Pseudobulb

“ On blowing through a pipe which is immersed in the water, a great number of bubbles are formed between the plates. If the bubbles are all large enough to reach across from one plate to the other, you will at once see that there are nowhere more than three films meeting one another, and where they meet the angles are all equal. The curvature of the bubbles makes it difficult to see at first that the angles really are all alike, but if you only look at a very short piece close to where they meet, and so avoid being bewildered by the curvature, you will see that what I have said is true. ”

“ If, on the other hand, the bubble were formed on the surface of a liquid, there would be precisely the same pressure on the bottom, only it would be acting on a medium which would give way to it; the liquid, therefore, would yield to the pressure of the air, and we should have the bubble as it were a little buried in the liquid by its own pressure. As the pressure increases with the smallness of the bubble, we should expect a small bubble to be very deeply buried, and a large bubble to be slightly buried. ”

“ You now see the image of the soap-bubble, which as long as the tube is closed remains unaltered in size; I open it, and it now at once contracts and disappears. This, then, conclusively proves that the air in the bubble was compressed. I will now go a step farther, and show that the amount of this compression depends on the size of the bubble. If it be large, the air is not so much compressed as if it be small. Let us consider what would happen if I formed bubbles at the two ends of a tube. ”

“ The air inside a bubble is generally under pressure, which is produced by its elasticity and curvature. If the bubble would let the air pass through it from one side to the other of course it would soon shut up, as it did when a ring was hung upon one, and the film within the ring was broken. ”

“ In pushing a bubble through a ring smaller than itself, by means of a flat film on another ring, it is important that the bubble should not be too large; but a larger bubble can be pushed through than would be expected. It is not so easy to push it up as down because of the heavy drop of liquid, which it is difficult to completely drain away. ”

“ If I blow in more air the caps become more curved, showing an increased pressure, and the sides bulge out even more than those of a sphere (Fig. 29) . I have now brought the whole bubble back to the spherical form. A little increased pressure, as shown by the increased curvature of the cap, makes the sides bulge more; a little less pressure, as shown by the flattening of the caps, makes the sides bulge less. Now the sides are straight, and the cap, as we have already seen, forms part of a sphere of twice the diameter of the cylinder. ”

“ If they were of the same size, evidently—the one pressing the air in one direction, and the other pressing it in the other with equal force no effect would follow. If, however, one bubble were smaller than the other, and what I have said be true, the small one would compress the air within it, and drive it from left to right (say) with greater force than the other would tend to drive it from right to left: hence the air would flow from the small bubble to the large one; the large one would increase, and the small one diminish. ”

“ There is, however, an opposite danger which may imperil the bubble when the surface-viscosity is great; and that is, that the liquid does not flow down in a straight line or regular curve, but in irregular masses, which every now and then tear away from each other. Now, these ruptures make little holes in the surface of the liquid; and when a hole is made the surface-tension tends to tear the liquid away, and to make it bigger. If the liquid has a very considerable surface-tension, the small holes in the surface may be so instantly turned into large ones that the bubble may burst. ”

“ But the resistance offered to its motion becomes greater the faster it moves; hence you have the bubble rising more quickly, the water being obliged to get more quickly out of the way, and finding more and more difficulty in doing so, and having, when the bubble gets very near the surface, less space between the bubble and the surface to flow away in. The result is, that the water cannot get out of the way, and therefore the bubble carries it up with it and forms a thin liquid film, which we see as foam upon the surface, through which the bubbles of air are rising. ”

“ These experiments are most conveniently made on a small scale. Pieces of thin brass tube, three-eighths or half an inch in diameter, are suitable. It is best to have pieces of apparatus, specially prepared with taps, for easily and quickly stopping the air from leaving either bubble, and for putting the two bubbles into communication when required. It should not be difficult to contrive to perform the experiments, using india-rubber connecting tubes, pinched with spring clips to take the place of taps. There is one little detail which just makes the difference between success and failure. ”

“ If a Bubble be blown with Water first made tenacious by dissolving a little Soap in it, 'tis a common Observation, that after a while it will appear tinged with a great variety of Colours. To defend these Bubbles from being agitated by the external Air (whereby their Colours are irregularly moved one among another, so that no accurate Observation can be made of them,) as soon as I had blown any of them I cover'd it with a clear Glass, and by that means its Colours emerged in a very regular order, like so many concentrick Rings encompassing the top of the Bubble. ”

“ If I again fill the pipe with water, and this time draw it rapidly out of the liquid, I shall leave behind a [Pg 82] cylinder which will break up into balls, as you can easily see (Fig. 36) . I should like now to show you, as I have this apparatus in its place, that you can blow bubbles of water containing paraffin in the paraffin mixture, [Pg 83] and you will see some which have other bubbles and drops of one or other liquid inside again. One of these compound bubble drops is now resting stationary on a heavier layer of liquid, so that you can see it all the better (Fig. 37) . ”

“ Now you know that coal-gas is lighter than air, and so a soap-bubble blown with gas, [Pg 128] when let go, floats up to the ceiling at once. I shall blow a bubble on a ring with coal-gas. It is soon evident that it is pulling upwards. I shall go on feeding it with gas, and I want you to notice the very beautiful shapes that it takes (Fig. 59, but imagine the globe inside removed) . These are all exactly the curves that a water-drop assumes when hanging from a pipe, except that they are the other way up. ”

