Text: (8.) Origin of the rotation.- It thus appears that in the mechanical rupture of a floating bubble forces are brought into play similar in kind and general direction to those operating on the ring-discharges from an orifice. Of these, one is evidently the tensional action of the liquid film driving the cloudy air through the opening, another is the resistance of the adjoining air into which the mass is impelled. The former corresponds in effect to the tension propagated through the reservoir in the previous experiments, the latter is the same in both cases; and we may consider the widening aperture of the bubble through which the air is thus as it were squeezed, as answering to the orifice on the top of the jar. This view of the mode of action of the film is confirmed by the very curious effect which results when the bubble is punctured on one side. Thus applying the point of the wire near the base we see the cloudy column rushing out in a nearly horizontal direction on that side. If we pierce the bubble at an intermediate height the air is projected obliquely upward toward the hand. It can hardly be doubted therefore that in all cases the bubble is destroyed by a progressive and regular enlargement of the opening first made, and that this takes place with sufficient slowness to allow the aperture to give direction to the extending force. (9.) Rings generated from spherical bubbles.-The effects just described are produced in a still more striking degree by the rupture of the bubbles of completely spherical form. In this as in the former experiments the utmost care must be taken to protect the adjacent air from agitation. With this view the bubble as soon as blown should be deposited at the bottom of the glass bowl on a piece of soft cloth or a flake of carded cotton, and left undisturbed for ten or fifteen seconds. On piercing it at the top with a very slender wire we see its contents suddenly projected up wards some inches, forming a cloudy column whose summit rolling over on all sides develops a beautifully distinct ring which at times ascends so rapidly as to separate from the general train. The effect is best seen with a bubble of from three-fourths to one inch in diameter. When the contiguous air is perfectly quiescent the ring retains its form even above the top of the bowl, affording a clear though momentary view of its twofold spiral and the resulting horizontal bands, as well as of the direction of its rotation. When pierced at the side instead of the apex, the bubble, as might be expected, projects its contents towards that side in the direction of the wire, showing though less perfectly the same configuration. Both effects are rudely indicated in fig. 5. As these experiments on the rupture of bubbles will be found to have much interest, I may suggest that they can be easily repeated by forming the bubbles with tobacco-smoke blown from the stem of a common pipe, from which they are readily transferred to the soapy water as spherical segments or to the cloth surface as entire spheres. But it must be remembered that even a slight motion of the contiguous air will defeat the observation. I need scarcely add that the tensional action of curved films of liquid, so strikingly shown in these phenomena, has been illustrated by Prof. Joseph Henry in a series of interesting experiments of which an account was published some years ago. In the present case the most important inference to be drawn from the results is that the destruction of the bubbles proceeds not from a subversion of cohesion irregularly throughout the film, but from a uniformly progressive enlargement of the opening first formed, by the continuous retraction of its edge. (10.) Of the rings produced by floating bubbles of phosphuretted hydrogen gas, when exploded.- In what has just been stated we mark the effects simply of the mechanical rupture of the bubble, calling into play as a motive agent the tensile force of the film, or what is equivalent, the expansive action of its contents released from the pressure under which they have been held. But in the present case the rupture has no sooner begun than a chemical action of great intensity sets in between the included gas and the air. This commencing usually at the apex and extending downwards and laterally in a symmetrical manner gives rise to a powerful expansive force having an obliquely upward direction on all sides. The products of the combustion are thus impelled into the contiguous and comparatively quiescent air under conditions very analogous to those of an energetic momentary discharge from the ring apparatus of the former experiments. The resistance at the sides of the ascending and spreading column, combining with the upward impulsion of the interior will, therefore, give rise to a similar rotation of the cloudy mass, rolling it up into the spirally constructed ring in which it presents itself at the close of the explosive action. It will be remarked that these explosive rings are more rapid in their expansion as well as their rotation than the rings produced by mechanical discharge, a result due doubtless to the greater energy of the