THE DAGUERREOTYPE. 173 



speedily than that which has been preserved in darkness 

 the temperature and every other condition being the 

 same. 



The phenomena of the Daguerreotype involve many 

 strange conditions. A plate of silver, on which a slight 

 chemical action has been established by the use of iodine, 

 is exposed to the lenticular image in the camera 

 obscura. If allowed tb remain under the influence of 

 these radiations for a sufficient length of time, a faithful 

 picture of the illuminated objects is delineated on the 

 plate, as shown by the visible decomposition and darken- 

 ing of the iodized surface. The plate is not, however, 

 in practice allowed to assume this condition ; after an 

 exposure of a few seconds the radiant influence is- cut 

 off, and the eye cannot detect any evidence of change 

 upon the yellow plate. It is now exposed to the vapour 

 of mercury, and that metal in a state of exceedingly fine 

 division is condensed upon the plate; but the conden- 

 sation is not uniformly spread upon its face. The 

 deposit of mercurial vapour is hr exact proportion to the 

 amount of chemical action produced. Is the change, 

 by which this peculiar power of condensation is effected, 

 a chemical, calorific, electrical, or merely a molecular 

 one ? The evidences, at present, are not sufficient to 

 determine the question. It has lately been suggested, 

 that the mercury acts chemically only, and effects the 

 full decomposition of the iodide of silver; and that the 

 picture is due to this, and not to the deposition actually 

 of the mercury vapour. In all probability we have the 

 involved action of several forces. We have some ex- 

 periments which show, clearly enough, that mercury is 

 deposited in proportions which correspond with the 

 intensity of solar action. A chemically prepared surface 

 is not necessary to exhibit this result. A polished 

 plate of metal, of glass, of marbje, or a piece of painted 

 wood, being partially exposed, will, when breathed upon, 

 or presented to the action of mercurial vapour, show 