“ I called one of the bubble forms, if you remember, by its name, catenoid; this is produced when there is no pressure. The dotted curve in the second figure B is this one; and to show that this catenary can be so drawn, I shall roll upon a straight edge a board made into the form of the corresponding section, which is called a parabola, and let the chalk at its focus draw its curve upon the black board. There is the curve, and it is as I said, exactly the curve that a chain makes when hung by its two ends. Now that a chain is so hung you see that it exactly lies over the chalk line. ”

“ That the jet we now see should be directed downwards rather than upwards may, I think, be explained in a general way as follows:—The great initial momentum of the sphere causes it to continue in rapid motion after the bubble has closed, thus the sphere acts as a sort of piston, which by increasing the length of the air-tube diminishes the pressure in it and so sucks in the bubble, which is driven down by the greater atmospheric pressure above. ”

“ The atmospheric pressure on a bubble six inches in diameter is over fifteen hundred pounds, and yet the fragile film lies safely between the opposing forces of nature—the pressure of the outer air, the spring of the inclosed cushion within it, the downward pull of gravity, the upward push of the buoyant atmosphere, and the molecular forces in the film itself: so long as the bubble lasts ”

“ The bubble is generally creased by the weight of the liquid along these channels. It must be remembered that the base of the bubble is in a state of oscillation, and that the whole is liable to burst at any moment, when such figures as 6 and 7 of the previous series will be seen.
SERIES V.
The Splash of a Drop, followed in detail by Instantaneous Illumination.
The Size of Drop and Height of Fall are the same as before, but the hollow shell (see figs. 4 and 5 of the previous Series) does not succeed in opening, but is left as a bubble on the surface. ”

“ Why, because the water of which it is composed has an attraction of particle for particle:—so great, indeed, that it gives to this bubble the very power of an india-rubber ball; for you see, if I introduce one end of this glass tube into the bubble that it has the power of contracting so powerfully as to force enough air through the tube to blow out a light (fig. 22) —the light is blown out. And look! see how the bubble is disappearing, see how it is getting smaller and smaller. ”

“ As the temperature increases these "beaded bubbles winking at the brim" give place to much larger bubbles, which are formed at the bottom of the vessel, and which rise a little way in the liquid, and then contract and disappear in a most mysterious manner, producing a hissing or simmering sound. But as the heating goes on, these bubbles, which consist of steam, rise higher and higher in the liquid, till at last they reach the surface and escape, producing a bubbling agitation, or the phenomena of ebullition. ”

“ Bubbles can be set to float upon the vapour without any difficulty. They may be removed in five or ten seconds by means of one of the small light rings with a handle, provided that the ring is wetted with the soap solution and has no film stretched across it. If taken to a light at a safe distance the bubble will immediately burst into a blaze. If a neighbouring light is not close down to the table, but well up above the jar on a stand, it may be near with but little risk. To show the burning vapour, the same wide tube that was used to blow out the candle will answer well. ”

“ If a soap-bubble be blown up, and set under a glass, so that the motion of the air may not affect it, as the water glides down the sides and the top grows thinner, several colours will successively appear at the top, and spread themselves from thence in rings down the sides of the bubble, till they vanish in the same order in which they appeared; at last a black spot appears at the top, and spreads till the bubble bursts. Hence it follows that the colours of a body depend in some degree upon the thickness and density of the particles that compose it ”

“ Soap-bubbles, blown with hydrogen gas, mixed with atmospheric air, will take fire, on presenting a lighted taper, and give a slight explosion. The ascension of these bubbles demonstrates, that the gas is lighter than atmospheric air, and it is its extreme levity that fits it for the purpose of filling balloons. It may be made twelve times specifically lighter than atmospheric air, by passing it over dry muriate of lime, in order to absorb the moisture it may contain, provided the gas be free from carbon, or carbonic acid. ”

“ In short, to have bubbles in perfection you should blow them in an ice room! As it is not every one that can obtain the use of a meat storage [521] safe for bubble blowing, let us make our first experiment in a cool room. Begin with working up a good stiff lather; and the better the soap the better the suds for our purposes. Use your tube as you would your pipe, and blow downwards into the basin steadily and strongly. Take a good breath of air to begin with, and hold it for a second or two. Keep the point of the tube downwards until you have fixed on the disc in the way shown in the sketch. ”

“ Froth consists of bubbles, each a tiny globular portion of air, bounded on the upper surface by an exceedingly thin film of liquid; the bubbles retain this position and condition until the struggle ends as a victory for one of the belligerents. The ascensive power of the air within the bubble overcomes the cohesive power of the liquid film or covering, and 'the bubble bursts'—our beverage loses its frothy 'head.' The length of time that elapses before the air escapes by the bursting of the bubble depends chiefly on the viscidity of the liquid. ”

“ Fun blowing glass bubbles. Smooth one end of a piece of No. 2 tube and allow it to cool. Close the other end in the blowpipe flame, turn it slowly, and heat until it is very hot. Take the tube out of the flame, put the smooth end into your mouth quickly, and blow as hard as you can (Fig. 11) . Do you get a fine big glass bubble which bursts with a pop?
7If you get only a small bulb at the first trial, heat the end, and try again. Do you find that the bulb shrinks when heated but blows out again readily? ”

“ Why is it so easy to blow bubbles with some liquids, and so hard to form them with others? Why does a bubble when blown at the end of an open tube gradually contract and disappear? Why, when it bursts, does it not still remain a liquid film, but is shattered to an almost imperceptible dust? ”