_The Development and Properties of the Image._--The latent image of the Daguerreotype is developed in a manner different from that of the humid processes generally,--viz. by the action of Mercurial vapour. Mercury, or Quicksilver, is a metallic fluid which boils at 662° Fahrenheit. We are not however to suppose that the iodized plate is subjected to the vapour of Mercury at a temperature at all approaching to 662°. The cup containing the Quicksilver is previously heated by means of a spirit-lamp to about 140°, a temperature easily borne by the hand, in most cases, without inconvenience. The amount of Mercurial vapour evolved at 140° is very small, but it is sufficient for the purpose, and after continuing the action for a short time the image is perfectly developed. There are few questions which have given rise to greater discussion amongst chemists than the nature of the Daguerreotype image. Unfortunately, the quantity of material to be operated on is so small, that it becomes almost impossible to ascertain its composition by direct analysis. Some suppose it to consist of Mercury alone. Others have thought that the Mercury is in combination with metallic Silver. The presence of the former metal is certain, since M. Claudet shows that, by the application of a strong heat, it can actually be volatilized from the image in sufficient quantity to develope a second impression immediately superimposed. It is a remarkable fact that an image more or less resembling that developed by Mercury can be obtained by _the prolonged action_ of light alone upon the iodized plate. The substance so formed is a white powder, insoluble in solution of Hyposulphite of Soda; amorphous to the eye, but presenting the appearance of minute reflecting crystals when highly magnified. Its composition is uncertain. For all practical purposes the production of the Daguerreotype image by light alone is useless, on account of the length of time required to effect it. This was alluded to in the third Chapter, where it was shown that in the case of the Bromo-Iodide of Silver an intensity of light 3000 times greater is required, if the use of the Mercurial vapour be omitted. _M. Ed. Becquerel's discovery of the continuing action of rays of yellow light._--Pure homogeneous yellow light has no action upon the Daguerreotype plate; but if the iodized surface be first exposed to white light for a sufficient time to impress a latent image, and then _afterwards_ to the yellow light, the action already commenced is _continued_, and even to the extent of forming the peculiar white deposit, insoluble in Hyposulphite of Soda, already alluded to. Yellow light may therefore in this sense be spoken of as a _developing_ agent, since it produces the same effect as the Mercurial vapour in bringing out to view the latent image.