have appeared equally among fishes. This was the capital point to establish. The fishes of the celebrated locality of Monte-Bolca had, it is true, been referred by Volta, without exception, to species actually living in the Mediterranean; but it was easy to see that the determinations of the author of the Ittiolitologia Veronese were not the result of sufficient study; many naturalists had pointed out his errors even by a simple comparison of the plates. In order to be perfectly certain in this respect, it was of importance to compare the originals themselves. This M. Agassiz has done, with the greatest detail, in the Museum of Paris, where the collection of Count Gazzola, and the greater part of the originals of Volta's work, are preserved. He was not long in discovering that all the species were new, and that about the half belonged even to extinct genera.

M. Agassiz has arrived at nearly the same results with regard to the fossil fishes of another deposit, equally celebrated, the species of which had also been regarded as identical with those of our own times; I mean the fishes of Oeningen. The formation of Oeningen is a fresh-water deposit of more recent date than Monte-Bolca. The fishes it contains are very similar to those which now live in the Lake of Constance, and almost all belong to the same genera. Now, when we consider how little our Leucisci or poissons blancs differ from each other, we might fear that the analytical method employed by Agassiz would not be sufficient. Fortunately, the fishes of this locality are in general admirably preserved, so well that we can study the details of their skeleton with as much precision as that of a living species. From the minute comparison our author has made of these fossils with the fishes of the lake of Constance and the basin of the Rhine in general, it appears that not only are these fossils different from their living analogues, but also that they equally differ from the fossil species of the other great hydrographical basins, and in particular, from the species of Ménat in the basin of the Rhone. Now, in order that it could happen thus, it is necessary to admit that at the period of the deposition of these formations, the two basins of the Rhone and the Rhine were already separated; for if they had communicated with each other, and if the fishes which now in

habit them were the direct descendants of the fossils of Oeningen and Ménat, it would follow that we ought no longer to meet with species peculiar to them either in the basin of the Rhone or in that of the Rhine. Now, every thing leads us to believe that the lake of Constance, as well as the greater part of the Swiss lakes, were produced by dislocations posterior to the deposition of the tertiary formations; and that being the case, how could the fishes of Oeningen survive catastrophes which have produced such modifications in the form of the surface of Switzerland? The consequence of these facts is obvious. If we succeed in demonstrating that certain basins, like certain terrestrial regions, are inhabited by peculiar species not found elsewhere in contemporaneous deposits, we must thence conclude that the creation has been not only renewed at different geological epochs, but also that the successive creations have been more or less local; that is to say, the species have been created in the places which they inhabit, and that a limit has been assigned to each which it does not overpass, as long as it remains in its natural conditions. It is only man, and a small number of species he has associated with him, that are not subject to this general law. And as the migrations of these same species have taken place under the direct influence of man, we may thence conclude that they did not take place in the anterior ages.*

On the Cause of the Electricity of Steam. By G. A. ROWELL, Esq. Communicated by the Author.†

The cause of rain, evaporation, and atmospheric electricity, having engaged my attention for many years, I endeavoured, in two papers read before the Ashmolean Society, 1839‡ and 1841,§ to shew that evaporation is caused by the increase of the surface of particles of water by expansion, and that thus

* From Bibliothèque Universelle de Génève, No. 100, p. 334-356. ↑ Read before the Ashmolean Society, February 26. 1844. Vide Report of the British Association, Glasgow Meeting. § London and Edinburgh Philosophical Magazine, vol. xx., p. 45.

having a greater capacity for electricity, they are buoyed up by their coating of electricity as a bullet may be buoyed up in water by a coating of cork, and that no evaporation at low temperatures could go on without electricity; that the vapour so raised into the air, when condensed, becomes surcharged with electricity, and thus remains suspended until the surcharge escapes, either as lightning, or else imperceptibly, to the earth, when the remaining coating of electricity being insufficient to buoy up the particles of vapour, they fall as rain, &c.; and that it is possible to cause rain at will by raising electrical conductors to the clouds, by means of balloons, and thus enabling the surcharge of electricity in the clouds to escape to the earth.

