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58 THE ADVANCE OF PHOTOGRAPHY
Moser has remarked that light has a certain action on almost all surfaces. He covered smoothly polished surfaces of silver, ivory, and glass with perforated screens, and exposed them to the light. After this he breathed upon them, or exposed them to the action of the vapour of mercury, and found that the vapour was condensed most powerfully where the light had reached the surface. Accordingly, Moser established the proposition : Light reacts on all bodies, and its action can be made visible by the greater condensation of vapours on the parts exposed to light.
Chemical Changes produced by Light. — The chemical changes effected b} light are far more numerous than the plrysical, and their study is the special province of photo-chemistry.
Before passing to the more complicated phenomena of photography, we must make the reader acquainted with the simpler phenomena of the action of light.
It is remarkable that many elements present themselves in quite different forms, so that it might be supposed they were different substances. The yellow, inflammable, poisonous phosphorus, soluble in ether, and formerly used in the manufacture of matches, is changed by heating in a closed vessel, into a reddish substance difficult to kindle, not poisonous, and insoluble. This is, however, phosphorus, and passes by melting into the state of common phosphorus.
It is an interesting fact that this transformation of yellow into red phosphorus is effected not only by heat, but also by light. If yellow phosphorus be exposed for a long time to the light, it becomes red.
The oxygen of the air is also susceptible of similar changes. Ordina^ oxygen is a colourless and inodorous gas. By the action of electricity, however, it is easily changed into another kind of gas, distinguished by a peculiar smell — the so-called sulphurous smell of lightning.