OBJECTIONS TO THE CHEMICAL THEORY ANSWERED. 723

Second Objection.-Supposing caverns to exist near or about the focus of a volcano, and these to be filled with air, the communication between such hollows and the atmosphere must be stopped up so soon as lava came to be emitted. Hence the action would be discontinued for want of air to carry it on.

Reply. I cannot ure to myself a cavern in the interior of the earth so hermetically sealed as to be impermeable to air when a partial vacuum took place within it. Even if the lava were to seal the inouth of the crater, and thus put a stop to the emission of solid or liquid matter, it could hardly spread itself over the whole internal surface of the cavern so as to close all its fissures and bar the ingress of air; nor if it did, could it prevent new ones from being formed, so soon as the rock began to contract.

Now admitting this communication with the external air to exist, it is surely a waste of time to calculate the size of a cavern which could supply oxygen enough for any supposed amount of lava, since whatever air was lost by oxidation in and about the cavern itself, would be immediately supplied by the atmosphere in connexion with it.

Third Objection.-Volcanic actions are deep-seated, but the chemical processes assumed are of such a nature as to be developed only near the surface.

Reply.—No reason is assigned, why chemical actions, excited by the access of air and of water to metallic bodies of a highly combustible nature, may not take place at any depth whatsoever.

Fourth Objection. If these chemical actions were supported by atmospheric air, vast volumes of nitrogen must be evolved in consequence. Now the amount of nitrogen observed is after all but inconsiderable.

Reply.—This objection implies a more thorough acquaintance with volcanic phænomena than we are as yet entitled to assume.

How do we know what amount of nitrogen may be evolved during the more active periods of volcanic action ?

How can we be justified, à priori, in laying down what proportion of the effect produced is due to atmospheric air, and how much to water?

The more philosophical mode of treating the subject seems to me to be, to content ourselves with endeavouring to account for the products which we have hitherto succeeded in detecting. Amongst these are nitrogen, sulphurous acid and sulphuretted hydrogen. The two former clearly imply atmospheric air, the latter water, to have been present.

The amount of all the three collectively taken, Bischof contends to be inadequate to the effects of which I regard them as the results; but can he, especially after Pilla's observations, assure himself that the quantity of one or of all may not be greater than has yet been determined ? and can he ac. count even for the amount ascertained to be emitted without having recourse to some such hypothesis as the one I have proposed ?

Even of nitrogen, the quantity emitted during languid volcanic action seems greater than can be explained by any decomposition of organic matter, bating other objections to such a solution-witness the thermal waters of Bath, where the amount is no less than 250 cubic feet in twenty-four hours--whilst of sulphuretted hydrogen the quantity disengaged must exceed what we can account for by the decomposition of sulphates, if we suppose such beds of sulphur as those in Sicily to have resulted from its decomposition,-not to allude to the fact, that the sulphates themselves can best be attributed to the sulphuric acid generated owing to a previous decomposition of sulphuretted hydrogen.

How difficult too is it to suppose such an amount of organic matter, as must be assumed in order to explain, according to Bischof's theory, the evolution of nitrogen no less than of sulpburetted hydrogen, existing for so many ages in the very focus of volcanic heat! These however are points that have been before sufficiently insisted upon.

Fifth Objection. If lava were formed by the union of the metallic bases assumed with oxygen derived from water, the quantity of the latter decomposed would be immense, and the quantity of hydrogen emitted equally so. Now as this hydrogen comes to the surface in union with sulphur, the amount of the latter concerned in the process must have been greater than can well be conceived. Moreover sulphur cannot be united to hydrogen by direct chemical means, and lastly, according to this hypothesis, the disengagement of sulphuretted hydrogen ought to precede the emission of lava, instead of following it, as is more commonly the case.

Reply.— The first part of the objection has been already met. It is not necessary to suppose that the whole of the sulphur finds its way to the surface, since sulphurous acid and sulphuretted hydrogen might be formed at the same time, and thus, by the mutual action of these bodies one upon the other, water might be reproduced and sulphur deposited.

With regard to the doubt expressed as to the possibility of an union of sulphur with hydrogen taking place, is not the latter, I would ask, at the instant of its disengagement, in precisely the same nascent condition, as when sulphuric acid liberates it from a moistened metallic sulphuret ? Is it not even possible that the alkaline or earthy metalloids may exist as sulphurets in the interior of the earth? With regard to the last point, as to the order of succession in which the phænomena take place, there seems no necessity for supposing the lava emitted to be exactly the same as that which has just been generated, nor do I know but that sulphuretted hydrogen may exist among the gaseous products by means of which the stones and ashes which precede an eruption are ejected from the crater.

