Considerations on Volcanos, the probable causes of the phenomena, the laws which determine their march, the disposition of their products, and their connexion with the present state and past history of the Globe; leading to the establishment of a new theory of the earth

Preface

. The object of the following Essay is to throw some light on those phenomena which consist in the development of subterranean activity in the form of Volcanos and Earthquakes, the investigation of which appears to me of primary importance to the progress of Geological science. The science of the History ofthe Globe has, I conceive, for its object — the examination of the nature of the in organic substances observable within the limits of our planet and its atmosphere, with a view to discover its origin and the changes it has undergone. It is thus naturally divided into two branches, viz. I. The examination of these substances and their mutual relations. II. The establishing conclusions from the results of this examination, as to their origin, and the changes they have undergone. The first division is properly called Geognosy.

The term Geology should of right be restricted to the second. Geognosy embraces the subordinate sciences of Mine ralogy, Chemistry, and the study of the relative positions of Rocks (Geognosie positive of Humboldt.) a

PREFACE.

. Geology has for its business a knowledge of the pro cesses which are in continual or occasional operation within the limits of our planet, and the application of these laws to explain the appearances discovered by our Geognostical researches, so as from these materials to deduce conclusions as to the past history ofthe globe. The surface of the globe exposes to the eye of the Geognost abundant evidence of a variety of changes which appear to have succeeded one another during an incalcu lable lapse of time. These changes are chiefly, I. Variations of level between different constituent parts ofthe solid surface ofthe globe. II. The destruction of former rocks, and their reproduction under another form. III. The production of rocks de novo upon the earth's surface. Geologists have usually had recourse for the explana of these changes to the supposition of sundry violent and extraordinary catastrophes, cataclysms, or general revolu tions having occurred in the physical state of the earth's surface.

As the idea imparted by the term Cataclysm, Catastrophe, or Revolution, is extremely vague, and may comprehend any thing you choose to imagine, it answers for the time very well as an explanation ; that is, it stops further in quiry. But it has also the disadvantage of effectually stopping the advance of the science, by involving it in obscurity and confusion. If, however, in lieu of forming guesses as to what may have been the possible causes and nature of these changes,

PREFACE.

. V we pursue that which I conceive the only legitimate path of geological inquiry, and begin by examining the laws of nature which are actually in force, we cannot but perceive that numerous physical phenomena are going on at this moment on the surface of the globe, by which various changes are produced in its constitution and external characters ; changes extremely analogous to those of earlier date, whose nature is the main object of geological inquiry. These processes are principally, I. The Atmospheric phenomena. II. The laws ofthe circulation and residence of Water on the exterior ofthe globe. III. The action of Volcanos and Earthquakes. The changes effected before our eyes, by the operation of these causes, in the constitution of the crust of the earth are chiefly — I. The Destruction of Rocks.

II. The Reproduction of others. III. Changes of Level. IV. The Production of New Rocks from the interior of the globe upon its surface. Changes, which in their general characters bear so strong an analogy to those which are suspected to have occurred in the earlier ages of the world's history, that, until the processes which give rise to them have been maturely studied under every shape, and then applied with strict impartiality to explain the appearances in question ; and until, after along investigation, and with the most li beral allowances for all possible variations, and an unlimited series of ages, they have been found wholly inadequate VI

PREFACE.

. to the purpose, it would be the height of absurdity to have recourse to any gratuitous and unexampled hypo thesis for the solution of these analogous facts. The study of the processes by which these effects are at present produced on the surface of the globe forms there fore a most important, but unfortunately most neglected, branch of Geology. It comprehends the following divisions, viz. I. Meteorology, or the knowledge of the atmospheric phenomena, including the decomposition of rocks by the air and light, Electricity, Magnetism, &c. II. Hydrology, or the study of the laws that regulate the conduct of water on the surface of the globe, and its mechanical effects on the solid parts of that surface. III. That of the phenomena produced on the exterior of the globe by the development of its internal and subterranean activity.

Each of these is equally necessary for a due apprecia tion ofthe effects which such agents are capable of pro ducing, during an unlimited series of ages, on the constitution of the globe; and at the same time offers in itself a subject of extreme interest. The following work is an attempt to pursue the last branch of study ; and, however imperfect, I am not with out hope that it may be found useful in clearing up much of the obscurity and doubt which at present exists as to the nature and mode of action ofthe volcanic forces. The opportunities which I have enjoyed for the pro secution of this study were sufficiently favourable to claim for me some confidence in the general facts I advance on my own authority.

