one swift slash of his sword. But for downright mechanical ingenuity commend us to the "sensation" which forms the prime feature of an American play called "Josiah Allen's Wife," first produced at Providence, R.I., in May 1882. With the early unfolding of the meagre plot it quickly becomes obvious that an important letter must be taken a long distance in a proportionately short time. Samantha— who gives the play its name-undertakes the perilous task, and she is seen to get into an old country waggon and start the horse at a brisk pace. Almost the entire second act is taken up with the performance of this journey without the waggon and its occupants ever leaving the stage. This scene, the illusion of which was simply perfect, was managed after the following adroit manner. The horse, which was one accustomed to brick-yard service, trotted for the nonce on a sort of large treadmill placed at the rear of the stage and completely hidden from the prying eyes of the spectator by a frontage of stone wall. This and the background were in reality a couple of panoramas deftly connected with the treadmill by smoothly working machinery; the mere trotting of the horse set everything in motion and caused forests, rivers, and country houses to fly by with surprising celerity and wonderful realistic effect.

W. J. LAWRENCE.

SCIENCE NOTES.

SOLIDIFICATION BY PRESSURE.

HE fact that gases may be compressed until they become liquid, and further compressed into the solid state, appears like a contradiction of what is stated in the preceding note concerning the ffuidity of solids produced by pressure. The following considerations will, I think, remove, or at any rate diminish, this difficulty.

The gaseous condition of matter is evidently due to the repulsive energy of heat. When matter is self-repulsive, striving to fling itself away from itself in every direction, and can only be kept within bounds by pressure exerted from without, it is a gas. When it is self-attractive, clinging inwards upon itself in every direction, it is either a liquid or what we call a solid, according to the amount of outside pressure to which it may be subjected. If this outside pressure moves it upon itself without tearing it asunder or breaking it, the properties of a liquid are displayed.

When we compress a gas or a liquid to a reduced volume its temperature is raised, and if this temperature exceeds that of its surroundings heat is lost by it, or, otherwise stated, some of its repulsive energy is abstracted by the surrounding matter which it warms. The pressure is thus an operation of abstracting heat or repulsive energy, and, as it appears to me, it is simply this abstraction of heat or diminution of repulsive energy that negatively effects the liquefaction or solidification, by permitting the attractive energies of the matter that were previously overpowered by the repulsive energy, to operate in holding it together. The evolution of the so-called "latent heat" when vapours are condensed by simple cooling is a similar transmission of repulsive energy to surrounding matter.

As far as I am aware, no gas has ever been liquefied or solidified without the abstraction of heat, and I suspect that any attempt to liquefy a gas under conditions preventing such abstraction would fail, however great the pressure. Such condition would be fulfilled if all the surroundings of the gas while under pressure were raised in temperature by outside means in the same degree as the gas itself rose

in temperature in consequence of the pressure. I here speak, of course, theoretically; the soundness of the theory can only be proved or disproved by experiment.

While on this subject I may add that M. E. H. Amagat is engaged in experiments on the compressibility of fluids. In a preliminary paper read before the Academy of Sciences on August 23, he stated results of a pressure of 3,000 atmospheres on water. Its volume was reduced one-tenth, and its coefficient of compressibility one-half.

I

THE INNER STRUCTURE OF THE EARTH.

HAVE been rather surprised to find that Sir J. William Dawson, in his Inaugural Address as President of the British Association, still favours the hypothesis that the earth has a solid central nucleus lying within a plastic undercrust; a constitution which he compares to a drupe or stone-fruit, such as a plum or peach, the stone representing the solid nucleus, the fruity pulp the plastic undercrust, and the skin the outer crust on which we live.

Sir J. W. Dawson does not appear to be acquainted with the experimental researches of W. Spring described in my notes of August 1882, February 1883, and November 1883, which prove that the metals and other solids become fluid when submitted to sufficient pressure.

The pressure communicated by Spring's apparatus imitates artificially (as I explained in the February and November notes) that to which such materials are subjected in the interior of the earth, and the experiments generally demonstrate as a matter of physical fact, without the intervention of molecular hypothesis or recondite mathematical calculations, that "the enormous pressure of the superincumbent mass must liquefy all the inner material of the earth, whatever be its temperature."

