346 "WELLS'S NATURAL PHILOSOPHY. 



If the plate were then left without further care, the imago formed would 

 eoon fade away, and leave no trace on its surface. In practice, the plate is 

 not exposed to the influence of light sufiicicntly long to form upon its sur- 

 face an image visible to the eye, but the picture is develojjed, or brought out 

 and rendered permanent by exposure to the vapor of mercury. This metal, 

 in a state of very fine division, is condensed upon and adheres to those por- 

 tions of the surface of the plate which have been subjected to the influence 

 of the chemical action. "Where the shadows are deep, there is scarcely a 

 trace of mercury ; but where the lights are strong, the metallic dust is do- 

 posited of considerable thickness. This deposition of mercury essentially com- 

 pletes and fixes the picture. 



The reason why the vapor of mercury attaches itself only to those portions 

 of the plate which have been affected by the chemical influence of light is not 

 definitely known: in all probability, we have involved the action of several 

 forces. It is not, however, necessary that a surface should be chemically pre- 

 pared to exhibit these results. A polished plate of metal, a piece of marble, 

 of glass, or even wood, when partially exposed to the action of light, will, 

 ■when breathed upon, or presented to the action of mercurial vapor, show that 

 a disturbance has been produced upon the portions which were illuminated ; 

 whereas no change can be detected upon the parts kept in the dark. 



That the luminous principle is not necessary for the success 

 What 6^6"- of ii^Q photographic process, may be proved by the experi- 

 that light is mcnt of taking a daguerreotype in absolute darkness. This 

 forth"*prodn'(> '^^^ ^^ accomplished in the following manner: — A large pris- 

 tion of a pho- matic Spectrum is thrown upon a lens fitted into one side of a 

 Bult'?^"° "^' dark chamber; and as the actinic power resides in great ac- 

 tivity at a point beyond the violet ray, where there is no light, 

 the only rays allowed to pass the lens into the chamber are those beyond the 

 limit of coloration, and non-luminous; these are directed upon any object, and 

 from that object radiated upon a highly sensitive photographic surface. In 

 this way a picture may be formed by radiations which produce no eflect upon 

 the eye. 



- 706. There are many reasons for supposing that each of the 



do the three three principles, light, heat, and actinism, included in the solar 

 {""""olar rav ^^^' ^^^rcise a distinct and peculiar influence upon vegeta- 

 fxert on rege- tion. Thus the luminous principle controls the growth and 

 tation coloration of plants, the calorific principle their ripening and 



fructification, and the chemical principle the germination of seeds. Seedt 

 which ordinarily require ten or twelve days for germination, will germinato 

 under a blue glass in two or three. The reason of this is, that the blue glass 

 permits the chemical principle of liglit to pass freely, but excludes, in a great 

 measure, the heat and the light. On the contrary, it is nearly impossible to 

 make seeds germinate under a yellow glass, because it excludes nearly all 

 the chemical influence of the solar ray. 



