have taken place, according to some authors, in the tertiary epoch? This is a point we cannot now decide. However this may be, it is not less true that the eruptions which have furnished the materials of the Taviglianaz sandstones, may have been accompanied with disengagements of sulphurous acid and hydrochloric acid, and exercised an influence on the formation of the dolomites of Savoy analogous to that which the pyroxenic eruptions of the Tyrol have exerted on the origin of the dolomite of that country.

On the Colour of Water. By Professor BUNSEN.

The hot springs which occur in many parts of Iceland, and are especially remarkable at Reykir, are, says that excellent observer Bunsen, characterised by extreme beauty. In the depths of the clear unruffled blue waters of these basins, from which rises a light vapour, the dark outlines of what once formed the mouth of a Geyser may be faintly traced amid the fantastic forms of the white stalactic walls. Nowhere can the beautiful greenish-blue tint of water be seen in greater purity than in these springs.

A few remarks on the causes from which they are derived will hardly be superfluous.

Chemically-pure water is not colourless, as is usually supposed, but naturally possesses a pure bluish tint, which is only rendered visible to the eye when the light penetrates through a stratum of water of considerable depth. That such is the fact, may easily be shewn by taking a glass tube, two inches wide and two yards long, which has been blackened internally with lamp-black and wax to within half an inch of the end, the latter being closed by a cork. Throw a few pieces of white porcelain into this tube, which, after being filled with chemically-pure water, must be set vertically on a white plate, and looking through the column of water (of two yards) at the pieces of porcelain, which can only be illumined from below by white light, we shall observe that the objects will, under these circumstances, acquire a pure blue tint, the in

tensity of which will diminish in proportion as the column of water is shortened, so that the shade of colour becomes at length too faint to be perceived. This blue coloration may also be recognised when a white object is illuminated through the column of water by sunlight, and seen at the bottom of the tube through a small lateral opening in the black coating. The blue tint so frequently observed in water cannot, therefore, be regarded as in any way strange. The question therefore arises, why this blue colour is not seen everywhere, and why it should not occur in many seas? Why, for instance, the lakes of Switzerland, the waters of the Geysers in Iceland, and in the South Sea Islands, should exhibit every shade of green, whilst the waters of the Mediterranean and Adriatic are occasionally of so deep a blue as to vie with indigo? These questions are easily answered, since clearness and depth are the primary, if not the sole requirements for imparting to water its natural colour. Where these fail, the blue tint will likewise be wanting. The smallest quantity of coloured elements which the water may take from the sand or mud of its bottom, the smallest quantity of humus held in solution, the reflection of a dark and strongly-coloured bottom, are all sufficient to disguise or alter the colour of water. is well known, that the yellowish-red colour of the waters which traverse the lower group of the trias formations depends upon hydrated oxide of iron, contained in the mud of the variegated sandstone. From a similar cause, the vast glacier streams of Iceland, which, in these desolate regions where there are neither roads nor bridges, the traveller finds, to his discomfort, that he must ford, are rendered opaque and milk-white from the detritus of dark volcanic rocks, which, crushed into a white powder by the overwhelming mass of the descending glaciers, are carried to the sea, in the form of white mud and sand, and again deposited there in vast deltas.

It

In like manner, the natural colour of the small lakes in the marshy districts of northern Germany is concealed by the black tint imparted by the dissolved humus derived from the turf. These waters often appear brownish or black, like the water in most of the craters of the Eifel and Auvergne, where

the sombre volcanic rocks obstruct the reflection of the incident light. It will, therefore, easily be understood that it is only where these disturbing influences do not exist that the colour of the water will be seen in all its beauty. Amongst the places at which this requirement is most completely fulfilled, we may especially instance the Blue Grotto at Capri, in the Gulf of Naples. The sea is there most remarkably clear to a very great depth, so that the smallest objects may be distinctly seen on the light bottom at a depth of several hundred feet. All the light that enters the grotto, the entrance of which is only a few feet above the level of the sea, in the precipitous rock opening upon the surface of the water, must penetrate the whole depth of the sea, probably several hundred feet, before it can be reflected into the grotto from the clear bottom. The light acquires, by these means, so deep a blue colouration from the vast strata of water through which it has passed, that the dark walls of the cavern are illumined by a pure blue radiance, and the most differently coloured objects below the surface of the water are made to appear bright blue.

An equally remarkable example of this fact presents itself in the glaciers of Iceland, as well as in those of Switzerland, which shews that water does not lose its original colour even when in a solid condition. At the distance of many miles, the eye may distinguish, on the flat heights of the "Jokull," the boundaries that separate the bluish ice of the glaciers from the white inaccessible plains of snow that rise to the summit of these mountains. On a closer examination of these glaciers, one is surprised to observe the purity and transparency of the ice, which often appears to be wholly free in large masses from vesicles of air and foreign admixtures, whilst its vast fissures and cavities are coloured all shades, from the lightest to the darkest blue, according to the thickness of the strata through which the light has penetrated.

