is still more curious, the very ones, the metals, which it was most difficult or apparently impossible to charge simply by rubbing them, now receive more electricity than all others on mere approximation of the excited tube. Here are two capital discoveries made at the very threshold of his labor.

This is not altogether unusual, as many a later investigator can testify. Long study and thought produces a sort of mental polarization that somehow dulls the perceptive faculties, or results in an intellectual inertia which renders it difficult for the mind to turn itself out of the path in which it has been moving. And, as a consequence, the power of original thought, of invention, is apt to weaken even in those most highly gifted with creative genius, unless the brain-work be differently directed for a time, or wholly intermitted for a period of rest. There seems to be no exhaustion of energy, for the thinking mechanism may continue its operation, although fruitlessly, with even greater assiduity than ever. It is rather a new condition of the apparatus which causes a change in the quality of its accomplishment. Therefore when a new mind-not polarized-attacks the problem, it is very apt not only to perceive solutions which evade the recognition of those which have long grappled with it, but to see the most prominent and general ones first. It is an incident of progress, and apparently a necessary one, that obstacles shall be attacked by a succession of new minds; and it constantly happens that a new mind without experience is often more potent in overcoming them than one rich with accumulated knowledge.

Gray had almost instantly discovered that electricity would pass from the excited body to one not excited: from the glass tube to the cork. Dufay also at once finds all bodies capable of electrification. Gray was halted by doubts as to the effect of the physical conditions of the body to which the charge is communicated. So Dufay similarly pauses because of misgivings as to the influence of color-these not

DUFAY'S EXPERIMENTS.

48: of his own suggestion, but because Gray had said that among electrified bodies physically alike, those which are red, orange, or yellow, attract very much more strongly than those which are blue, green or purple. Dufay saw in this not merely a possible cause of error in his future researches, but a suggestion that there might be a relation between electricity and light, if the former had a capacity for color selection. For both reasons, he proceeds.

His initial experiments seem to confirm Gray decisively. Of nine suspended ribbons (black, white and the rainbow colors), the rubbed glass tube attracts the black first and the red last. White gauze and black gauze intercept the electric virtue, while gauzes of the rainbow hues, the red especially, allow it to pass. Dufay presses on to the broader question, fully believing that he is on the track of a startling discovery.

If color alone exercises the effect, it can make no difference, he argues, whether the hue be natural or artificial: whether it appear on the rose-leaf or on a painting. So he tries the flowers-and the signs fail. The scarlet gera

nium responds to the attracting glass as readily as does the purple pansy-the green leaves as quickly as the white petals of the lily. Perhaps there is something in the inherent quality of these vegetable substances which interferes. Clearly the crucial test requires pure color, and that is only in the rainbow.

He directs a sunbeam through a prism, and spreads it out into its gorgeous spectrum, and distributes therein white ribbons, so that the sun paints one red, another orange, another yellow, and so on through nature's color box. But the ribbons act like the flowers. No one of them responds to the electric pull any more than does another. The notion that electric attraction could tear the sunbeam to pieces, and change it from white to red by drawing out the blue rays, was only a delusion.

Then Dufay went back to his colored ribbons and wet them-and their differences vanished. He heated his

gauzes and the virtue went through black or red with equal facility. He had been misled by the dressing which the makers had put in the ribbons to give them body: that was all the color exerted no influence.

Perhaps this left him in something of a questioning attitude toward Gray's other conclusions, for he begins to investigate long-distance transmission anew; and finally reaches the conclusion that the substances which are most difficult to electrify-such as metals or wet objects-best convey the virtue; while on the other hand, those easiest excited-amber or silk-can hardly be got to convey it at all. He puts up a packthread line 1256 feet long, and wets it; and the electricity traverses it with the same freedom with which it nowadays runs along wet telegraph poles, or escapes from the wires which touch the dripping foliage. For Gray's silk supports, he substitutes glass tubes and masses of Spanish wax, and thus, for the first time, uses solid insulators upon an electric line of communication. The new principle destroyed the non-electric and the electric as distinctive significations-it made non-electrics into "conductors," and electrics into "non-conductors."