forces by which they are developed. As might be expected, rings of the latter kind approaches more nearly to the former in these particulars when a vigorous impulse is applied in producing them. In most cases the wreath of phosphoric acid is so opaque as to preclude the examination of its interior structure, but when more diluted, we are able to trace within it, as in the other rings, the two-fold coil of cloudy and clear atmosphere. In experimenting with the explosive bubbles every one must have noticed the frequent occurrence of irregularities and failures in the production of the ring. This is a natural consequence of the rupture beginning on the side or at more than one spot, instead of commencing at the apex of the film and accordingly in such cases it will be remarked that the flame and smoke dart out laterally, producing a broken and almost formless wreath. III. Of the formation of liquid rings. The production of liquid rings by a succession of drops, although only of occasional occurrence, has doubtless often been observed in the course of laboratory manipulations. Yet so far as I am aware no attempt has hitherto been made to determine, the structure and movement of these rings, or the precise conditions under which they are generated. My attention having been called to this class of effects by some remarks of Prof. Horsford on the rings formed by precipitated sulphate of lead, I was led after various trials with this and other precipitates to discard the chemical action as irrelevant to the particular effect in view, and do employ as a dropping-liquid water charged with some coloring substance, either suspended or dissolved. Among the materials thus used may be mentioned chromate of lead, carbonate of lead, sulphate of lead, sulphate of baryta, cobalt blue and dilute solution of sulphate of indigo. Of these, the two first and the last yield perhaps the most perfect results. 11. Production of liquid rings by drops.- A convenient apparatus for these experiments consists of a globular pipette (fig. 6) of about two inches in diameter, mounted on an arm which is capable of turning easily in a horizontal plane, and a large cylindrical vessel filled with clear water nearly to the brim. The beak of the pipette, about an inch in length, has a smoothly ground aperture of one-tenth inch. The upper tube, bent at right angles, is fastened into a flexible pipe about eighteen inches long, to the outer end of which is adapted, by means of a small stopcock and short gum-elastic tube, a slender tub of glass drawn to a fine capillary bore. In order to charge the pipette, we revolve the horizontal arm into a convenient position, and bring the small vessel containing the colored liquid up to the beak. Then slipping off the coupling tube we apply the lips at the stopcock, pump up the charge and quickly closing the stopcock replace the coupling and capillary tube. After one or two drops have fallen the flow ceases and the pipette may be brought round over the centre of the reservoir. By opening the stopcock either partially or wholly we have perfect control over the rate of discharge, and can make the drops succeed each other as slowly as we please. It should be observed that air bubbles carried by the drop into the liquid produce an irregular motion, destructive of the desired effect. Hence the beak should be placed only a short distance above the surface. An interval not exceeding one and a half inches usually answers very well, but at a distance of from one half to one inch admirably uniform results are obtained. Indeed, it is not necessary to the effect that the drop should reach the surface with any sensible velocity, as a well-formed ring will be produced by simply laying the drop upon the water from the closely approached beak of the pipette. As it is essential to a perfect experiment to have the liquid of the reservoir as motionless as possible, its mass should be large, and it should be allowed to come to rest after each drop before the next is allowed to impinge upon it. Operating with these precautions, and viewing the result from a point a little lower than the level of the fluid, we see that the drop, soon after merging in the liquid, gives origin to a ring of exquisite symmetry, which rotates and enlarges as it descends, precisely after the manner of the gas-rings already described. In some cases the ring, continuing unbroken through the whole depth, spreads out on the bottom a flat annulus of the heavy coloring matter. But usually, after reaching a distance of four or five inches, it breaks up with a peculiar outward uncoiling motion, forming at intervals of the circuit flattened spaces, from the outer points of which the pigment is seen dropping in numerous slender streams. 12. Motions and structure of the liquid rings.