influences on numerous cupriferous veins, whose development is also subordinate to them; 2d, Containing accidentally oligistic iron enclosed in globular shapes in their masses, and sulphuret of nickel disseminated in contemporaneous crystals; 3d, Presenting relations of contact with the multiplied repositories of oligistic iron.

The schalstein of Dillenburg, the blatterstein of the Harz, and the gabbro of Italy, rocks which we have assimilated, as resulting, the whole three, from metamorphic influences developed at the point of contact with the masses of trap, have a common character of the most striking kind, which is the strong red colour they impart to a great portion of the rocks they contain. In their normal state, these rocks are green, and exhibit, in a more or less distinct form, the characters of the trap masses to which they happen to be subordinate, establishing the passage between the eruptive rock and the upraised stratified rocks. The proportion of protoxide of iron which they contain in this normal state, prevents us supposing the reddening here to be the result of a simple superoxidation of pre-existing iron; the iron is superadded, and in such quantity that the rocks are connected by transitions and relations of contact to concentrations of pure ores.

In order to account for the reddening of these metamorphic rocks, we have, therefore, strong reasons to admit the same theoretic explanation as for the generation of the subordinate ores; and this explanation necessarily extends to the simple reddened clay-slate of Dillenburg, the red flinty slate of the Harz, the galestri of Tuscany, and those red jaspers which the Italian peasants so expressively name mattoni (bricks). Now, in the present state of our geological knowledge, we can only ascribe this generation of oligistic iron to subterranean emanations; these emanations have followed the outbursting of the trap-rocks, since we find the products of them in certain veins which intersect the traps; they are, therefore, to the trap rocks, what the products of Solfataras now are to the volcanoes of the present period.

Concentrations of oligistic iron in highly crystalline repositories such as that of Rio in the island of Elba, leave no doubt upon the mind. We can conceive the posterior arrival

of these ores, under a form sufficiently subtile to penetrate into all the fissures of a formation, to saturate all the mineral mass, and become insulated in wide veins. This Rio repository bears, indeed, all the characters of a slow generation, by the prolonged action of vapours analogous to those which bring the oligistic iron into the craters of volcanoes. The lustre, the geodes incrusted with crystals, the perfect isolation of the crystals of pyrites which formed special groups, and the corrosion of these pyrites, which are often transformed into oligistic iron; all these details seem to combine in indicating the prolonged action of metalliferous vapours. We see that, in many cases, the oligistic iron; when in the micaceous form which, under the hammer, affords a light and brilliant powder, is posterior to the oligistic iron, which is compact or in binoternary crystals. Is not this same subterranean action further evident in the semi-crystalline repository of Framont, which has produced the repositories of the Harz and Nassau, which differ from it only in their less crystalline nature? Ought not the lithoid oligistic iron, which impregnates the schalstein, blatterstein, and gabbro, be ascribed to the same causes, which are here marked by the same conditions of position, and finally the red hue of the stratified rocks, such as the red clay-slate, the gallestri, mattoni, &c.

By generalising this theory, we shall be led to even more extended conclusions. In certain sedimentary formations, we find earthy oligistic irons concentrated or disseminated in the red-coloured rocks. Formations of the old and new red sandstone, the sandstone of the Vosges, the varied coloured marls, and generally the gypseous and saliferous marls of the secondary or tertiary formations, present us with numerous and well-developed examples, either of the general or partial coloration of deposits by oligistic iron. Among these deposits we find beds of concentrated ores, compact or oolitic (Lavoulte, Laverpillère, Privas, &c.,) and in these beds shells, themselves transformed into compact or even crystalline ores.

What are the phenomena which could have accumulated in particular beds, or disseminated, through entire formations,

such considerable quantities of peroxide of anhydrous iron; while we cannot well conceive the iron deposited from waters, in any other state than that of hydrated peroxide? When we examine the immense quantity of oligistic iron disseminated through red-coloured arenaceous formations, we can form only two hypotheses; either this mass of peroxide has been derived, like the other arenaceous elements, from the pre-existing rocks; or, it has been superadded, by means of special phenomena, in those same basons where the sedimentation took place. The first of these suppositions is scarcely admissible; and we are led, by every thing that has been previously said, to have recourse to the phenomena of subterranean emanations, contemporary with the deposits, and mingling their products with those of the sedimentation.

In support of this hypothesis, we may mention the remark made by M. Elie de Beaumont, that the presence of stratified dolomite, gypsum, anhydrite, and rock-salt, almost always concurs with the red colour of the deposits. Now, all have nearly agreed in regarding all these substances as originating in metamorphic actions contemporaneous with the deposits in which they are formed.

Thus, throughout the whole duration of geological times, the interior of the globe should appear to us as a centre of continuous emanations, which have sent enormous masses of iron to the surface; these emanations mingling their anhydrous products, sometimes with those of sedimentation, at other times interposing themselves under the form of concentrated repositories, in the rocks elevated by the eruptive masses.-Amédée Burat.*

On an Equation between the Temperature and the Maximum Elasticity of Steam and other Vapours. By WILLIAM JOHN MACQUORN RANKINE, Civil Engineer. (With a Plate.) Communicated by the Author.

In the course of a series of investigations founded on a peculiar hypothesis respecting the molecular constitution of matter, I have obtained, among other results, an equation giving a very close approximation to the maximum elasticity of vapour in contact with its liquid at all temperatures that usually occur.

As this equation is easy and expeditious in calculation, gives accurate numerical results, and is likely to be practically useful, I proceed at once to make it known, without waiting until I have reduced the theoretical researches, of which it is a consequence, to a form fit for publication. The equation is as follows:

Where P represents the maximum pressure of a vapour in contact with its liquid :

t, the temperature, measured on the air-thermometer, from a point which may be called the ABSOLUTE ZERO, and which is

274° 6 of the centigrade scale below the freezing point of water.

462° 28 of Fahrenheit's scale below the ordinary zero of that scale, supposing the boiling point to have been adjusted under a pressure of 29.922 inches of mercury, so that 180° of Fahrenheit may be exactly equal to 100 centigrade degrees.

461°.93 below the ordinary zero of Fahrenheit's scale, when the boiling point has been adjusted under a pressure of 30 inches of mercury, 180° of Fahrenheit being then equal to 100° 0735 of the centigrade scale.

The form of the equation has been given by theory; but three constants, represented by a, B, and y, have to be determined for each fluid by experiment.

The inverse formula, for finding the temperature from the pressure, is of course

It is obvious that for the determination of the three constants, it is sufficient to know accurately the pressures corresponding to three temperatures; and that the calculation will be facilitated if the reciprocals of those temperatures, as measured from the absolute zero, are in arithmetical progression.

In order to calculate the values of the three constants, for the vapour of water, the following data have been taken from M. Regnault's experiments :

These data give the following results for the vapour of water, the pressures being expressed in millimètres of mer