PREFACE.

. VII My attention was first directed to the phenomena of Volcanos in the winter of 1818, which I passed within sight of Vesuvius, then in permanent eruption. In the course ofthe next year I visited Ætna and the Lipari Isles, particularly Stromboli, where I spent some days. After having examined the phenomena of these active volcanos, I turned my attention to the extinct vents of Italy and France ; passed six months in Auvergne, the Velay, and Vivarais, contiuually occupied in geological researches, and afterwards revisited Vesuvius (which I reached just in time to witness the stupendous eruption of October 1822), examined the Ponza Isles, and all the different volcanic districts of Italy from thence to the Euganean Hills, returning to England by those ofthe Rhine, the North of Germany and the Eiffel.

In addition to these original sources of information, I have availed myself of many ofthe facts that are to be met with in the works of Humboldt, Von Buch, Beudant, Brieslak, Brocchi, Monticelli, De la Torre, Mackenzie, Bory de St. Vincent, and other writers who have published descriptions of phenomena or countries of a volcanic nature. The short sketch of a History of the Earth with which I have been tempted to terminate this essay, may probably be considered as premature ; but the conclusions to which I arrived in the body ofthe work, on the nature of the vol canic phenomena, unavoidably led to their generalization in this form ; and since the theory is confessedly imperfect, requiring much further development, room is left for the corrections and improvements that may be hereafter suggested by other researches, hydrological and meteorolo gical, which should perhaps have been made to precede even the rough draft of such a h:story. The Appendix will be found to contain a list of all the volcanos known to be at present in habitual activity, taken

PREFACE.

. VIII in great part from that published at Paris in the Annuaire du Bureau des Longitudes, for the year 1824; to which Messrs. Humboldt and De Buch contributed. To this list has been added brief accounts of some of their most remarkable eruptions and other contemporaneous phenomena, from the descriptions of eye-witnesses. An article is also appended to the work, on the remarkable volcanic eruption of Xorullo in 1756, in which I have taken the liberty to dispute the inferences drawn by M. de Humboldt from the traditionary account of the phenomena, and the appearances at present visible on the spot. I trust the theory I have ventured to put forth will be considered impartially and without prejudice; and, if so, I think it can hardly fail to satisfy a candid and enlightened reader. For myself, when the idea was first urged upon me by the same chain of inductive evidence which is given in the work, I was incredulous; but the more I investigated ils results, the more closely did they appear to correspond with, and explain the problematical appearances of the earth's crust, which have so long presented a stumbling block to Geology.

Just as these pages are going to press, I have met with the work of Mr. Knight on a new theory of the earth, which I had not previously known. I observe that some of his ideas as to the mechanical deposition of gneiss, mica-slate, &c, and the crystallization of rocks by the admixture of an aqueous vehicle in very small proportions, correspond with mine. We differ however toto cœlo on most points. He supposes mountains to have been origi nal protuberances ofthe granitic surface ofthe globe, and

PREFACE.

. IX does not admit of the subsequent elevation of their strata; and notwithstanding this, he conceives the strata hardened by heat. He imagines that the admission of water to the metallic nucleus of the globe produces volcanos, &c. ; ideas which are completely at variance with the opinions as to the nature and modes of operation of the subter ranean forces, developed in the following Essay. TABLE OF CONTENTS. -♦»■ CHAP. I._Page 1. Descriptive Account of the Volcanic Phenomena. §. 1. Introductory. — Definition of Volcano, and of Lava. §.2. Number of such vents known on the globe. Their real number probably much greater. Irregularly and generally dispersed over the globe's surface. §. 3. Their division into subaerial and subaqueous. §. 4. Subaerial eruptions ; distinction according as they take place either from a new or an habitual vent. §. 5.

