96 CORRELATION OF PHYSICAL FORCES. 



polarised ray, instead of passing through water, be made to 

 pass through oil of turpentine, the definite direction in which 

 it is polarised will be found to be changed ; and the change of 

 direction will be greater according to the length of the column, 

 of interposed liquid. Instead of being an uniform plane, it will 

 have a curvilinear direction, similar to that which a strip of 

 card would have if forced along two opposite grooves of a rifle- 

 barrel. This curious effect is produced in different degrees by 

 different media. The direction also varies ; the rotation, as it 

 is termed, of the plane being sometimes to the right hand 

 and sometimes to the left, according to the peculiar molecular 

 character of the medium through which the polarised ray is 

 transmitted. These effects will be presently reverted to. 



Light is, perhaps, that mode of force the reciprocal rela- 

 tions of which with the others have been the least traced out. 

 Until the discoveries of Niepce, Daguerre, and Talbot, very little 

 could be definitely predicated of the action of light in producing 

 other modes of force. Certain chemical compounds, among 

 which stand pre-eminent the salts of silver, have the property 

 of suffering decomposition when exposed to light. If, for 

 instance, recently formed chloride of silver be submitted to 

 luminous rays, a partial decomposition ensues ; the chlorine 

 is separated and expelled by the action of light, and the silver 

 is precipitated. By this decomposition the colour of the sub- 

 stance changes from white to purple. If now, paper be 

 impregnated with chloride of silver, which can be done by a 

 simple chemical process, then partially covered with an 

 opaque substance, a leaf for example, and exposed to a strong 

 light, the chloride will be decomposed in all those parts of the 

 paper where the light is not intercepted, and we shall have, 

 by the action of light, a white image of the leaf on a purple 

 ground. If similar paper be placed in the focus of a lens in a 

 camera-obscura, the objects there depicted will decompose the 

 chloride, just in the proportion in which they are luminous, 

 and thus, as the most luminous parts of the image will most 

 darken the chloride, we shall have a picture of the objects 

 with reversed lights and shadows. The picture thus produced 

 would not be permanent, as subsequent exposure would 

 darken the light portion of the picture : to fix it, the paper 



