LIGHT. 113 



over the vapour of heated mercury, the mercury attaches it- 

 self to the portions affected by light, and gives them a white 

 frosted appearance ; the intermediate tints are less affected, 

 and those parts where no light has fallen, by retaining 

 their original polish, appear dark ; the iodide of silver is 

 then washed off by hyposulphite of soda, which has the 

 property of dissolving it, and there remains a picture 

 in which the lights and shadows are as in nature, and the 

 molecular uniformity of the metallic surface enables the 

 most microscopic details to be depicted with perfect accu- 

 racy. By using chloride of iodine, or bromide of iodine, 

 instead of iodine, the equilibrium of chemical forces is ren- 

 dered still more unstable, so that images may be taken in an 

 indefinitely short period a period practically instantaneous. 



It would be foreign to the object of this essay to enter 

 upon the many beautiful details into which the science of 

 photography has branched out, and the many valuable discov- 

 eries and practical applications to which it has led. The 

 short statement I have given above is perhaps superfluous, as, 

 though they were new and surprising at the period when these 

 Lectures were delivered, photographic processes have now be- 

 come familiar, not only to the cultivator of science, but to 

 the artist and amateur ; the important point for consideration 

 here is that light will chemically or molecularly affect mat- 

 ter. Not only will the particular compounds above selected 

 as instances be changed by the action of light ; but a vast 

 number of substances, both elementary and compound, are 

 notably affected by this agent, even those apparently the most 

 unalterable in character, such as metals : so numerous, in- 

 deed, are the substances affected, that it has been supposed, 

 not without reason, that matter of every description is altered 

 by exposure to light. 



The permanent impression stamped on the molecules of 

 matter by light can be made to repeat itself by the same 

 agency, but always with decreasing force. Thus a photo- 