Examples of the first kind — their rarity. §. 6. Habitual vents. §. 7. Apparent irregularity of their phenomena. §. 8. Their condition in one of the three phases — viz. of permanent eruption, of moderate activity, or of prolonged iutermitfences. §. 9. Phase of permanent eruption of rare occurrence — Examples — Stromboli, Nicaragua, &c. §. 10. Phase of moderate activity, of frequent occurrence — Examples from Vesuvius, Ætna, volcanos of the Pacific, Mexico, and Quito, &c. §. 11. Phase of prolonged intermittences — most general condition of habitual vents. §. 12. Uniformity of its phenomena. — Detailed description of them, under their most ordinary characters. — Examples of paroxysmal eruptions. CONTENTS. §. 13. Generally succeeded by quiescent intervals, during which the crater often becomes a solfatara. §. 14.

Instances of volcanos in the third phase. §. 15. Occasional passages from one phase into another. CHAP. H.— Page 17. Fxplanation ofthe Volcanic Phenomena. _. 1. Their moving power, the expansive force of aeriform fluids generated by the action of heat in a subterranean body of lava. §. 2. The phenomena of Stromboli, Ætna, the Isle de Bourbon, &c. and the constant expulsion of liquefied fragments (scoriæ) prove the existence and ebullition of such a mass. _. 3. Its liquidity very imperfect, not amounting to fusion. Appearance of lava when proceeding from a source in the flank of a volcano during some tranquil eruption — radiates very little heat. _. 4. Opinion of Dolomieu confirmed as to its liquidity being caused by some other circumstance than fusion, from its crystalline texture both when suddenly and slowly cooled. §. 5.

Nature of its liquidity. The suspension of its crystalline particles in a fluid. §. 6. Aeriform, the same which explodes from the mouth of the vent, and elevates the lava by its expansive force ; aqueous vapour or steam ; proved to be this by direct experiment, and by the torrents of rain which fall from the clouds evolved when a volcano is in eruption — the escape of this vapour causes the instantaneous consolidation of the lava. §. 7. Recapitulation. §. 8. Examination of the effect which must be produced in a crystalline substance of this nature, by diminution of pressure or increase of temperature ; partial vaporization of the water ; disaggregation ; intumescence, and liquefaction of the mass — vapour collects into parcels and bubbles, and finally rises to the surface — condition of such rise.

CONTENTS. XIII _. 9. Comminution of grain, by intumescence, or disintegration. §. 10. Effect produced by a diminution of temperature, or increase of pressure, considered. §.11. Recapitulation. §. 12. Application of these laws to a mass of lava confined in a solid state, and at an elevated temperature, beneath the outer crust of the earth. §. 13. Hypothesis as to the continual accession of caloric from below to such a mass, rendered probable by observations in mines, &c. The secondary and schistose rocks inferior conductors of heat to the crystalline and granitoidal. §. 14. Hence, the caloric will be concentrated in these lower beds, and continually augment their temperature, and with it their expansive force. The overlying rocks will at length necessarily yield to this force. §. 15. At this time the expansive force will be greatest in the lowest part of the crystalline bed.

The upper parts will be therefore raised en masse, and in a solid state. Rupture and dislocation attendant on this forcible elvation, occasioning earthquakes. §. 16. Fissures formed, some opening outwardly, some downwards towards the heated lava. The latter allow of its expansion and liquefaction, by which the fissure is filled with intumescent lava. This is the case both with the fissures formed in the upper strata of the heated crystalline mass, (contemporaneous veins) and in those broken through the overlying strata of schistose, sedimentary, or other pre-existing rocks. (Injected veins or dykes.) Immediate consolidation of the matter filling these veins, by which the fractured rocks are repaired and strengthened. §. 17. An interval of tranquillity succeeds, until a similar expansion again occurs.

These processes must succeed one another continually in such circumstances. §. 18. At length, perhaps, some fissure broken entirety through the overtying rocks so as to open directly into the atmosphere alone. — Consequent ebullition and intumescence of lava up this opening — circumstances which accompany and modify its rise. — Sometimes checked by the predominance of the repressive force before the lava reaches the aperture, so as to occasion an abortive eruption, vapour alone being discharged from the fissure. CONTENTS. But if the width of the fissure and other' circumstances permit it, the lava reaches the mouth of this orifice, and a volcanic eruption commences. §. 19. §.20. Examination of the laws that regulate the development of the eruptive force; moderated by the repressive force, con sisting of, 1. the supported column of liquid lava ; 2. the reaction of the vapour generated from the impediments to its expansion ; 3. the external pressure on the surface of the intumescent lava.

