Instead, therefore, of having recourse to the supposition, of the cloud in Tuscany having been produced by any other kind of exhalations from the earth; we may venture to believe, that an immense cloud of ashes, mixed with pyritical dust, and with numerous particles of iron, having been projected from Vesuvius to a most prodigious height, became afterwards condensed in its descent;--took fire, both of itself, as well as by means of the electric fluid it contained;--produced many explosions;--melted the pyritical, and metallic, and argillaceous particles, of which the ashes were composed;--and, by this means, had a sudden crystallization, and consolidation of those particles taken place, which formed the stones of various sizes, that fell to the ground: _but did not harden the clayey ashes so rapidly as the metallic particles crystallized_; and, therefore, gave an opportunity for _impressions to be made_ on the surfaces of some of the stones, as they fell, by means of the impinging of the others.
Nor does it appear to me, to be any solid objection to this conclusion, either that Vesuvius was so far distant; or that the cloud came from the north.
For, if we examine Sir William Hamilton's account of the very eruption in question,[D] we shall find, that he had reason to conclude, that the _pine-like_ cloud of ashes projected from Vesuvius, at one part of the time during this eruption, was twenty-five or thirty miles in height; and, if to this conclusion we add, not only that some ashes actually were carried to a greater distance than _two hundred miles_;[E] but that, when any substance is at a vast height in the atmosphere, a very small variation of the direction of its course, causes a most prodigious variation in the extent of the range of ground where it shall fall; (just as the least variation in the angle, at the vertex of an _isosceles_ triangle, causes a very great alteration in the extent of its base;) we may easily perceive, not only the possibility, but the probability, that the ashes in question, projected to so vast an height, were first carried even beyond _Siena_ in Tuscany, northward; and then brought back, by a contrary current of wind, in the direction in which they fell.