A Manual Of Photography

238 PRACTICE OF PHOTOGRAPHY. is experienced in operating in a humid medium, especially when we come to the mercurial operation, which requires, to raise a suitable vapour, a temperature of 122° F. " This vapour, which first heats the air contained in the apparatus, produces on the metal a dew which weakens the image. It is very evident that this humid layer is very injurious; since if, for example, the plate, on leaving the camera obscura, be breathed on two or three times, the mercurial vapour can no longer cause the impression to appear. “ The water which is condensed, even at the slightest diffe- rence of temperature between the surface of a body and the surrounding air, contains in solution, or in suspension, a non- volatile substance, which might be called atmospheric dust; and as soon as the equilibrium of temperature is established between the air and the surface of the body, the humid vapour which was condensed on it is volatilised, and depositing on it the dust which it contains, goes on to be re-saturated in the air with a fresh quantity of this impure substance. " In order as much as possible to neutralise this effect, the temperature of the plate may be kept higher than that of the surrounding air, during each of the operations. But it is im- possible to cause this heat to reach to 322° F., in order for it to be of the same temperature as the vapour of mercury, since, if the plate be exposed to that degree of heat after the operation of light in the camera obscura, the image will be altered. " I first tried to absorb the humidity of the air in the mercu- rial box by the usual means, such as lime, &c. ; but these means are insufficient, and only complicate the process, without giving a good result. Another means which has been proposed coin sists in vapourising the mercury under the pneumatic machine; by this process, truly, the dew on the plate is avoided, but the pressure of the air, which is indispensable to the impression, is suppressed. The results thus obtained, also, are always wanting in purity. " The following is the process at which I have stopped, because it is very simple, and because it obviates the two incon- veniences above mentioned; that is to say, it frees the silver as much as possible from all dirt or dust, and neutralises the humidity produced by the elevation of temperature in the mer- curial box. By the first of these two effects it increases the promptitude, and by the second it renders the lights much whiter (especially by the application of M. Fizeau’s chloride of gold) : these two effects are always certain. The promptitude given by this process is to that hitherto obtained as 3 to 8 : this proportion is accurate.