Laws deduced from hence by which the energy and duration of an eruption are determined. §.21. §.22 Causes which usually produce a variation in the energy of the eruption. The enlargement of the vent at first augments it ; but the quantity of matter protruded tends to obstruct the vent, and check the eruption. General law, that the development of volcanic action universally tends to its own extinction, by augmenting the opposite force of repression. — 1. By increasing the length of the column of liquid lava pressing on the focus. 2. By the weight and consolidation of the lava produced outwardly en masse. 3. By the accumulation of fragmentary ejections within and around the vent. From these causes the repressive force at length regains the predominance over that of ex pansion, the crisis of the eruption is past, its violence diminishes progressively, and it is at length wholly checked.

Escape of the dilated matters on the surface of the globe. §.23. §.24. Lava must ahvays rise en masse to within a short distance of the mouth of the vent. §.25. Phenomena resulting from these laws as apparent on the surface of the globe, or volcanic eruption. Commence with earthquakes — after which, either lava alone, or elastic fluids alone, or both together, are expelled from the vent. The two first cases rare, the last most frequent. Explosions of vapour and fragmentary ejections — emissions of lava en masse — crisis of eruption — 'gradual decrease in violence, lava ceases to flow, scoriæ continue to be ejected, oscillations in the eruptive force — trituration of fragments and scoriæ — expansive force finally stifled — eruption ter minates. — The repressive force has regained the predomi nance and continues to increase. §.26 §. 27 Condition of the dilated mass below, or focus, at this time.

CONTENTS, XV Its temperature inferior to that ofthe surrounding crystalline mass. — It abstracts therefore caloric from thence. §.28 §. 29, If this accession of caloric keeps pace exactly with the increase of the repressive force, the eruption is perma nent. §. 30. If not, the continual increase ofthe expansive force in the lower parts of the crystalline bed, resolidifies the upper parts, and seals up the vent. But the expansive force of the focus continues to increase, and perhaps at length overcomes the resistances opposed to it. If it break out repeatedly in the same direction, it produces an habitual volcano, and finally a volcanic mountain. If the repressive force prevail till the focus is equalized in temperature to the stratum in which it lies, it shares in the general expansive force of that stratum. §. 31 §.32 This is continually increasing, and must at length find a vent.

Generally on the same spot as before, and hence the frequency of habitual volcanos. §.33, §. 34 If on a fresh point, probably on the continuation of the original fissure. §.35, Distance of new from old vent dependent on local circumstances. §.36 General remarks on the struggle kept up between the op posite forces of expansion and repression, beneath every habitual volcano — where they are in equilibrio the volcano is in 1st phase — where they oscillate frequently about an equilibrium, in 2nd — where the oscillations are on a large scale, in the 3rd. §. 37. Passages of these phases into one another ; they may co exist in the same volcano. Consideration of the circumstances which determine the mutual relations of these opposite forces. — Conduct of the expansive force in any individual lava bed, uniform. — The irregularities must therefore proceed from changes in the repressive force, and first, for the examples of complete equilibrium. — Stromboli. — The peculiar form of its crater alone occasions the permanence of its phenomena. — Vol cano of Bourbon, its figure. — Lava boils over the lips of the crater down the slope of the cone. §. 38.

Interesting comparison of the phenomena of Stromboli and Bourbon. §. 39. CONTENTS. XVI §. 40. The duration of the intermittences of a volcano is determined by the size and form of the crater. §. 41. Influence of atmospheric changes on the eruptions of a permanently active volcano ; confirmed by observation. §. 42. Similar influence exerted in the production of earthquakes. §. 43. Conclusion, that the quiescent intervals of volcanos are occasioned by the obstruction of vents by the matters produced, and but for this, the eruptions would be all tranquil and permanent. §. 44. But in conformity with the law announced above, the greater number of vents obstruct themselves, and force the caloric to accumulate below till it break out in a fresh direction. §. 45. Circumstances which produce the shifting of the draught of caloric from one vent to another — the focus abstracts caloric from its enclosing walls, and from the lower parts of the lava-bed — neighbouring vents mutually retard the activity of each other. — The activity of one may cause the extinction of another. — Other modes of escape for caloric, thermal springs, &c. §. 46.

