WHEN the man of science deals with some hitherto unexplained phenomena of Nature, he first collects all the available facts, and then casts about in his mind for some hypothesis which will bring them into harmony with one another. The latter process has been well called the "scientific use of the imagination," and is illustrated by the history of every physical discovery. Thus the theory of gravitation, which brought so many terrestrial and celestial phenomena into harmony with law, was, in the first instance, a leap of the imagination on the part of Newton. The undulatory theory of light sprang from the effort of a great mind endeavoring to co-ordinate a vast body of facts, which fell into order under its application. In the same way Dr. Joule imagined that heat is a mode of motion among the ultimate particles of matter; and the truth of his brilliant guess" was shown by the competence of the theory to explain all the phenomena which heat exhibits.

Among these guesses of genius, many have been futile; others have failed after carrying their authors far on the road toward explanation; while there are some provisions which were correct, though they were not capable of proof at the time when they were made. Perhaps no physical philosopher ever possessed a more forward-looking mind than Faraday, and certainly none ever made more use of the scientific imagination. Many of his views only waited for verification until the means of research had been enlarged or improved, and among these his anticipations in regard to what he termed the radiant condition of matter have received remarkable confirmation in some results which have been obtained, and quite recently given to the world, by Mr. Crookes.

It was Faraday's belief that a form of matter existed whose subtlety surpassed that of gases as much as the gaseous surpassed the liquid, or the liquid the solid state of matter. Aided by physical apparatus such as Faraday could not command, Mr. Crookes is carrying out an inquiry of the utmost interest, and we propose to give a short account of the facts which he has lately laid before the

scientific world in connection with his researches on radiant matter.

Chemists have found that when two bodies, A and B, are capable of uniting together in several proportions, the several quantities of A which combine with a constant quantity of B stand to one another in very simple ratios. Thus the element oxygen will combine with the element chromium in proportions of I to 2, or 2 to 4, or 3 to 6.; but one element can only unite with a constant quantity of another element by well-defined steps or increments, standing to one another in simple numerical ratios.

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The scientific use of the imagination led Dalton to infer from this strange fact that elementary bodies are composed of ultimate particles, or atoms," each with a constant weight peculiar to itself, and that combination between two elements takes place by the juxtaposition of these atoms. This hypothesis harmonized a chaos of chemical facts, and made everything orderly where before all was disorder. Hence the "atomic theory" has come to form the basis of modern chemistry, and the unseeable "atom" takes its place in science as an undoubted physical fact.

Two or more atoms united together constitute a molecule. Thus one atom of the elementary body oxygen combines with two atoms of the element hydrogen to form a molecule of water, which substance we may therfore consider as composed of a vast number of molecules, each of which is again compounded of the elements oxygen and hydrogen united in the proportion of two to one.

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Dalton's law had long been accepted by chemists when Dr. Joule added another and equally important conception to the molecular view of matter. showed that heat, which the earlier philosophers considered as something having a material existence, ought properly to be regarded as a mode of motion. A rifle bullet striking an iron target has its motion of translation suddenly stopped; and, as a result, the lead is found to be heated. Joule proved that when the bullet is arrested its motion as a mass is converted into an unseen interior movement of its ultimate particles,

or molecules, of which molecular movement heat is the sensible manifestation.

In ordinary language we speak of hot and cold bodies as if the terms were absolute; but in calling ice cold and boiling water hot we simply refer them to the standard of our sensations; both boiling water and ice would seem hot to an organism whose temperature was below freezing point. Every terrestrial body possesses some absolute heat, or, in other words, the molecules which compose it are in more or less active movement; and upon their activity depends the temperature of the body.

Now the amount and character of this internal molecular movement which declares itself sensibly as heat varies in solids, liquids, and gases. The "scientific imagination" has revelled in picturing the kind and extent of movements which are executed by the ultimate particles of matter in each of these conditions. With solids and liquids we have at present no concern, but it is supposed that the molecules of gases are constantly moving forward in straight lines with uniform velocity until they impinge either against each other or against the walls of the containing vessel. Such a vessel may be likened to a hive containing a swarm of bees; and we readily see that upon the number of bees in the hive will depend the length of flight which each bee can make before coming into contact with another bee. Suppose the average distance a bee can fly without colliding with its neighbor is two inches, then we will call that dimension the "mean free path" of the bees. If we remove one half of the bees from the hive we double this free path, and we might go on reducing the number of bees until it extended from one end of the hive to the other.

The molecular conception of matter declares that this figure represents the state of things in every gaseous body, and bearing this in mind we turn for a few moments to consider the phenomena of the electric discharge through gases, which, as we shall presently see, occupy an important place in the research.