- The rotation of the liquid ring on its circular axis is directed upwards on the outer circumference and downwards on the inner; or viewed in relation to the progression of the mass, it presents an advancing movement of the inner and a retreating movement of the exterior periphery. It is thus identical with the rotation of a gas-ring impelled in a descending direction. The liquid ring, moreover, resembles that of gas in being composed of a coil of colored fluid enfolding a parallel uncolored coil. Indeed, when sufficiently translucent, it exhibits quite distinctly the horizontal bands which, in the case of rings of cloudy air, have been shown above to be an optical result of this twofold structure. The correspondence between the phenomena is rendered still more complete by the fact, that the liquid ring is followed by a residualy colored mass, which, when the forming impulse is feeble, remains attached to it as a train. 13. Formation of liquid rings by impulsive discharge.- The observed identity of motions and structure in the two classes of rings led me to attempt the production of liquid rings by a mechanical process similar to that used in forming the rings of air. For this purpose I lowered the beak of the pipette so as to immerse it a little below the surface of liquid in the reservoir, and closing the stopcock applied a sudden and transient pressure of the fingers to the flexible tube. The experiment was eminently successful. The colored liquid thus discharged was seen to shoot downwards in the form of a very perfect ring, in all respects resembling those above described, except that its rotation and expansion were more rapid. Modifying the arrangement by attaching to the stop-cock a small but thick gum-elastic bag, I have found it easy to regulate the impulse so as to develop the rings as slowly as may be desired, and thus to reproduce with the colored liquid all the stages of the phenomena previously marked in the case of rings of air. In this way, by a gentle and rather gradual force, I can cause the escaping fluid to rise up intolateral volutes, or, increasing the action, to form the opening ring with its attached train; or, by a yet quicker and stronger impulsion, I can compel the ring to shoot rapidly away, leaving the train either to break up irregularly or to form by its own motion, a second smaller ring, as in the case of discharges of air. The rings thus formed in the midst of the liquid will, of course differ in size, according to the amount of liquid expelled at each impulse. As, however, this is never so small as a drop, and the velocity of the action is comparatively great, the rings thus formed are always larger than those resulting from the dropping process. They are, therefore, better suited for observations on the internal motions and duplicate structure of the liquid ring. It is essential in such observations, however the ring may be formed, to make use of a partially transparent liquid, such as a dilute solution of sulphate of indigo, or a thin mixture of cobalt blue, and to view the ring by a moderately strong light. In these circumstances the double spire and the horizontal bands become beautifully apparent. 14. Origin of the liquid ring, and of its rotation.- Considering the entire agreement in structure and motions of the liquid ring as compared with the mechanically formed ring of air, it is natural to conclude that similar conditions of force are concerned in their production. Such is evidently the fact with rings generated by the process last described, that is, when the orifice is at or below the level of the reservoir. Here, the impulse of discharge acting downward and laterally, and the resistance of the aperture and of the contiguous medium acting backwards, present a combination of forces precisely such as, according to the previous explanation, operates in the production of the rings of air. In regard to the formation of rings by the dropping process, the mechanical conditions, although apparently different, are such as I think would naturally give rise to the same combination of motions. In considering the mode of action of the drop it is proper to distinguish between the case of a drop which impinges upon the surface of the medium with a sensible velocity and that of one simply laid upon it from the beak of the dropping tube. The former strikes the medium with the acquired momentum, and penetrating into it as if through a circular aperture, forms an advancing column whose central parts are carried forward, while the sides are relatively retarded by the resisting action of the contiguous fluid. Under these conditions, the forces brought into play and their resulting motion must evidently be the same as in the case of an impulsive discharge of liquid from the immersed beak of the pipette. In the other case, that is, where the drop is simply laid upon the liquid plane, the gravity of the matter of the drop and the tension of the curved surface unite in giving a downward impulsion to is contents. At the same time, the surface of contact with the fluid beneath, forming, as it were, a rapidly enlarging circular aperture, secures the symmetry of the moving mass. This, as it advances, will of course be subject to the same moulding action of the impulsive and retarding forces which has just been described, and which, as we have seen, imparts to rings of air and water formed by mechanical discharge their identity of structure and motion.

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