Now if water were at any time admitted to them whilst in that condition, we know from the common principles of chemistry, that a great evolution of gaseous matter must take place, and that the combination of the oxygen of the water with these inflammables would give rise to heat, sufficient to account for the liquefaction of the surrounding rocks, and all the other phænomena attendant on an eruption.

Such being the opinions respecting volcanos, which appear, at first sight, to possess the greatest share of probability, let us next inquire somewhat more minutely into the arguments that may be advanced in their support.

And first with regard to the former hypothesis, I may remark, that it is chiefly favoured by the general occurrence of sulphur and of gases containing it in almost all volcanos; from whence it might seem a natural inference, that on the presence of this substance the phænomena themselves depended. Whether the same be the case with petroleum, which Breislac regards as the chief agent in the process, will appear hereafter; but whichever of these substances be the cause of the volcanic operations, it will at least be admitted, that there would be little difficulty in imagining means by which either of them might be brought into a state of combustion. When however we examine more narrowly into the analogies between the effects of volcanic fires, and those which we know to result from the combustion of either of these materials, we are soon brought to confess the inadequacy of such an hypothesis to account for the facts before us. What resemblance for example do the porcelain-jaspers and other pseudo-volcanic rocks, as they are improperly termed, which we observe in coal mines, that have been for centuries in a state of inflammation, bear to the lavas and the ejected masses of a genuine volcano ; or where do we observe from them the same evolution of aeriform fluids, and of streams of melted materials which are so characteristic of the latter? The difference would

appear

still more striking, if I were to enter into other particulars with respect to the depth and geological position which must be assigned to the seat of the volcanic action; but these will be more in place at a later period of this Lecture.

The remaining hypothesis which I have to consider, has at least this advantage, that it supposes the agency of bodies which do not exist in nature on the surface of the globe; and this circumstance gives it at least a superiority over other explanations, in a case where the phænomena to be accounted for are of a description altogether different from any that result from other natural processes placed within the sphere of our observation. The individual therefore who maintains, that volcanos arise from the access of water to the metals of the earths and alkalies, is exempt from the necessity of pointing out, as the advocates of the contrary hypothesis ought to do, some process going on near the surface of the earth, resembling in kind at least, if not in degree, the phænomena which he attempts to explain.

Now if volcanos have arisen from this latter cause, the necessity of water to excite the combustion seems to imply, that they would be met with rather in the neighbourhood of the sea, than on the elevated table lands of extensive continents; and the existence of substances capable of decomposing that fluid indicates, that the process must have gone on at a depth sufficiently great to have precluded the access of air, which would have long ago imparted to them the very principle, to the absorption of which the volcanic action is attributed. Hence the rocks, which appear to proceed from the focus of a volcano, ought to be derived rather from granitic and other of the older formations, than from those of modern date; and the gases evolved during the process ought to consist, in part at least, of those which we know to be given out, when water is made to act upon the alkaline and earthy bases.

In order therefore to ascertain the degree of probability belonging to this theory, I shall consider, first, the geogra phical situation of volcanos; secondly, the character of the

aeriform fluids evolved by them in different stages of activ ity; thirdly, the nature of their lavas and ejected masses; and fourthly, the depth from which they appear to have emanated.

Before however I enter upon these questions, it is right that I should define more accurately than has hitherto been done, what appearances are to be considered as establishing the existence of volcanic action, inasmuch as the ideas entertained on that head are singularly various and indefinite, and thus have occasioned much of the difference of opinion that exists on these questions. Some for instance are unwilling to admit earthquakes as any probable indication of subterraneous fire, whilst others not only include these, but go so far as to class hot-springs, gaseous exhala, tions like those of the Pietra Mala, and the eruptions of mud and petroleum commonly called "Salses" amongst volcanic phænomena.

With regard however to the first of these, I apprehend, that those who coolly examine the facts that have been collected on the subject, will scarcely entertain any other difference of opinion, than as to whether their connection with volcanos is universal; for in some instances earthquakes have occurred so immediately antecedent upon vol canic eruptions, and are so manifestly derived from the very same centre of action, that we want no better proof to esta blish an identity of origin.