But, after all, I am quite willing to admit that difficulties, to which none but conjectural answers can be returned, may be started to this as well as to every other hypothesis. If what we propose be an approximation to the truth, it is as much as can be expected, and the only fair way of estimating its claims, is to compare the difficulties that beset it with those which may be alleged against any other rival explanation.

Now the presence of an unlimited supply of organic matter for ages in contact with substances undergoing intense heat-the evolution of so much sulphuretted hydrogen without decomposition of water-the constant disengagement of nitrogen without any mode of accounting for the simultaneous abstraction of oxygen—the production of heat being promoted, instead of being checked, by the access of water—and other particulars which will easily occur to my readers—undoubtedly require an explanation before we can consent to espouse the opposite theory.

Sixth Objection. There could not be chlorine enough evolved to unite

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with so enormous a quantity of hydrogen as that which my hypothesis assumes; nor could the latter unite with oxygen, without occasioning such a consumption of that element as would render the air in the immediate neighbourhood of the volcano unrespirable.

Reply.—The muriatic acid evolved is a sufficient index of the amount of chlorine which has combined with hydrogen. Estimated by this standard, its volume must be considerable, as may be understood from the copious evolution of muriatic acid during volcanic eruptions, but no one has ever contended that it was commensurate with the amount of hydrogen evolved. The remainder of the latter either comes to the surface as sulphuretted hydrogen, or unites with oxygen derived both from the air which enters the interior of the earth, and from the sulphurous acid at the same time generated, thus contributing to reproduce an equivalent amount of water.

Seventh Objection.—Lava, when freshly emitted, and in a liquid or ignited state, contains no traces of the bases of the earths and alkalies, or even of metallic iron.

Reply. With all due deference to the excellent chemists who have originated this objection, I am at a loss to conceive how potassium, sodium, calcium, &c., could be expected to occur in the midst of a lava-current. That which I examined in 1834 was evolving at the very time water and muriatic acid. How could such combustibles exist in contact with these oxidizing agents ? Even metallic iron, under such circumstances, could scarcely be found; but, on the other hand, the presence in volcanic products of this same metal in the state of magnetic iron instead of a peroxide, conveys to my mind a strong presumption, that hydrogen has exerted that deoxidizing influence which Sir Humphry Davy and Professor Bischof demand as indications of its presence.

In reply to Dr. Bischof's concluding remarks, as to whether the defenders of the chemical theory suppose granite, trachyte, basalt, porphyry, to have all resulted from the same chemical processes which they imagine to be going on in volcanos at present, I may refer to my 40th chapter both for a reply, and for the grounds on which the affirmative of this question may be supported. I would moreover ask, whether, if it be true that the gases observed to issue from a volcano during the several phases of its action, confessedly imply the existence at the present time of chemical processes of such a nature as, if carried on on a sufficiently extensive scale, would account, both for the heat of the globe, and for the various volcanic phænomena which the earth presents, it be not more philosophical to regard these processes as the main instruments of the changes brought about, than to ascribe the eruptions of the volcano to one set of causes, and the gases, &c. which accompany or rather follow after them, to another independent series of operations ?

Page 658.

On the Tertiary Lavas. M. Menard de Groye, in his account of the volcano of Beaulieu, seems to have adopted in some degree the same ideas with those expressed above, with respect to the cause of the differences between basalt and lava. They

are, he says, precisely such as we might expect, from the one being submarine, the other produced in the open air :

“L'air de vétusté, qui se voit empreint, si l'on peut dire, sur toutes ces terrains trappéens; la destruction de tout cratère, s'il y en eut parmi eux; cette stratification, qu'on leur assigne pour ordinaire, mais que je n'ai presque jamais bien reconnue ; leur alternation, observée en Saxe, dans le Vicentin, dans le Derbyshire et ailleurs, avec des couches de sable, de pierre calcaire, et ces corps hétérogènes coupées même quelquefois dans le basalt et dans la wacke; la fragilité qui se fait remarquer généralement dans les matières trappéens; leur état plus crystallin ou du moins plus grenu; la stratification très rare parmi elles; toutes ces singularités si inexplicables dans d'autres hypothèses deviennent faciles à concevoir, et à expliquer, dans celle que nous proposons.”—Menard de Groye, Journ. de Physique, vol. lxxxii.