Gray had shown how one line may electrify another placed near it. Dufay varies this by placing two short lines, respectively six and eight feet in length, end to end with an air-space intervening. When the gap is a foot wide he says that the attraction, despite the shortness of the lines, is as weak as if the virtue had traversed the continuous length of 1256 feet. Nevertheless it seems to him that the charge can escape from line to air, and therefore he says, coining the word, the necessity is apparent that the transmitting cord should be "insulated."1

He has meanwhile remarked that if he touches the ball hanging at the end of his electrified line, it refuses to attract; the electricity, he says, being dissipated through him to the floor. But suppose he touches it with a small

"Que la corde dont on se sert pour transmettre au loin l'électricité soit isolée."

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483

body, itself insulated. Then the ball loses only a part of its electricity, which goes to the last-named body. Consequently he says, the volume of the electrified ball must be considered. If too large, the virtue reaching it becomes too extended to act quickly; if not large enough, it will not take all that is brought to it by the cord. These were the first perceptions of the distribution of an electric charge on a conductor. Gray had found it resident on the surface.

Dufay now emulates the English philosopher in suspending people by silk lines and electrifying them; but he soon discards children and suspends himself. Then he compares the sensation caused by an electrified tube near his face to that of a spider-web drawn over it, and for the first time feels the pricks and burns of the electric sparks as they dart from his fingers. He believes them to be fire, and, as such, altogether different from the hitherto seen glow.

His is a nimble mind, and it leaps from one subject to another with marvelous rapidity. But this is necessary; for he is not only breaking a new path, but rebuilding the old one. As he meets a new problem he discovers that the vantage ground from which he must proceed is infirm. That necessitates re-examination of the foundation facts; and in this way he finds himself side by side with Von Guericke, contemplating the singular behavior of the feather which the sulphur globe drives away, and which, nevertheless, like the moon, always turns the same face. Dufay lets fall some gold-leaf upon his excited tube and sees it repelled in the same way, avoiding the tube as he chases the fragments around the room. But if, meanwhile, he rubs the tube, the leaf comes to it and goes away from it alternately, following the motion of the hand. When the leaf touches the tube, he says, it becomes electrified thereby by communication. Yet obviously it is repelled. Therefore all electrified bodies first attract bodies that are not electrified, communicate to them their own electricity, and that done, repel them. Nor will the latter

be again attracted until, having touched some other body, the acquired electricity is lost. This, which Von Guericke saw, is now explained by Dufay.

But Dufay went a little further and imagined a whirl, a field of force, around the tube, and figured to himself the action going on there and not in the body of the tube. The attracted body, on touching the tube and becoming electrified, acquires a field of its own, the two fields repel, and so long as that of either body remains the same, the relative position of the two is unchanged. But if the field of the attracted feather, for example, is dissipated, the feather falls back to the tube; if the field of the tube is varied, as it is by the hand moving from one end of the tube to the other, then the feather swings to and fro, following the changes caused.

It is while examining the repulsive action of the glass tube that Dufay accidentally notes an effect which he says disconcerted me prodigiously ;" and well it might, for it seemed to be subversive of every conclusion which he had hitherto formed concerning the behavior of electrified bodies. He is watching a bit of gold-leaf float in the air under the repulsion of his excited glass tube. It occurs to him to see what it will do when subjected to the action of two electrified bodies; and therefore he rubs a piece of gum-copal and brings it to the leaf. To his utter astonishment the leaf, instead of retreating from the electrified gum, as it certainly did from the electrified glass, adheres to it. He tries the experiment again and again, but in every instance the leaf is drawn by the gum or by amber or by Spanish wax, while it is repelled by the glass tube. Yet a second glass tube or a piece of rock crystal brought near the leaf exercises the same repelling effect as the original tube.

This was Dufay's most important discovery. "I cannot doubt," he says, "that glass and crystal operate in exactly the opposite way to gum-copal and amber; so that a leaf repelled by the former because of the electricity which it