So long as the caloric passes off in these modes, in the ratio in which it is received below, the general expansive force remains invariable. If not, the force increases, and must at length gain the predominance over the repressive forces, and produce elevations of superficial strata, earthquakes, &c. Distinction between the general or primary, and local or secondary, expansive forces — the activity of one of these forces inversely proportionate to that of the other. §. 47. The laws thus determined hold good, whatever the scale of magnitude of the phenomena they give rise to — whether the elevation of continents or of a few square yards of rock a quiescence of centuries, or of a few minutes, &c. CONTENTS. XVII CHAP. III.— Page 67. Disposition of Volcanic Products on the Surface ofthe Globe. §. 1.

Circumstances which occasion a subaerial eruption. §. 2. Explosive projections of scoriæ and fragments, which falling accumulate into a cone, around the orifice of the Volcano — Laws which determine its figure, dimensions, crater, ridge. §.3. Disturbing causes, viz. J. Unequal horizontal dimensions of vent — more than one cone produced from the same fissure — their interference. — Examples from Auvergne, &c. — 2. Inequalities of pre-existing surface. 3. Violent or prevailing winds — instance the Antilles. 4. Solid obstacles — instance Puy Chopine. 5. Contemporaneous emission of lava from the same vent — Its weight and cohesion on congelation often, 1. prevent any subsequent explosions, or, 2. force their escape by a fresh vent — If not, the residuum of lava is hollowed out. — Examples, &c. §. 4.

Composition of a simple volcanic cone. — Nature of composing fragments. 1. Scoriæ — their shape and structure, and laws which determine them — difference of feldspathic and basaltic scoriæ accounted for — Their different forms and sizes. 2. Fragments of pre-existing rocks, of all sorts and ages. — Their alteration ; decomposition, and recomposition of minerals in them. — Trituration of these fragments. — Internal explosive energy by which they are projected, accounted for. §. 5. Structure of cone. — Stratification — disposition of strata by which they may be recognised even when much degraded. — Structure influenced by the contemporaneous emission of lava. b CONTENTS. CHAP. IV.— Page 85. Laws ofthe Protrusion and Disposition of Lava, en masse, on the Surface ofthe Earth. §. 1. Preliminary notice on mineral nature of lavas — and difference of specific gravity and texture, by which their fluidity is determined. §. 2.

Its protrusion and congelation of surface — depends on the size of the grain. §. 3. Disposition of lava determined by, 1. the force of expulsion. 2. Its fluidity. 3. The external circumstances that may modify the permanence of its fluidity. — Consideration of these circumstances. — Effect of obstacles in checking the course of a lava current. — According to these laws lava is dispersed either in sheets, streams, hummocks or domes. §. 4. Examples adduced. — Remarkable difference generally observable in the disposition of the trachytic and ferruginous lavas. — Its cause, confirms our hypothesis on the nature of the fluidity of lava. §. 5. Procedure of lava poured on the earth's surface continued — occupies fissures — advances by its own gravity independently of the expulsive force — arched gutters formed by its subsidence. — Examples from Ætna and Bourbon — great effect produced on a current by meeting with obstacles comparatively insignificant; difficulty with which it swerves from a straight line — reasons for this — crystals slide over one another with ease, in the direction of their longest axis, hence the greater fluidity of clinkstone lavas in that direction alone — curious procedure of lava when it meets with a perpendicular obstacle, as a wall, &c— Explanation of this — its effect on dry grass, shrubs, trees, and fragments of other rocks ; on marshy ground ; when it enters the sea or any body of water, it progresses more slowly below water, but in a similar manner. — Water heated by it. — Destruction of fish. — Monte Bolca.

CONTENTS. XIX CHAP. V.— Page 110. Consolidation of Lavas. §. 1. Is effected either by the condensation or escape of its fluid vehicle. §. 2. The condensation takes place either by increased pressure or diminished temperature. — Distinction between the results of the condensation of the fixed and free vapour, accompanied by a partial recrystallization, which is facilitated by slowness of condensation. — Laminar arrangement of such crystals in planes perpendicular to the pressure. §. 3. Those vapours only will be condensed which cannot effect their entire escape: this cannot take place in some instances, as in dykes, &c. But where the lava is exposed to contact with air or water, it can and does, in one or both of two modes. 1. By ascent in bubbles through the liquid lava. The more fine-grained the lava, the more spherical the bubbles; their elongation, compression, and vertical extension — enlargement : velocity of their ascent determined, — proportion of vapour which escapes to that which remains enclosed.