In other cases the evidence, though not quite so direct, is perhaps as cogent as can be obtained in most questions of this description.

When for instance we observe two volcanic districts, both subject to earthquakes, which are ascertained to have a connection with the volcanic action going on, and find that an intermediate country, in which there are no traces of the operation of fire, is agitated by subterraneous convulsions, similar in kind, but stronger in degree than those which occur in the more immediate vicinity of the volcanos; have

we not reason to conclude, that the same action extends throughout the whole of the above space, and that it is this which produces in the intermediate country the effects alluded to, which are only the more alarming from the absence of any natural outlet, from which elastic vapours

might escape?

Now in proof of the former of these positions, it may be scarcely necessary to do more than appeal to the case of Etna or Vesuvius, which rarely return to a state of activity, after a long interval of comparative quiescence, without some antecedent earthquake, which ceases so soon as the mountain has established for itself a vent.* Such was the case before the celebrated eruption of 79 in Campania, and in that of Etna in 1537, where, says Fazzello, noises were heard, and shocks experienced, over the most distant parts of Sicily. In such cases no one would doubt the connection between the volcano and the earthquake.

* Humboldt gives us the following series of phænomena, which presented themselves on the American Hemisphere between the years 1796 and 97, as well as between 1811 and 1812.

1796.-September 27, Eruption in the West India Islands; volcano of Guadaloupe in activity.

November..... The volcano of Pasto begins to emit smoke.

.... December 14.

1797.-February 4.

Destruction of Cumana by earthquake.

Destruction of Riobamba by earthquake.

1811.-January 30. Appearance of Sabrina Island in the Azores.

creases particularly on the 15th of June.

It in

May......... Beginning of the earthquakes in the Island of St. Vincent, which lasted till May, 1812.

... December 16. Beginning of the commotions in the valley of the Missisippi and Ohio, which lasted till 1813. Earthquake at Carracas.

.....

December.

....

Destruction of Caraccas; earthquakes which con

tinued till 1813.

Eruption of the volcano in St. Vincents'; and the same day subterranean noises at Caraccas, and on the banks of the Apure.

Pers. Narr. Vol. IV,

See also Gemellaro on the Meteorological Phænomena of Mount Etna, extracted in the Journal of Science. Vol. 14. 1813.

The second point seems established, by considering the tremendous earthquakes which ravaged Calabria, and those mentioned by Humboldt as intervening between, and in the line of, the volcanos of Columbia, Quito, and Chili. Von Buch has well shewn, in his paper on Lancerote, the comparative immunity enjoyed by Teneriffe from those convulsions of nature which agitate the neighbouring islands, destitute of that great natural chimney or safety valve afforded by the Peak of Teyde.

If it be objected, that earthquakes are too general to be referred to this one cause, I may answer, that the preceeding details have shewn volcanos to be phænomena, which (taking in those extinct as well as recent) are seen in almost every part of the globe that borders upon the sea, and that earthquakes, like volcanos, though felt undoubtedly even in the centre of large continents, seem to produce their most frightful effects in countries not very far removed from the

ocean.

Thus the most tremendous instances that we read of are those of Lisbon, where traces of antient lavas are discover. able, Asia Minor near the Catacecaumene, and the Caraccas situated between the volcanos of the Antilles and those of Columbia.

Dr. Stukeley, who wrote on the causes of earthquakes, about the middle of the last century, at a time when the physical geography of the globe was much less generally explained, might have reason for believing that volcanos are too rare to account for a phænomenon so general as that which formed the subject of his enquiry; but at present this at least hardly can be said to furnish an objection, especially when we recollect the vast distances, to which sound and other vibratory motions may be propagated along the substance of solid bodies, and therefore are not obliged to place the seat of the action at that vast depth at which Dr. Stukeley imagines it must reside, considering the radius over which its effects are perceived.

Phil. Trans. for 1750.