Effects of Heat and Light on Inorganic Bodies. 413 



oily ink, this is rejected from the damp places and only adheres to the 

 asphalt that is, to the picture, thus a stone giving impressions is ob- 

 tained. Another process uses chromium and gelatine. 



Chloride of iron is another compound reduced by light to a hypo- 

 chloride of iron, a bleaching process. If paper be prepared with a 

 coating of chloride of iron, and exposed to light in a camera, the coat- 

 ing will be divided into two parts ; the part affected by the light becom- 

 ing hypo-chloride of iron. Ked prussiate of potash will enter into com- 

 bination with this hypo-chloride, while it will not with the chloride, so if 

 a picture made by this process is immersed in a solution of the red 

 prussiate of potash, all the light parts of the object taken will be repre- 

 sented in blue color, which, however, will not stand, but must be altered 

 chemically to another color. Chloride of copper and chloride of uran- 

 ium are also reduced by light to t^po-chlorides. The action of light is 

 thus shown to be very powerful and rapid on certain bodies. On other 

 bodies the action is more deliberate while not less effectual in the long 

 run. If the crystals of red sulphuret of arsenic are exposed to the ac- 

 tion of light for some months, they become disintegrated and fall into 

 powder. Glass is also sensitive to light, the texture of most kinds be- 

 ing changed in a few days' exposure. An inscription of brass letters 

 on a mirror in Berlin, was effaced after having remained some years. 

 But the texture of the glass under the letters was found to be different 

 from the rest of it, and yellow marks penetrated too far below the sur- 

 face to be rubbed off. Glass containing manganese is strongly affected 

 by the light. Manganese is sometimes made a constituent of glass for 

 the purpose of discoloring it. But the action of the light undoing some 

 of the chemical unions in the body of the glass, oxygen and manganese 

 are left free to form the new combination of dark colored oxide of man- 

 ganese, which then gives to the glass the dark color. In other minerals 

 colors are sometimes changed by light. The Siberian Topaz is faded 

 out from the fine golden yellow color it has in nature. A fine one, six 

 inches high, in the Berlin mineralogical museum, was spoiled in that 

 way. (Vogel.) 



It is said the reason why bees exclude the light from their hives, is 

 that honey, exposed to the light, rapidly crystallizes, while it does not 

 in the dark. Camphor and Iodine crystals are formed, in a glass jar or 

 bottle, by exposure to light in the same way. 



But we are to observe that bodies are susceptible not merely to light 

 in general, but that the different tone and pitch of the various kinds of 

 light affect different bodies in different degrees. Indigo waves of light 

 produce the ;reatost impression on the photograph plates as they are 

 commonly prepared. Chloride of silver by itself, is most sensitive to 

 violet, but is little affected by blue. Bromide of silver is affected by 