Of the latter, part escrpes in the 2nd mode, viz. by percolation through the pores and crevices of the already solid exterior. Laws which determine the porosity of lava, varies directly with the average size and irregular arrangement of the crystalline particles. This process proceeds from without inwardly, with a rapidity proportioned to the porosity ofthe rock. — Recapitulation. — Compactness of a lava-rock varies inversely with the proportion of vapour which effected its escape by percolation — this proportion varies directly with the size and irregularity ofthe particles ; that which escapes in bubbles, with the specific gravity of the lava, ceteris paribus ; that which remains, and is condensed, directly with the size and conformable arrangement of the particles, and inversely with their specific gravity. §. 4.

Propositions as to the conduct of lava protruded on the earth. I. If of extremely fine grain, and low specific gravity ; its superficial congelation is rapid, its internal slow ; great fluidity, spherical air-bubbles, sometimes elongated horizontally or vertically ; crystals the same — examples — the scoriæ of such lavas is pumice — concretionary process b XX CONTENTS. which accompanies its slow solidification — pearlstones, radiated or not ; variolites. These concretionary parts elongated in the direction of the movement ; their subsequent disintegration by diminished pressure, produces veined or marbled rocks — examples. §, 5. — II. A higher specific gravity with a fine grain increases the fluidity of the lava, and its extent of lateral spread ; bubbles rise with greater force ; rude and ragged surfaces of such lava currents. — Spherical bubbles confined beneath this surface — cavernous blisters — glassy coating of these and the surfaces, pseudo-stalactitic projections — analogy to the blisters of loaves — lower part of the current propoitionately compact ; subsequent denudation of the upper and scoriform parts in ancient currents. §.

C. — III. A high specific gravity with a coarse grain, occasions low fluidity, and increased bulk — a porous texture — great contraction, and wide fissures of retreat — shattered and fragmentary surface of such streams — examples. Irregular and often angular, cellular cavities of such lavas — examples. §. 7. — IV. A lower specific gravity, with a coarse grain, wholly prevents the vapour from uniting into bubbles — extreme porosity of such lavas — great bulk of currents — examples, trachytes, lava sperone, &c. occasional concretionary separation of parts ; Piperno of Naples, &c. — elongation of these parts in direction of motion. §. 8. — V. A low specific gravity, with a still coarser grain, may produce rather a compactness of texture, very little of the vapour being free, and the greater part therefore condensed — examples from large grained trachytes, syenites, &c. If crystals non-conformably arranged, the fluidity is at its minimum. — If conformably, it maybe considerable in the direction ofthe plane surfaces of the crystals. §. 9. — VI.

When little disintegration has taken place, and the lava intumesces only by the expansion of its fixed vapour, by the condensation of this it returns to its former compactness, as may have been the case with syenite, granite, &c. §. 10. The aqueous vapours that escape from a lava as it consolidates are accompanied by mineral substances, which become more abundant as the lava cools, and the quantity of aqcous vapour exhaled diminishes. Deposition of these substances at the opening ofthe fissures; their nature. — Difficulty of distinguishing those which are formed in situ from CONTENTS. XXI those which are merely volatilized — Decomposition of some of the ingredients of lava by its intense heat, as the elasticity of the contained vapour diminishes.— Specular iron, a pro duct of volatilization — other crystallizations formed in the same manner — others deposited from water in the pores or vesicles of the rock, zeolite family, amygdaloidal kernels, agates, &c. — enhydrous — effects of infiltration — changes produced on lava-rocks by acid vapours, particularly the sulphuric.

Aluminite. Solfataras, why common to volcanic craters in a state of quiescence. §. 11. Origin of thermal springs from the condensation of the vapours escaping from a subterranean mass of lava. — Eruptions may be obviated, and the temperature of the focus diminished, or rendered uniform, by this placid escape of caloric. — Mineral substances held in solution, and afterwards deposited by them. — Intermittent hot-springs — Geysers.— Formation of thermal springs by atmospheric water filtering through heated lava. — Jorullo. — Bertrich-bad. §. 12. Permanent gasses evolved from lavas — mephitic exhalations — Lago d'Averno. §. 13. Duration of the process of consolidation — circumstances that determine it. — Examples of the time during which lavas may retain a high internal temperature. CHAP. VI.— Page 134.