The great effect produced on the vegetable kingdom by the heat-rays may be judged of from the fact, that almost all the plants which exhibit the remarkable phenomena of irritability, almost approaching to animal life, are confined to those regions where the heat is extreme. On the banks of the Indian rivers grows a plant in almost constant motion. In the hottest of the conservatories at Kew is a curious plant, whose leaflets rise by a succession of little starts. The same house contains Venus's fly-trap. Light seems to have no effect in quickening their movements; but the effect of increased heat is at once seen. They exhibit their remarkable powers most during the still hot nights of an Indian summer.
Heat is of essential importance in the production and ripening of fruit. Many trees will not bear fruit in our cold climate, which are most productive in the sunny south. Animal as well as vegetable life is in great measure dependent on heat. Look at the insect tribes. The greater number of them pass their winter in the pupa state. Hidden in some sheltered nook, or buried in the earth, they sleep on, until the warmth of returning spring awakens them to life and happiness; and if, by artificial means, the cold be prolonged, they still sleep on, whereas, if they he exposed to artificial heat, their change is hastened, and butterflies may be seen sporting about the flowers of a hothouse, when their less favored relatives are still wrapped in the deepest slumber. To judge of the influence of heat on the animal and vegetable economy, we need but contrast summer and winter--the one radiant and vocal with life and beauty, the other dark, dreary, and silent.
The third constituent of the sunbeam is actinism--its property being to produce chemical effects. So long ago as 1556, it was noticed by those strange seekers after impossibilities, the alchemists, that horn silver, exposed to the sunbeam, was blackened by it. This phenomenon contained the germ of those most interesting discoveries which have distinguished the present age; but, in their ardent search for the philosopher's stone and the elixir of life, they overlooked many an effect of their labors which might have led them to important truths.