410 Dynamic Theory. 



Horn Silver, or silver chloride, which is a native form of silver, is 

 turned a dark violet by exposure to light for a few minutes. For this 

 reason it has been much emplo}'ed in photography. A number of sub- 

 stances that can be shaken apart by the action of light, are used in pho- 

 tography and other arts. Some of these will now be mentioned. Lunar 

 caustic is silver nitrate. Anything smeared with a solution of it will 

 turn black when exposed to the light. If a piece of paper be smeared 

 with it and a flat object, like a leaf, be laid upon it and exposed to the 

 light, all the parts not covered will be turned black, thus making a pro- 

 file of the leaf. This is called a silhouette. Paper prepared with a so- 

 lution of Red pruss iate of potassa, and citrate of iron and ammonia is 

 turned blue on exposure to the light. This paper is used for copying 

 maps and other drawings. A tracing of the drawing is first made on 

 transparent paper or tracing cloth, and this is laid on the sensitive paper, 

 secured in a suitable frame and exposed to sunlight. All the parts not 

 protected by the lines of the drawing are turned blue and become insol- 

 uble in water. The drawing is then washed, the water carrying off all 

 the chemicals that were protected by the lines of the drawing, leaving 

 them the original color of the paper white. 



Asphaltum ( or the bitumen of Judea, &c. , ) "is soluble in ethereal 

 oils, such as oil of turpentine, oil of lavender, besides petroleum, ether 

 and others. " ( Herman Vogel.) A film of this poured over a plate soon 

 dries, becoming a light brown film of asphaltum. When this is exposed 

 to light it looses its property of solubility in these ethereal oils. If a 

 picture be taken on a plate prepared with this, and then the plate cov- 

 ered with oil of lavender, the oil will dissolve away all the asphaltum 

 that was not exposed to the light. By taking advantage of this property, 

 pictures were first taken in the camera in 1826. Copper plate engrav- 

 ings have been made with the asphaltum process by exposing the plate 

 to an acid after the lines of the picture had been laid bare through the 

 film of asphaltum. The acid would eat the lines on down into the cop- 

 per while it has no effect on the parts protected by the asphaltum. 



Chloride of silver, Iodide of silver, and Bromide of silver are all very 

 sensitive to the action of light, and in consequence are of great use in 

 photography. The action of the light upon these compounds is to shake 

 them down to hypo-chlorides, hypo-bromides and hypo-iodides, com- 

 pounds with molecules containing only half the quantity of chlorine, 

 bromine and iodine ; and the action is accompanied by a change of color. 

 The chloride of silver is white, the hypo-chloride violet. Bromide of 

 silver is light yellow, the hypo-bromide is a yellowish gray. Iodide of 

 silver is yellow, and hypo-iodide green. These changes of color in- 

 dicate that a change has taken place in the molecular constitution of the 

 body, and it may be incidentally remarked that a change of color under 