He then goes on to consider the third class, namely that of incomplete immersion, when a volcano is bathed in water at its base, whilst its summit is elevated above the waters, in which class he places those which I have called Tertiary volcanic rocks, such as Beaulieu, Vicentin, Meisner, &c.

But I cannot agree with this author in attributing the formation of columnar basalts to their being elevated above the waters ; nor can I admit the fact to which he appeals, namely that the basalt at Beaulieu passes into greenstone in the lower part, as a proof that the former was ejected above, the latter below, the level of the then existing water. Such an idea accords indeed very well with what I have elsewhere said with regard to the dependence of crystalline arrangement upon pressure, but unfortunately it happens that the basalt is seen quite as frequently passing into greenstone in its upper as in its under part, as is the case in the very instance of the Meisner, to which Menard alludes.

I believe I have stated fairly the theory proposed by M. Menard de Groye, to explain the distinction between basalt and lava, and have enabled my readers to judge how far my ideas have been anticipated by this author ; for although I did not read his memoir until after my treatise was composed, he has of course a fair claim to priority, where the opinions are the

It will be seen however, that though Menard finds himself obliged (as I conceive every geologist will be, who examines with attention the tertiary volcanic rocks) to make a distinct class of them, yet he does not explain the cause of their differences on the same principles as myself. According to him, the differences arise from their being formed partly above, partly below the surface of water; an explanation which may indeed account for some of the differences, but does not appear to me altogether to supersede the necessity of imagining that they have been in certain instances formed under a body of water less considerable than the ocean. With respect to basalt, his views are diametrically opposite to mine, as he considers it to be so formed in consequence of the absence of water, whereas I have explained its compactness from its being produced underneath that fluid. Indeed M. Menard does not appear in any part of his memoir to allude to the difference in the state of compression produced by the presence or absence of water.

Dr. Boué, in his memoir on Germany, has also alluded to a similar

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distinction (Journ. de Phys. vol. xcv.) of volcanic products, which he divides into those caused by volcanos burning in the open air, and by the same more or less submarine, or burning under water.

Under the head of those partially submarine he includes trachytes and many

basalts, and this division obviously corresponds with my second class of tertiary volcanic products. The rocks included by Dr. Boué and by myself are nearly the same.

Dr. Boué has also noticed in common with myself the following distinctions between submarine and subaërial volcanos : viz. that they originate from dykes, form mountains of inferior height, and are associated with tuffs possessing a strong degree of aggregation. He also remarks the greater frequency of crystalline infiltrations, and the more decided changes effected upon the surface of the ambiguous rocks. It is satisfactory to find my observations thus confirmed, or rather anticipated, as it proves that they have not been imagined for the sake of propping up an hypothesis, but that the principles laid down by Sir J. Hall admit of a more extended application than appears, so far as we can collect from his writings, to have been anticipated by their author.

Page 665. On the Oscillations of Opinion with respect to the Origin of Trap. That I am warranted in speaking as I have done of the oscillations of opinion which have prevailed respecting the origin of basalt, will be evident from the following passage in Daubuisson's account of the basalts of Saxony, translated by Neill :-" It appears,” says the celebrated chemist of Berlin *, “that naturalists are recovering by degrees from the volcanic illusion. It is about fifty years since a French naturalist revived the opinion concerning the volcanic origin of basalt, and he lived to see almost all Europe adopt his sentiments. Bergman, the first of the chemists who employed himself with diligence and success in examining mineral substances, and who, to an intimate acquaintance with the effects of heat, joined an extensive knowledge of mineralogy, could not bring himself to consider basalt as a product of volcanic eruptions. The Swedes adopted his view of the question. It is scarce forty years since everybody in Germany considered basaltic mountains as ancient volcanos. Werner lifted the neptunian standard; and now, among all the German mineralogists of any reputation, I know but one (Voigt) who still retains the old doctrine. We have already seen in how decisive a manner Klaproth has pronounced on the subject : he, of all the German chemists, has had most opportunities of observing the effects of fire on mineral substances, and he has besides studied the history of basaltic mountains with that correctness for which he is remarkable. In Ireland Mr. Kirwan was a supporter of the volcanic doctrine; but the numerous chemical experiments which he made on minerals, and other considerations, led him to a change. Dr. Mitchell, one of the very best mineralogists, Mr. Jameson, the author of the Mineralogical Travels in Scotland, and the greater part of British naturalists, consider basalt as having been produced in the humid way.”

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