296 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



surface of the plate ? How are the images formed ? What is the 

 mode of action of the light, and how do the images, invisible at first, 

 though formed, become visible in all their details ? 



We have already seen that the result of exposing the silvered 

 plate to the vapours of iodine is the formation of a chemical compound, 

 iodide of silver. It is this compound which covers the originally white 

 surface of the metal with a tint which varies, according to the thick- 

 ness, from straw colour, golden or orange, red and violet to blue. Let 

 us remark, to begin with, that this phenomenon of colouring is not 

 due to the colour of the iodide of silver, which is pale primrose, but to 

 an action in which the interference of the rays of light plays the prin- 

 cipal part, as we have seen in the chapter in the Forces of Nature 

 dealing with the colours of thin plates. M. Dumas has measured 

 the weight of the coating of iodine formed on the surface of a 

 daguerreotype plate, and he has made an approximate estimate of the 

 thickness of the coating itself. This summary is so curious that we 

 will quote from the celebrated chemist's account : " A plate of 5760 

 square millimetres in surface having been brought to a -straw-coloured 

 tint by exposure to iodine vapour, was placed on a very delicately 

 adjusted balance, and the weight exactly ascertained ; there was a 

 decided increase of weight, but it did not amount to half a milli- 

 gramme. When the shade deepened to golden, the weight increased 

 to the half milligramme. By prolonging the duration of the action 

 of the iodine vapour beyond the necessary time, by quadrupling it, 

 for example, I obtained very appreciable effects in the balance : an 

 increase of two milligrammes in weight. I supposed the quarter of this 

 quantity would have sufficed to give the whole surface sufficient iodine 

 to produce the image. But on calculating the weight of iodide of 

 silver which this iodine represents, and the volume of iodide cor- 

 responding to this weight, the thickness of the coating of iodide of 

 silver deposited on the surface of the plate is arrived at. It amounts 

 to less than the millionth part of a millimetre." 



When the metal plate, covered with iodide of silver and bromide 

 of silver, has been submitted to the action of the accelerating sub- 

 stances, what happens when it is impressed by the light ? What 

 influence have the waves of light on the sensitive coating? On this 

 point opinions differ. According to M. Dumas, whose opinion was 

 circulated when Dagueire's discovery was made public, the action of 



