328 Common Science 



of blotting paper or on a paper towel, so that the water will 

 be absorbed. Spread the remaining white paste of silver 

 chlorid (AgCl) out over the blotter as well as you can. Cover 

 part of it with a key (or anything that will shut off the light), 

 and leave the other part exposed. If the sun is shining, put 

 the blotter in the sunlight for 5 minutes. Otherwise, let as 

 much daylight fall on it as possible for about 10 minutes. 

 Now take the key off the part of the silver chlorid (AgCl) 

 that it was covering and compare this with the part that 

 was exposed to the light. What has the light done to the 

 silver chlorid (AgCl) that it shone on? 



What has happened is that the light has made the 

 silver (Ag) separate from the chlorine (Cl) of the silver 

 chlorid (AgCl) . Chemists would write this : 



AgCl->Ag + Cl. 



That is, silver chlorid (AgCl) has changed into silver 

 (Ag) and chlorine (Cl). Chlorine, as you know, is a 

 poisonous gas, and it floats off in the air, leaving the fine 

 particles of silver behind. When silver is divided into 

 very tiny particles, it absorbs light instead of reflecting 

 it ; so it looks dark gray or black. 



How photographs are made. All photography de- 

 pends on this action of light. The plates or films 

 are coated with a silver salt, usually a more sensi- 

 tive salt than silver chlorid. This is exposed to the 

 light that shines through the lens of the camera. 

 As you have learned, the lens brings the light from the 

 object to a focus and makes an image on the film or 

 plate. The light parts of this image will change the 

 silver salt to silver ; the dark parts will not change it. 

 So wherever there is a white place on the object you 

 are photographing, there will be a dark patch of silver 



