past three years, or thereabouts, Signori Bartoli and Papasogli have been gradually progressing, using a variety of solutions with varying effects. A vast field of further research is evidently opened.

Others are now at work in the same direction. D. Tomassi and Radiguet have a paper in the Compte Rendus describing a battery of which the elements are carbon, lead peroxide, and common salt, all very cheap materials, and suggesting the direction in which to look for substitutes for the gold and platinum plates of Bartoli and Papasogli.

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"HEAT CONSIDERED AS A MODE OF MOTION."

FIND that this title of Dr. Tyndall's book, and his historical remarks, have led many students to form erroneous notions on the history of the subject. In the preface to the first edition he says: "To the scientific public. the names of the builders of this new philosophy are already familiar. As experimental contributors, Rumford, Davy, Faraday, and Joule stand prominently forward. As theoretic writers (placing them alphabetically), we have Clausius, Helmholz, Kirchoff, Mayer, Rankine, Thompson." He also mentions Regnault, Seguin, and Grove.

From the context I infer that by the "new philosophy" he probably means that which Grove has broadly designated "The Correlation of Forces"; but the majority of readers suppose that Tyndall is describing as a new philosophy the idea that heat is a mode of motion, not the "imponderable" substance, the caloric, which Rumford and Davy experimentally controverted.

The fact is, that this view of heat as a mode of motion is but a revival of the conception of Descartes, Bacon, Boyle, and Newton. In the article "Heat" in the "Complete Dictionary of Arts and Sciences," by the Rev. Temple Henry Croker, Dr. Thomas Williams, and Samuel Clark (1764), Dr. Williams says, "Heat in the body that communicates it is only motion; in the mind, a particular disposition of the soul." "The Cartesians,” he says, assert heat to consist in a certain motion of the insensible particles of the body, like that whereby all the parts of the human body are agitated by the motion of the heart and blood."

Bacon, in the treatise "De Forma Calidi," concludes from a number of facts "that heat in bodies is no other than motion, so and so circumstanced; so that to produce heat in a body, nothing is required but to excite a certain motion in the parts thereof."

Boyle contends for the same, specifying as evidence the heating

of a piece of iron by continual hammering, "the forcible motion of the hammer impressing a vehement and variously determined agitation on the small parts of the iron, which, being a cold body before, grows, by that superinduced commotion of its small parts, hot." He further adduces the case of driving a nail into wood. So long as the nail advances but little heat is produced, "but when it is once driven to the head, a few strokes suffice to give it a considerable heat; for while, at every blow of the hammer, the nail enters further into the wood, the motion produced is chiefly progressive, and is of the whole tending one way; but when that motion ceases, the impulse given by the stroke being unable to drive the nail further on, or break it, must be spent in making a various, vehement, and intestine commotion of the parts among themselves, wherein the nature of heat consists."

Dr. Williams very distinctly anticipates the modern explanation of the difference between light and heat, i.e. between luminous and obscure radiations.

Fire," he says, "differs from heat only in this, that heat is a motion of the particles of a body with a lesser degree of velocity; and fire, a motion with a greater degree of velocity, viz. such as is sufficient to make the particles shine."

By "fire" he means luminous radiation, as the following shows : "There seems to be no other difference between fire and flame than this that fire consists in a glowing degree of velocity in the parts of a body, while yet subsisting together in the mass; but flame is the same degree of velocity in the particles dissipated and flying off in vapour; or, to use Sir Isaac Newton's expression, flame is nothing else but a red-hot vapour."

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THE ORIGIN OF THE MOON'S IRREGULARITIES.

VERY interesting address on the subject of "Pending Problems of Astronomy" was delivered by Professor Charles A. Young to the American Association for the Advancement of Science on September 5 last. Among other subjects was that of the misbehaviour of the moon in not proceeding exactly as it should, according to the lunar theory. As Professor Young says, "The motions of the moon have been very carefully investigated, both theoretically and observationally; and in spite of everything, there remain discrepancies which defy explanation. We are compelled to admit one of three things either the lunar theory is in some degree mathematically incomplete, and fails to represent accurately the gravitational VOL. CCLVIII. NO. 1850.

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action of the earth and sun and other known heavenly bodies, upon her movements; or some unknown force other than the gravitational attractions of these bodies is operating in the case; or else, finally, the earth's rotational motion is more or less irregular, and so affects the time-reckoning and confounds prediction."

I read through his further examination of these three alternatives, in which he confesses his failure to obtain any solution of the riddle, but do not find that he has at all considered a source of disturbance which to me appears most obvious.

