easy to do, the form of the oval changes. At right angles the oval film, is four times longer than it is broad. As the angle between which the oval film stands is increased, it widens till it is nearly square. If the rectangles could be made to lie exactly one upon the other, the oval film would fill up the space. Now, when the angle of the two wire frames is made narrower instead of wider, the oval narrows till, at 45°, it is a line, and in one moment the system has changed: the oval stands between the wider angle just across its old position and at right angles to it.
A still more remarkable change takes place when a bubble is blown upon the oval film, the lines being at right angles to each other. When it reaches the proper size, all the films disappear, and a hollow curvilinear cube is formed, each side curving out from the wires which define its vertical edges. At the top and bottom the wires make a cross on the film; in each of these triangular spaces four summits appear; colored rings form around them; a black spot shows in the center of each summit, and the bubble bursts. If the wires are held straight up and down when the bubble bursts, the old system of films will start into being again, as if it had left its ghost behind it to recover the elements which the bubble had appropriated.
Dr. Sloane, in his "Home Experiments in Science," gives some beautiful figures. A wire is bent in a spiral, with one end turned straight up through the middle like an axis. Dipped in the fluid, it gives a single spiral film curving around the central wire as a spiral staircase curves around its central pillar. He also gives some very simple and interesting experiments showing the traction of films, requiring no special apparatus or fluid, and so within the reach of every one. All the frames used in this article were made of thin copper wire bent into shape with the fingers or a pair of pliers. Of course, if the wires are soldered instead of being twisted together, and are covered with a thin film of paraffin by rubbing a so-called wax candle on them and then holding the frame above but not too near a bed of coals, the films will last longer; but that is the only difference.
The wonderful traction of films is shown by the recent experiments with oil upon the waves in a storm. The oil, of course, does not still the waves, but it converts the combing waves, so dangerous to navigators, into a comparatively harmless swell. It is the traction of the film which prevents the wind from drawing the water up the incline of the wave and sending it jetting upward to fall over in a comb. A film of oil 1200000 of an inch in thickness will hold the wave of water driving before a gale so that it can not break into spray.
The closing words in Brewster's experiment on the revolving rectangles of wire bring us to another remarkable though famil-
