We are thus led to the conclusion that the grand and terrible appearances displayed at Vesuvius and other volcanoes in a state of violent eruption do not differ in any essential respect from the phenomena which we have witnessed accompanying the miniature outbursts of Stromboli. And we are convinced, by the same considerations, that the forces which give rise to the feeble displays in the latter case would produce, if acting with greater intensity and violence, all the magnificent spectacles presented in the former. In Vesuvius and Stromboli alike, the active cause of all the phenomena exhibited is found to be the escape of steam from the midst of masses of incandescent liquefied rock. The violence, and therefore the grandeur and destructive effects of an eruption, depend upon the abundance and tension of this escaping steam. There is one respect in which volcanic phenomena are especially calculated to excite the fear and wonder of beholders--namely, in the sudden and apparently spontaneous character of their manifestations. Eclipses were regarded as equally portentous with volcanic eruptions till astronomers learned not only to explain the causes which gave rise to them, but even to predict to the second the times of their occurrence. If we were able in like manner to warn the inhabitants of volcanic regions of the approach of a grand eruption, the fear and superstition with which these events are now regarded would doubtless be in great part dispelled. The power of prediction is alike the crucial test and the crowning triumph of a scientific theory. But, although natural philosophers are able to assign the causes to which the grand operations of volcanoes are due, and also to explain all the varied appearances which accompany them, they have not as yet so far mastered the laws which govern volcanic action as to be able to predict the periods of their manifestation. That these operations, like all others going on upon the globe, are governed by great natural laws we cannot for a moment doubt. And that, in all probability, more careful and exact observation and reasoning will at some future time lead us to the recognition of these laws, every student of nature is sanguine. But at the present time, it must be confessed, we are very far indeed from being able to afford that crowning proof of the truth of our theories of volcanic action which is implied in the power of predicting the period and degree of intensity of their manifestations. [Sidenote: ERUPTIONS AND THE INTERVALS BETWEEN THEM.]