Effects of Heat and Light on Inorganic Bodies. 411 



the same light indicates such molecular change in any body. A glass 

 plate is prepared with one of the above compounds of silver or a mixture 

 of them, and on being exposed ( in a camera ) to the light reflected from 

 some body that is to be " taken," the brighter rays will most reduce the 

 silver compound to the hypo-compound, and in some cases the ^ part 

 of a second is long enough to effect the change. This compound being 

 thus suddenly torn to pieces by the action of the light, its pieces are free 

 to form new combinations. A " bath " is made by mixing a solution of 

 silver with a diluted solution of green vitrol from which a gray powder 

 of metallic silver is precipitated. The picture that has been exposed to 

 the light is immersed in this bath, and the free precipitate of silver finds 

 its way to the parts of the plate on which the original coating of iodide 

 of silver, &c. , was most decomposed, and forms with it a new com- 

 pound. The picture is next subjected to a pouring bath of hyposulphite 

 of soda, which has an affinity for the original iodide of silver, &c. , that 

 composed the original coating of the plate, but which has no power over 

 the new compound formed of the silver precipitate. All that remains 

 of the original coating being thus removed, the parts changed by the 

 light only remain. The picture thus formed is the negative, those parts 

 which were black in the object taken being represented by naked places 

 on the glass, while the light colored parts are represented by parts of 

 the film of corresponding shape remaining on the plate. When this 

 negative is laid on sensitized paper and exposed to sunlight, of course 

 the positive picture formed on the paper is reversed, and the dark and 

 white parts correspond with those of the original object. The paper 

 used for the positive picture is prepared with white of egg upon which 

 is formed a film of chloride of silver, by first moistening the paper with 

 a solution of chloride of sodium, or common salt, and then laying it on 

 a bath formed of a solution of silver, the silver joins the chlorine and 

 the coating of silver chloride is formed. The process takes a minute, 

 and when the paper is dry it is sensitive to light. 



' ' Chloride of iron is not sensitive to light, but chloride of iron dis- 

 solved in ether is sensitive to light because the liberated chlorine unites 

 at once chemically with the ether. " ( H. Vogel. ) So iodide of silver 

 by itself is but little sensitive, but mixed with nitrate of silver it is very 

 sensitive because the liberated iodine has a body present with which it 

 can make new combinations. So, in general, bodies which unite readily 

 with iodine increase the sensitiveness of iodide of silver when mixed 

 with it. < ' Among these bodies may be enumerated extract of copper, 

 extract of tea, morphine, tannin. " 



Ohromate of potash is another compound shaken up by light and used 

 sometimes to make pictures. After paper prepared with it has been ex- 

 posed to the light, it must be simply washed in clear water which dis- 



