The advance of photography : its history and modern applications (1911)

2 THE ADVANCE OF PHOTOGRAPHY which may be shorter or longer, as the heating power of the sun is stronger or weaker. In addition to these two effects of sunlight, there is a third, which generally requires a still longer period to make itself noticed, and cannot be directly perceived either by the eye or by the sense of feeling, but only by the peculiar changes which light produces in the material world. This is the chemical effect of light. Physical v. Chemical Change. — If we take a piece of wood, and bend or saw it, we change its form ; if we rub it, it becomes warm — we change its temperature, but it still remains wood. These changes, which do not affect the substance or matter of a body, we term physical. But if we set fire to a piece of wood, strong-smelling gases ascend, ashes are deposited, and a black residue remains, which is totally different from the wood. By this process a new substance — charcoal — has been produced. Material changes of this kind we term chemical changes ; — and such chemical changes are, in an especial manner, produced by heat. If, for instance, we heat a bright iron wire red hot, it undergoes apparently only a physical (not a material) change. But if Ave allow it to cool, we find the bright rod has become dull and black ; that it has acquired a brittle, black surface, which easily breaks away on bending the rod, and differs entirely from the bright, tough, flexible iron. Here again a chemical change — that is to sa}^, a change of substance — has taken place ; the iron has been converted into another body, into iron scale, because it has combined with a component part of the surrounding air — with oxygen. Chemical changes of this kind are not only produced by heat, but also by light. Fading of Coloured Fabrics. — It has long been known that when the colours with which fabrics are dyed are not the so-called fast colours, they fade in the light — that is, become paler. In this case the colouring matter changes