The electric spark passes with difficulty through the mixture of gases forming our atmosphere, and air is consequently called a non-conductor or dielectric." A flash of lightning, or the

spark from an electrical machine, exhibits the violence with which electricity is discharged through air; but it passes readily through a high vacuum, exhibiting very beautiful phenomena.

Most persons have seen the wellknown Geissler's vacuum tubes, and are acquainted with the appearances presented by the discharge. The space separating the positive and negative poles is filled with violet-colored light, but a close inspection shows that the immediate neighborhood of the negative pole is surrounded by a dark region. The violet light is produced by collisions among the molecules of gas left after exhaustion, which are excited into abnormal activity by the passage of the electric current. These molecules, as will afterward appear, stream out from the negative toward the positive pole, and the small dark space around that pole represents their mean free path, or, in other words, the distance which they traverse before coming into contact with their fellows.

So at least Mr. Crookes's imagination led him to suppose; and in the verification of this brilliant guess, he has made the world acquainted for the first time with that radiant condition of matter which Faraday prevised. Faraday, as we have said, wanted means to prove the truth of his anticipation, for the vacuum of an ordinary Geissler's tube is far more perfect than any which could be produced in his day by the old-fashioned air-pump, while it falls. very short of the extreme exhaustions. used in Mr. Crookes's research. Very perfect vacua were required by that gentleman in the construction of his radiometer, and special means were devised for their production; these have been further improved during the recent investigations, and exhaustion can now be carried to almost absolute perfection.

With these means at hand, Mr. Crookes. found that, as the vacuum in a Geissler's tube improved, the dark space surrounding the negative pole increased, and he was ultimately enabled to carry exhaustion so far as to make this dark space equal to the whole length of the tube.. In other words, he removed so many of the molecules forming the inclosed gaseous matter that, like the bees in the hive, they could pass from end to endi

without collision. When this point was reached the residual matter within the tube exhibited entirely new phenomena; the violet light of the electric discharge disappeared, and that which has been well called radiant energy took its place.

It was soon found that a particular degree of exhaustion was most favorable for the display of this new force. The vacuum might be so perfect that the discharge would not pass at all; on the other hand, it might be too low, in which case the violet light of the ordinary discharge was seen. The best point is reached when one millionth part of the atmospheric pressure remains in the tube. Very high figures convey no meaning to the mind; it is easy to speak of a million, but difficult to realize what the word signifies. Let the reader therefore imagine that the barometric column of mercury representing the pressure of our atmosphere is one mile in height, then one millionth of that pressure will be represented by a space one sixteenth of an inch in length.

Still there is matter remaining in the tube, and when this is excited to rapid movement by the passage of an electric current, the tube becomes brilliantly phosphorescent from the impact of the flying molecules on its walls. Different kinds of glass glow with different-colored light. English glass shines with blue, German glass with yellow, and uranium glass with green radiance. Substances of known phosphorescent properties glow with an intense light when exposed in the tube to the battering of these ultimate particles of matter. The diamond shines with a new lustre, ruby emits a deep red light, colorless alumina glows red like the ruby, and in particular a substance known as "Becquerel's phosphorescent sulphide" shines with marvellous brilliancy.

It may well be asked whether we have to do with material bodies at all in tubes from which matter has been practically eliminated, and how we can speak with certainty of the impact of actual particles in a vacuum so nearly absolute. Theory, however, assures us that we have by no means got rid of all the molecules originally contained in the tube; on the contrary, their name is legion even after reaching these high exhaustions; and if

we follow Mr. Crookes through his experimental demonstrations, we find good reasons for believing what theory declares.

As we have already stated, the excited particles move away from the negative pole, and they do so in straight lines whose direction is at right angles (or normal) to the bounding surfaces of the pole. If this be, for example, a flat metal plate standing vertically, the molecules rush away from it in horizontal streams; if the pole be concave, the streams converge; if convex, they diverge; and if spherical, they radiate in all directions.

In an ordinary vacuum tube the electric discharge seeks the shortest path between the two poles, and no matter how sinuous that course may be made by the glass-blower, the current follows every curve into which the glass is bent. Radiant matter behaves in quite another way; the particles are not discharged by the nearest route from one pole to the other, but they ray out from the negative pole in straight lines which are persistently normal to its surface, no matter what may be the position of the positive pole in the tube. The molecules are incompetent to turn a cornerthey behave, indeed, just like a stream of bullets-and if their energy be excited in a V-shaped tube having a pole at each of its upper extremities, these molecular bullets are projected only along one leg of the V; they cannot turn the corner, and do not seek the positive pole like an ordinary electric discharge.