In my essay on The Fuel of the Sun," written above sixteen years ago, I pointed out how those ever-active upheavals of the outer layer of solar matter--the solar prominences-must eject vast quantities of the metallic vapours, of which the spectroscope tells us this layer is composed, and that when thus ejected into the cooler regions beyond the sun they must become condensed as metallic hail, and that these never-ceasing but very variable projections of such material sufficiently account for the phenomena of the corona and the zodiacal light, as well as the occasional fall of lumps of solar matter (meteoric iron) upon the earth.

This was a bold speculation at the time, fairly open to criticism as too bold; but all that has been added to our positive knowledge since it was written is confirmatory.

We now know that the velocity of ejection of the matter of the prominences is in many cases sufficient to fling solar material beyond the orbit of the earth, and ordinarily far beyond the then known. limits of the corona.

We now know (especially by the recent American observations of the eclipse of July 1878) that the corona, instead of extending merely two or three hundred thousand miles beyond the solar surface (as was supposed when I wrote), is, as I predicted, actually continuous with the zodiacal light. Professor Langley saw it with the naked eye extending in a continuous stream to a distance of twelve diameters of the sun, and adds: "The twelve diameters through which I traced it under these circumstances I feel confidence in saying were but a portion of its extent." The other independent observations made at other less favourable stations confirm this. Twelve solar diameters amount to more than ten millions of miles, and this extension quite reaches the visible base of the zodiacal light.

The mass of matter thus projected must be very great, quite sufficient to account for the mysterious perturbations of the orbit of Mercury which Leverrier supposed to be produced by an intra-Mercurial planet, which has been named "Vulcan," but has not been

found, nor seen during any solar eclipse, as it should be if it actually exists.

Not only is the quantity of matter sufficient to do something, but the irregularity of its distribution is such as should produce the irregularities of disturbance displayed by the moon.

The width of the outstream of twelve diameters in length was (as shown in Langley's drawing) about one million of miles. The out-stream in the opposite direction was but of half the length, and somewhat wider and spreading. In the other directions the length of extension was but two or three millions of miles. Similar and strictly corresponding variations are displayed in drawings accompanying the other reports. I should add that Langley's observations were made on Pike's Peak of the Colorado Mountains, at an elevation of 14,000 feet "through the peculiarly dry air of Colorado."

That Professor Young, who has done so much in solar observations, especially in demonstrating the enormous velocity and magnitude of these solar ejections, should so entirely overlook this enormous quantity of matter extending so irregularly about the sun, and of course exerting its gravitation on Mercury, Venus, the Earth, and the Moon, is very curious, and I venture to predict that he and all the other astronomers who, like our ex-Astronomer Royal, are struggling so profoundly with the lunar theory, will continue to be baffled until they do take it into account.

Professor Young's blindness to this very effective source of disturbance comes forth still more strikingly when, in the course of the same address, he struggles with the problem of the disturbance of Mercury, and says that "It has been surmised that the cause may be something in the distribution of matter within the solar globe, or some variation in gravitation from the exact law of the inverse square, or some supplementary electric or magnetic action of the sun, or some special effect of the solar radiation, sensible on account of the planet's proximity, or something peculiar to the region in which the planet moves; but thus far no satisfactory explanation has been established."

W. MATTIEU WILLIAMS.

TABLE TALK.

EXPANSION OF LONDON.

S yet no idea of the probable expansion of London to be witnessed in the next score years, seems to dawn upon those who take charge of our internal government, and the necessary enlargements and repairs are made without the slightest reference to our coming needs. Again and again, when the one side of a street is pulled down, as in the case of Gray's Inn Lane, a few more yards are added to meet present necessities, and residents are encouraged to put up more or less costly buildings on each side as though the ultimate requirements of London were met. By a constant experience I know that our main thoroughfares are now overcrowded and blocked to the extent of being dangerous. A walk up the Strand to a man who has lost the elasticity of youth and who is in haste is a task of danger. At the present, when the theatres close, the place is a thoroughfare only in name. To meet the requirements of this century even, the Strand should be as wide through all its length as Charing Cross. It should have-like Unter den Linden in Berlin-two separate carriage-ways for vehicles going in different directions, and a foot-way between: if possible a boulevard with trees and with light foot-bridges by which passengers might avoid the traffic. Let any man disposed to scoff at this idea compare, if he can, the difference between the Strand of to-day and that of a score years ago, and think what another score years at our accelerating rate of increase will do. It is no part of our business to look too far ahead and make provision for a remote posterity. Let those, however, who are aware of the power of figures reckon up our present rate of progress and see how long it will take with no unforeseen check to give London a population of twenty million.

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PAYNE COLLIER'S DIARY.

N the interest of all lovers of literature it is to be hoped that the "Old Man's Diary," a copy of which sold under special conditions for £150 at the recent sale of Collier's library, will be