The discovery of the electricity of steam I considered a strong support of these opinions; but a theory having been proposed by Dr Faraday, who explained the electricity of steam as caused by the friction of particles of water carried along by the steam rubbing against the solid matter of the passage through which the steam is escaping from the boiler, -the following is an attempt to shew that the electricity of steam is not caused by friction, but by its expansion, on escaping from the boiler, thus carrying off electricity, and rendering the boiler (if insulated) negative, the steam again giving off its positive electricity when condensed; and that the phenomena of Dr Faraday's experiments will support this hypothesis.

One experiment, which I believe tells against the theory of friction is as follows:-" An insulated wire was held in the stream of steam issuing from a glass or metal tube, about half an inch from the mouth of the tube, and was found to be unexcited; on moving it in one direction, a little further off, it was rendered positive, on moving it in the other direction, nearer to the tube, it was negative." In addition to this, both Mr Armstrong and Mr Pattison, in their experiments, found the greatest development of electricity at some distance from the boiler, in some cases five or six feet.

I cannot conceive how this phenomenon can take place if the excitement is caused by friction of the particles of water in the tube, as in that case I believe the strongest development of electricity would be at the mouth of the tube or

boiler; but it fully agrees with the hypothesis that the phenomenon is caused by the expansion and contraction of the particles of steam.

All the experiments on the subject shew, that the steam within the boiler is not electrified, and that the electrical development takes place on its escape from the mouth of the tube. At this point, there is an enormous expansion of the steam; and it then takes up its portion of electricity, according to its expanded surface, in the same proportion as the electrical state of the boiler, or rather the issue tube. If the boiler or tube be insulated, they will be rendered negative; the steam at this point is so also; but, as it begins immediately to condense, it is, at a short distance, neutral; and, on a further condensation, and consequent diminution of surface, the steam becomes positively electrified.

The cause of the increase, through friction, of the electricity of steam, is probably from its bringing a greater quantity of the steam in contact with the issue tube; thus enabling a greater portion of the steam to take up its coating of electricity than could be the case if escaping from a round smooth aperture; as, in that case, owing to the non-conducting powers of high pressure steam, only the exterior particles of the column of steam could take their full coating of electricity.

The presence of water in the tube may increase the electrical development, by rendering the connecting and issue tube a better conductor of electricity from the boiler to the mouth of the issue tube.

The necessity for the issue tube being a good conductor of electricity is shewn by the experiments of Dr Faraday, who says, "A metal, glass, or box-wood tube, well soaked in distilled water, being used for the steam issue, the boiler was rendered well negative, and the steam highly positive; but if a quill or an ivory tube be used, the boiler received scarcely any change, and the stream of steam is also in a neutral state."

This must be owing to the difference in the conducting power of the various tubes, and not to the difference in the friction they occasion, as metal, wood when well soaked in water, and glass, from its becoming damp from the steam, are good conductors, and would supply the escaping steam with electricity; but quill and ivory being non-conductors, and hav

ing a tendency to resist dampness, would prevent the supply of sufficient electricity to cause any strong development.

Every insulated substance held in the current of steam from ivory or quill tubes became negatively charged, from the steam taking off a portion of their electricity.

That electricity cannot be obtained from currents of low pressure steam, may be accounted for by the increased conducting power of steam in this state preventing any development of electricity in the condensed steam, by conducting the electricity back to the boiler the instant any accumulation takes place even the addition to high pressure steam of any saline or other substances (which increases the conducting power of water) prevented electrical development.

It is difficult to account for the absence of electricity when the valve of the boiler was lifted, in Dr Faraday's experiments, as both Mr Armstrong and Mr Pattison performed most of their early experiments from the safety valves of several boilers, and Mr Armstrong states that on one occasion "the engine was rendered intensely negative by a copious emission of steam from the valve." It may be owing to the small pressure on the boiler used by Dr. Faraday.

With respect to the cause why oil of turpentine, olive oil, &c., renders the steam negative, I can form no opinion, but believe that any substance which would reduce the conducting power from the boiler to the mouth of the tube, in any great degree, would render the stream of steam negative, by preventing the particles of steam obtaining their coating of electricity.

The increase of electricity, with the increase of pressure on the boiler, may be accounted for; as the expansion of steam on escaping from the boiler increases also with the pressure.