When the bubble is first blown, its form is elliptical, into which it is drawn by its gravity being resisted; but the instant it is detached from the pipe, and allowed to float in air, it becomes a perfect sphere, since the air within presses equally in all directions. There is also a strong cohesive attraction in the particles of soap and water, after having been forcibly distended; and as a sphere or globe possesses less surface than any other figure of equal capacity, it is of all forms the best adapted to the closest approximation of the particles of soap and water, which is another reason why the bubble is globular. The film of which the bubble consists is inconceivably thin (not exceeding the two-millionth part of an inch); and by the evaporation from its surface, the contraction and expansion of the air within, and the tendency of the soap-lather to gravitate towards the lower part of the bubble, and consequently to render the upper part still thinner, it follows that the bubble lasts but a few seconds. If, however, it were blown in a glass vessel, and the latter immediately closed, it might remain for some time; Dr. Paris thus preserved a bubble for a considerable period.
Dr. Hooke, by means of the coloured rings upon the soap-bubble, studied the curious subject of the colours of thin plates, and its application to explain the colours of natural bodies. Various phenomena were also discovered by Newton, who thus did not disdain to make a soap-bubble the object of his study. The colours which are reflected from the upper surface of the bubble are caused by the decomposition of the light which falls upon it; and the range of the phenomena is alike extensive and beautiful.[15]
Newton (says Sir D. Brewster), having covered the soap-bubble with a glass shade, saw its colours emerge in regular order, like so many concentric rings encompassing the top of it. As the bubble grew thinner by the continual subsidence of the water, the rings dilated slowly, and overspread the whole of it, descending to the bottom, where they vanished successively. When the colours had all emerged from the top, there arose in the centre of the rings a small round black spot, dilating it to more than half an inch in breadth till the bubble burst. Upon examining the rings between the object-glasses, Newton found that when they were only eight or nine in number, more than forty could be seen by viewing them through a prism; and even when the plate of air seemed all over uniformly white, multitudes of rings were disclosed by the prism. By means of these observations Newton was enabled to form his Scale of Colours, of great value in all optical researches.
Dr. Reade has thus produced a permanent soap-bubble:
Put into a six-ounce phial two ounces of distilled water, and set the phial in a vessel of water boiling on the fire. The water in the phial will soon boil, and steam will issue from its mouth, expelling the whole of the atmospheric air from within. Then throw in a piece of soap about the size of a small pea, cork the phial, and at the same instant remove it and the vessel from the fire. Then press the cork farther into the neck of the phial, and cover it thickly with sealing-wax; and when the contents are cold, a perfect vacuum will be formed within the bottle,—much better, indeed, than can be produced by the best-constructed air-pump.
To form a bubble, hold the bottle horizontally in both hands, and give it a sudden upward motion, which will throw the liquid into a wave, whose crest touching the upper interior surface of the phial, the tenacity of the liquid will cause a film to be retained all round the phial. Next place the phial on its bottom; when the film will form a section of the cylinder, being nearly but never quite horizontal. The film will be now colourless, since it reflects all the light which falls upon it. By remaining at rest for a minute or two, minute currents of lather will descend by their gravitating force down the inclined plane formed by the film, the upper part of which thus becomes drained to the necessary thinness; and this is the part to be observed.
Several concentric segments of coloured rings are produced; the colours, beginning from the top, being as follows:
1st order: Black, white, yellow, orange, red.
2d order: Purple, blue, white, yellow, red.
3d order: Purple, blue, green, yellowish-green, white, red.
4th order: Purple, blue, green, white, red.
5th order: Greenish-blue, very pale red.
6th order: Greenish-blue, pink.
7th order: Greenish-blue, pink.
As the segments advance they get broader, while the film becomes thinner and thinner. The several orders disappear upwards as the film becomes too thin to reflect their colours, until the first order alone remains, occupying the whole surface of the film. Of this order the red disappears first, then the orange, and lastly the yellow. The film is now divided by a line into two nearly equal portions, one black and the other white. This remains for some time; at length the film becomes too thin to hold together, and then vanishes. The colours are not faint and imperfect, but well defined, glowing with gorgeous hues, or melting into tints so exquisite as to have no rival through the whole circle of the arts. We quote these details from Mr. Tomlinson’s excellent Student’s Manual of Natural Philosophy.
We find the following anecdote related of Newton at the above period. When Sir Isaac changed his residence, and went to live in St. Martin’s Street, Leicester Square, his next-door neighbour was a widow lady, who was much puzzled by the little she observed of the habits of the philosopher. A Fellow of the Royal Society called upon her one day, when, among her domestic news, she mentioned that some one had come to reside in the adjoining house who, she felt certain, was a poor crazy gentleman, “because,” she continued, “he diverts himself in the oddest way imaginable. Every morning, when the sun shines so brightly that we are obliged to draw the window-blinds, he takes his seat on a little stool before a tub of soapsuds, and occupies himself for hours blowing soap-bubbles through a common clay-pipe, which bubbles he intently watches floating about till they burst. He is doubtless,” she added, “now at his favourite amusement, for it is a fine day; do come and look at him.” The gentleman smiled, and they went upstairs; when, after looking through the staircase-window into the adjoining court-yard, he turned and said: “My dear madam, the person whom you suppose to be a poor lunatic is no other than the great Sir Isaac Newton studying the refraction of light upon thin plates; a phenomenon which is beautifully exhibited on the surface of a common soap-bubble.”