While the lightning flash shows the brilliancy which the electric illumination can attain, it shows also the intense heat resulting from the electric discharge. This might, indeed, be inferred simply from the brilliancy of the light, since we know that this brilliancy can only be due to the intense heat to which the particles along the track of the electric flash have been raised. But it is shown in a more convincing manner to ordinary apprehension by the effects which the lightning flash produces where--in the common way of speaking--it strikes. The least fusible substances are melted. Effects are produced also which, though at first not seemingly attributable to intense heat, yet in reality can be no otherwise explained. Thus, when the trunk of a tree is torn into fragments by the lightning stroke, though the tree is scorched and blackened, a small amount of heat would account for that particular effect, while the destruction of the tree seems attributable to mechanical causes. It is, indeed, from effects such as these that the idea of the fall of thunderbolts has doubtless had its origin, the notion being that some material substance has struck the body thus shattered or destroyed. In reality, however, such destructive effects are due entirely to the intense heat excited during the passage of the electricity. Thus, in the case of a tree destroyed by lightning, the shattering of boughs and trunk results from the sudden conversion of the moisture of the tree (that is, the moisture present in the substance of the tree) into steam, a change accompanied of course by great and sudden expansion. The tree is as certainly destroyed by the effects of heat as is a boiler which has burst, though in each case the expansive power of steam directly works the mischief. It is the more useful for our present purpose thus to note at the outset both the illuminating and the heating power of the lightning flash (or rather of the electric discharge), because, as will presently be seen, the electric light, while in all cases depending on intensity of heat, may either be obtained in the form of a series of flashes succeeding each other so quickly as to be to all intents and purposes continuous, or from the incandescence of some suitable substance in the path of the electric current. Let us now consider briefly the general nature of electrical phenomena, or at least of those electrical phenomena which are related to our present subject.