The stream of excited particles may be arrested by an obstacle placed in its path. Mr. Crookes arranges a tube in such a manner that a small cross of thin mica can be made to stand erect within it at pleasure; when in this position the cross faces the negative pole, which consists of a flat plate of metal. Radiant matter stream out horizontally from the plate, and declares its presence by rendering the opposite end of the glass tube brilliantly phosphorescent. The cross being erected interposes an obstacle in the path of the particles, and its shadow" is at once projected dark on the glowing end of the tube.

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Here we have something very like demonstration of the material character of the discharge from the negative pole,

Advantage is taken of this fact to show. yet more conclusively the material character of radiant energy. A tube fitted with a small revolving disk furnished with peripheral vanes, like a water-wheel in miniature; the negative pole is a flat plate occupying one extremity of the tube; from this the discharge proceeds horizontally, striking against the little wheel in its passage. No movement follows, because under these circumstances the particles impinge equally on the vanes above and below the centre of rotation. But when the north pole of a magnet is applied in such a way as to deflect the stream a little from its horizontality, those vanes only are struck which lie on one side of the centre, and the wheel rotates with great velocity. On reversing the magnetic poles and throwing the molecular stream to the other side of the centre, the rotation of the disk is at once reversed in accordance with the changed direction of the particles.

It will be remembered that about two years ago the same investigator brought before the world the remarkable instrument now so well known as the radiometer. It consists of a light disk furnished with four peripheral vanes, hung on a delicate vertical axis, and inclosed within an exhausted glass bulb. When exposed to heat or light, the disk revolves more or less rapidly according to the amount of heat or light supplied.

Many explanations have been offered to account for this strange phenomenon, but it was reserved for Mr. Crookes

himself to furnish the true solution of the problem. He constructed a radiometer with vanes metallic on one side only, and so arranged matters that the disk formed the negative pole, placing the positive pole indifferently at any part of the exhausted bulb of the instrument. Upon the passage of the current, radiant matter streamed away from the metallic faces of the vanes, and the reaction of the discharge, like the kick of a gun against the shoulder, set the disk in rapid rotation.

He next found that radiant energy can be excited by light or heat as well as by electricity. For this purpose an exhausted bulb was provided having a light disk supported upon a vertical axis, and furnished with vanes inclined to the horizon at an angle of about thirty degrees. Beneath this disk, and encircling the axis, was a coil of platinum wire which could be heated to redness by the passage of an electric current. As the temperature of the wire rose, radiant energy was excited; streams of molecules issued from the inclined vanes, and rotation of the disk followed from their unbalanced reaction. Similarly it was demonstrated that light was competent, equally with heat and electricity, to produce the phenomena of radiant matter; but we should need a diagram to explain the more complex apparatus by which this was proved.

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In this way it was shown that the movements of the radiometer depend on radiant energy, and one of Nature's riddles was answered, as riddles always must be, by an effort of the imagination. Yet, as Professor Tyndall has said, "there are Tories even in science who regard imagination as a faculty to be feared and avoided rather than employed. are they who, having observed its action in weak vessels, are unduly impressed by its disasters. They might with equal justice point to an exploded boiler as an argument against the use of steam. Bounded by co-operant reason, imagination becomes the mightiest instrument of the physical discoverer; without this power, our knowledge of nature would be a mere tabulation of sequences. should still believe in day and night, summer and winter; but casual relations would disappear, and with them that science which is now binding the

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parts of nature together to an organic whole."

But to return. The experiments we have described leave little room for doubt that we have been concerned with the motions of actual particles of matter, that a real hail-storm of molecular projectiles caused these glass tubes to glow, the gems and other bodies to phosphoresce, the vaned disks to revolve; and in that hail we are brought face to face with the ultimate constituents of matter. We stand on the threshold of a new world. Atoms and molecules are only inferences from certain properties of matter, and their existence has been, and still is, stoutly denied. Their reality depends on the same kind of proof as favors those imagined pulses of a suprasensuous atmosphere called the æther which enable us to explain the phenom

ena of light; but in presence of Mr. Crookes's experiments we almost seem to see those inconceivably minute particles of matter which can never be brought within the actual ken of man, for he puts the unknowable and unseeable molecules into harness before our eyes, and sets them turning a wheel like a stream of water.

Such is an outline of the phenomena exhibited by matter in its radiant condition with which this research has made us acquainted. It furnishes the best objective evidence yet afforded in support of that theory of the molecular condition of matter which forms the basis of modern physics, and will undoubtedly lead future inquirers into new, unexpect ed, and fruitful fields of observation.Belgravia Magazine.

Temple Bar.