with its light or actinism. Nature has amply provided for the varying wants of plants; in the spring we may detect an excess of actinism in the solar rays; in the summer an excess of light, and in the autumn an excess of heat.
We have said that all bodies undergo a chemical disturbance when exposed to the solar rays, but it must not be supposed that this disturbance always manifests itself in a blackening, as in the case of horn silver. If a polished plate of metal, of glass, of marble, or even a polished surface of wood, be in part exposed to the influence of sunshine, it will, when breathed upon, exhibit the fact that a disturbance of some kind has taken place upon the portions illuminated, whereas no change can be detected upon the parts kept in the dark. But if we expose a chemically prepared tablet to the sunbeams in a similar manner, we may by a certain process render the effect produced on its surface permanent, and thus as it were fix a shadow.
The beautiful art of photography, or light-drawing, is based upon this marvellous fact. Everybody is familiar with the grand results of this art. Everybody has seen those wondrous pictures which neither pencil, brush, nor hand has touched, but which have been delicately traced by the magic sunbeam. We have ceased to look upon these pictures with astonishment, just as we have ceased to wonder at the locomotive, the electric telegraph, and the steamship. But in times gone by, had any one asserted
