the Chemical Rays and those of Radiant Heat. 201 



there is not anything like a direct superposition in the case, 

 and that the particles of amalgam and iodide lie as it were 

 side by side. 



(30.) 3rd. That 'when a ray of light falls upon the surface 

 of this, &c. &c. 



There is no difficulty in proving this directly, and the in- 

 direct evidence is copious. If we lay a piece of paper im- 

 bued with starch on an iodized plate, and expose it to the 

 sun, although the plate presently assumes a dark olive green 

 colour, the starch remains uncoloured. 



(31.) This dark substance is probably a subiodide of silver; 

 the iodine therefore which has been disengaged from it not 

 having been set free, must have necessarily united with the 

 adjacent metallic silver, — this, for very obvious reasons, there 

 is no difficulty in admitting. 



(32.) Now, therefore, when a photogenic impression ex- 

 isting on the surface of a plate in an invisible state is brought 

 out by the action of mercury vapour, we easily understand 

 how this is effected. No iodine is ever evolved. But each 

 atom of iodide of silver, that has been acted on by the light, 

 yields to the attraction of the mercury its atom of silver, and 

 the iodine thus set free unites with the metallic silver par- 

 ticles around it, reproducing the same yellow iodide by a di- 

 rect corrosion of the plate: the proofs that we have of this 

 are two in number. 



(33.) 1st. Dry some mucilage of gum-arabic on a Daguer- 

 reotype, just brought from the mercury-bath ; when it has 

 split up, we perceive that the white amalgam of silver is re- 

 moved, and an uniform coat of yellow iodide of silver, of the 

 very same thickness as at first, as is proved by its colour, is 

 left. 



(34.) 2ndly. Dry upon the same plate a solution of Russian 

 isinglass, and when it has split up, it will be seen that it uni- 

 formly rends off with it the yellow iodide, leaving the metallic 

 plate with an exquisite polish ; and wherever the light has 

 touched, there it is corroded. 



(35.) These two facts, taken together, prove that in mer- 

 curializing a plate no iodine is evolved, but that a new film 

 of iodide of the same thickness is formed, at the expense of 

 the metallic surface. 



(36.) From these facts we readily gather, that on the 

 presence of the metallic silver the sensitiveness of this pre- 

 paration mainly depends, for to the tendency which the light 

 has impressed on the elements of the iodide to separate, is 

 added the strong attraction of metallic silver for nascent io- 

 dine. 



