se- 

 ries of different coloured pieces of cloth upon snow exposed 

 to sunshine, the black cloth absorbing the most light, and de- 

 veloping the most heat, sinks more deeply in the snow than 