In the note of November 1883 I showed that there is no such thing as an absolutely rigid solid; that, for example, the fluidity of so-called solids is shown by every coin in one's pocket, the metal having flowed into all the channels of the die when subjected to sufficient pressure. Such a liquid as water flows in a like manner when subjected to the pressure of its own weight; the action is the same in both cases, but the degree of pressure demanded to effect it is different. Deep in the earth the gravitation of the material above excites a pressure far greater than that of the coining-press or Spring's apparatus.

Sir J. W. Dawson tells us that he bases his conclusions on "astronomical investigations "—that is, on the results of excessively recondite mathematical calculations, based on hypothetical explanations of excessively refined and minute astronomical observations. The unreliability of these results is admitted by him in a note to the report of his Address in Nature of September 2, where he says :— "Hopkins, Mallet, Sir William Thomson, and Professor G. H. Darwin maintain the solidity and rigidity of the earth on astronomical grounds; but different conclusions have been reached by Hennesey, Delaunay, and Airy. In America, Barnard and Crosby, Dutton, Le Conte, and Wadsworth have discussed these questions."

The pre-eminent authority on this particular subject is Airy. He has devoted many of the later years of his life—and from what I have heard him say at the Astronomical Society, he was then devoting more—to the Lunar theory, upon which these calculations primarily depend. As he says in one of his admirable articles in the “English Cyclopædia,' "1"the actual motion of the moon round the earth is one of the most complicated questions in astronomy." Its disturbance of the earth's movements due to the reaction of this motion on the equatorial protuberance is correspondingly complex, and this complexity is still further complicated and reduced to mathematical cobweb-splitting of the most infinitesimal character when an attempt is made to determine the magnitude of internal earth-tides or the earth's deformation by their theoretically demanded sub-disturbance of the disturbance.

Even if we admit the reliability of such highly elaborated calculations, they become blocked out of the question I have raised by the fact that we are as yet quite ignorant of the degree and character of the mobility and elasticity of the pressure-generated fluid matter.

There is no such complication in the conclusion derivable from the experiments I have quoted. They have not been refuted by any counter-experiments, or questioned in any way. Their testimony is absolutely direct and immediately to the point.

About the clearest, simplest, and therefore the ablest astronomical essay extant is the article "Gravitation," in this Cyclopædia. It was written by Airy for "The Penny Cyclopædia" nearly half a century ago. I read it when a boy, and recommend its careful perusal to all-to non-mathematicians as a grand subject usually put out of their reach, and to mathematicians as a lesson in the art of reasoning directly and vigorously by using the intellect unsupported on the customary crutches of technical formulæ.

A "WRINKLE" TO ARTISTS.

Y ast month's note on the properties of oxydized linoleine,

My the drying constituent of linseed oil, shows how peculiarly

it is fitted for its purpose as a permanent medium for pigments generally. Nothing at present known combines so many merits, or even approaches to such combination.

About a dozen years ago the collected works of Landseer were exhibited in London. I was shocked on observing that some of his finest works were miserably faded. This was notably the case with "The Sanctuary," a picture of a red deer that had just crossed a lake and was emerging with the water dripping from its fur. When this was first exhibited at the Academy I admired, among other details, the sparkling brilliancy of the water-drops, and the general suggestion of cool freshness throughout the picture. At the later exhibition all this had gone.

In naming this I am not selecting an exception, but a typical example of the early fate of the pictures of the majority of modern artists; some become faded, others wrinkled and reticulated with a network of cracks, even during the lifetime of the painter, while the works of the old masters remain with very little decay during many generations.

My opinion is that the turpentine of the medium is the chief offender, and that the true artist should discard it altogether as fitted only for the work of the house painter, of whom rapid drying and flatting are demanded.

Careful examination of the surface of the works of the older of the old masters has led me to doubt whether they used turpentine at all, and to conclude that their medium was linseed oil pure and simple, used so freely that the drying of their pictures must have demanded days or weeks, and a studio free from dust.

With such a medium every individual particle of the pigment matter is enveloped and sealed in a curiously imperishable transparent skin, which dries by gaining something, viz., oxygen, and therefore swells a little in thus drying, thereby compacting itself and embracing more firmly with loving æsthetic hug the precious colour particles that constitute by their arrangement the artist's pleasuregiving legacy to his fellow-creatures.

Not so the turpentine-diluted medium. In this case haif of the medium evaporates, leaving the poor particles of pigment half naked to their enemies.

Some painters have mixed varnishes, such as copal or mastic, with

VOL. CCLXI.

NO. 1870.

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