The blue tint of the cloudless and vapoury atmosphere is probably dependent on similar phenomena, if we are justified in concluding, from the colour of solid and fluid water, that aqueous vapour has a similar colour. On considering all these facts, we can scarcely doubt for a moment that the blue VOL. XLVII. NO. XCIII.-JULY 1849.

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colour of water is a peculiar and not accidental characteristic of that substance. This natural colour of water will also afford us an easy explanation of a light green tint which is even more strongly manifested in the crystal-like siliceous springs of Iceland than in the lakes of Switzerland; for the yellow colour derived from traces of the hydrated oxide of iron, in the siliceous sinter walls surrounding the water, blends with the original blue to produce the same greenish tint, which, in the Swiss lakes, is derived from the yellow bottom; the most different rocks experiencing a superficial decomposition from the continued action of water, and becoming tinged with yellow by the formation of hydrated oxide of iron. Hence it will be easily conceived that the blue, which continues to increase in intensity with the increased depth of the strata of water, may obliterate the action of this yellow reflection, and thus either weaken or wholly destroy this greenish tint. The green grotto on the shores of Capri affords a most striking proof of this fact. The green colour, which is produced by the reflection at an inconsiderable depth of water, from the yellowish limestone constituting the bottom and the walls of the grotto, illuminated by the light from without, wholly disappears in the enormous depths of the water of the blue grotto; there a pure blue colour takes the place of the green, observed in the shallower cavern, although the water and rocks are the same in both cases. *

Geological Changes from Alteration of the Earth's Axis of Rotation.t

Respecting a possible change of climate resulting from a change in the earth's axis of rotation,-an hypothesis which has from time to time engaged attention, as one which might serve to account for the occurrence of organic remains, supposed to be those of animals and plants requiring a higher temperature than that of the regions where such remains are found, we have had two communications. In one from Mr Saull, he calls attention to the undoubted evidences

* Vide Works of the Cavendish Society, vol. i., 1848.

↑ Sir H. J. De la Beche's (President of the Geological Society) Anniversary Address to the Society for 1849.

of the land being at intervals above and beneath the waters, and to changes of temperature over the same area. These effects he attributes to a change in the earth's axis of rotation, arising from astronomical causes, and describes the results which would follow from such conditions. As to the possibility of a change in the earth's axis of rotation, we had a paper from Sir John Lubbock, in which he first adverts to the revolution of a solid body on its principal axis, and its continuing to do so for ever, unless such solid body be acted upon by some extraneous force. He further observes that on this supposition no change of climate would obtain on any given latitude on the earth's surface, except from a change in internal temperature or the heat of the sun.

He then notices that a change of climate alone is not sufficient to account for geological changes, such as water covering a part of the earth's surface at one time and not at another: and remarks that the moon's attraction and the causes which produce the precession of the equinoxes do not modify these conclusions.

Sir John Lubbock then states, that "it is unlikely that when the earth was first set spinning, the axis of rotation should exactly coincide with the axis of figure, unless indeed it were all perfectly fluid.” He subsequently takes a period not so remote, when the earth, from the different fusibility of its component parts, might have been partly solid in irregular masses and partly fluid, and afterwards a still more advanced state, in which land and water irregularly occurred on its surface, suited to the existence of animal life, always supposing the axis of rotation not to coincide with the axis of figure. If any resistance exists, "the pole of the axis of rotation would describe a spiral round the axis of figure, until finally it would become, as at present, identical with it." Supposing a displacement of the axis, the movement of the water from one equator to another and the consequent changes of climate are pointed out. Glancing at friction on the surface of the earth rendering the invariability of geographical latitude, otherwise existing, not a necessary consequence, at our ignorance of the earth's structure beneath its crust,—and of the history of the changes effected during the process of cooling,-Sir John considers that "the utmost that can be accomplished by mathematics is to explain under what hypothesis a change of the position of the axis of rotation is possible or not." Adverting to the dictum of Laplace, that the changes on the earth's surface and in the relative positions of land and water cannot be accounted for by a change in the position of the axis of rotation, he observes that in this statement Laplace did not take into consideration either (1.), the dislocation of the strata by cooling, or (2.), the friction of the surface. Finally, our colleague, after admitting that if at any remote period the earth had been a homogeneous spheroid of any pure metal in a state of fusion, it would in cooling always rovolve about the principal axis of rotation, that of figure, considers that there is sufficient evi