ON PHYSICS, OR NATURAL PHILOSOPHY.

No. XXVIII.

(Continued from page 8.)

VIBRATION OF THE AIR IN SONOROUS PIPES.

Origin of the Sound in Wind Instruments.-In the different apparatus described in former lessons, the sound arose from the vibrations of solid bodies; the air was only the medium of conveyance. In wind instruments, constructed of pipes whose sides are sufficiently strong to resist vibration, it is the column of the air inclosed in the pipes which is alone the sonorous body. It is, in fact, demonstrable that the matter of the pipes has no influence on the sound, which is the same under equal dimensions, whether they are made of wood, glass, or metal, with the exception of the timbre or distinct quality of the sound. As to the manner of putting the air into vibration in pipes, wind instruments are divided into instruments with a mouth-piece, instruments with a rigid tongue, and instruments with a membraneous tongue.

Instruments with a Mouth-piece.-Iu instruments having a mouth-piece, all the parts of the latter are fixed. Fig. 140 Fig. 147.

Fig. 146.

body, as well as in others formerly investigated, there are portions which vibrate and portions which do not vibrate, that is, swells and nodes. When a pipe is closed at the extremity opposite to that where the passage is, it is almost evident that the stratum of air in contact with the bottom

Fig. 148.

belongs to a node of vibration; and it is equally manifest that the portion of air in the neighbourhood of the mouth belongs to a swell, for it is at this point that the disturbing cause exists. This node and this swell occur only when the pipe emits the lowest or fundamental sound. A change in the diameter of the pipe, in the dimensions of the mouth, or in the velocity of the current of air, will produce a series of sounds rising gradually higher and higher, and affecting the ratios of the different portions of the column of air in the pipe, as shown in fig. 149.

represents the mouth-piece of an organ-pipe; and fig. 147 represents that of a whistle or flageolet. In these two figures, the opening i is called the passage; by this aperture the air is introduced into the pipe. The opening bo is called the mouth, and its upper lip, b, is made with a feathered edge. The upper part of both figures represents the pipe, which may be either open or closed according to the nature of the instrument. In fig. 146 the foot, or extremity, P, is employed to fix the pipe on a blowing machine, as shown in fig. 134, p. 364, vol. iv. When a rapid current of air rushes through the passage, it strike against the upper lip, and produces a concussion which prevents the air from issuing in a continuous manner from the mouth, and causes it to proceed intermittingly. From this cause the pulsations which are transmitted to the air in the pipe make it vibrate and emit a sound. In order that the emitted sound may be clear and distinct, a certain relation must be established among the lips, the aperture of the mouth, and the size of the passage; and the pipe must have a great length in comparison with its diameter. The number of vibrations depends, in general, on the dimensions of the pipe and the velocity of the current of air. In the German flute the mouth is a simple lateral and circular opening. It is owing to the arrangement of the lips that the current of air is made to strike against the edges of this aperture; this is seen in the Pandian reed, and in the perforated key which is made to whistle.

The following representation (fig. 148) of an organ-pipe or flute, exhibits a side section and front view, in which i shows the extremity of the passage, and the mouth. Since the vibration of air contained in the pipes is really the cause of the production of the sound, we shall find that in this sonorous

VOL. V.

Sound 3.

Sound 5.

2 codes and 2 swelle.

3 nodes and 3 swells.

After the first or fundamental sound, that which is produced the most naturally allows the node and the swell at the extremities of the pipe to remain ; but a new node appears at a third part of the column of air reckoning from the mouth, and a new swell at a third part reckoning from the bottom. The higher sound which immediately follows forms an additional node and an additional swell. The sounds which correspond to these different states of the pipe are higher and higher. The manner in which the column of air is divided in a closed pipe, shows that the different sounds emitted are to each other as the series of odd numbers 1, 3, 5, 7, etc.

In a pipe open at both ends the arrangement of the nodes and swells is different. The bottom, which in the former case determined at once a node of vibration, no longer exists; consequently, a swell takes its place. Thus, there is at each extremity of an open pipe, a swell; it is necessary, therefore, that there should be at least one node in the interior. Experiment shows that this node is in the middle, and that it divides the column of air into two parts which vibrate in an inverted direction. Each of these parts is, of course, one-half of that which would vibrate if the pipe were closed at one end, and this explains the reason why the fundamental sound of the pipe open at both ends is the higher octave of the former. After this fundamental sound, there exist also, in pipes open at both ends, a scries of sounds which arise from

106

the subdivision of the column of air into aliquot parts, and which produce new swells and new nodes, as exhibited in fig. 150.

Fundamental sound,

or sound 1.

Fig. 150.

The existence of these nodes and swells is proved by introducing a piston into the pipe at each of the nodal points, and making holes in its side at each of the swell points; and when they are accurately adjusted, these different operations do not alter the height of the sound. The length of the column of air comprised between two nodes is called a concameration. The length of a concameration has been found by experiment nearly equal to the length of the wave of the sound produced by propagation in free air.

Instruments with a Rigid Tongue.-The air is put into vibration in instruments having a rigid tongue by a simple elastic piece of metal or of wood, which is put in motion by a current of air. This tongue acts in a manner easily comprehended, see figs. 151 and 152, which show a front and side view of it. Fig. 151.

Fig. 152.

F

Fig. 154.

M

Lis the tongue fixed at one extremity, in front of a rectangular opening, o, made in the side of the pipe, whether square or round. This tongue is constructed in such a manner that it is bent in a state of rest, and leaves the opening free to the admission of air. But if a current of air be produced in any manner so as to have a tendency to rush through the aperture o, in the direction indicated by the arrow, the tongue is forced against the side of the pipe, and for a moment it stops the flow of the current; then its elasticity restores it to its first position after this it is again impelled against the side of the pipe, and so on alternately, as long as the air exerts a tendency to rush through the aperture o. Thus we see that the impulse of the air must be made intermittingly, and when these intermittent impulses are sufficiently rapid, a sound arises which is stronger than that proceeding from the vibrations of the tongue or lamina alone.

This kind of tongue is used in hautboys, bassoons, clarionets, children's trumpets, and other simple instruments of this kind. Some organ-pipes have a mouth-piece like that already described, fig. 146; others have rigid tongues. Fig. 153 is a representation of one of the latter, with the arrangement best adapted for demonstration. It is mounted on the box of a blowing machine, and glass, inserted at E in the sides of the pipe, admits of the vibrations of the tongue being seen. A wooden apparatus, like a trumpet, is used for strengthening the sound. Fig. 154 represents the tongue outside of the pipe. It is composed of four pieces:-1st, a rectangular wooden pipe closed at the bottom, and open at the top, at o; 2nd, a brass plate, cc, pierced by a longitudinal aperture, called the channel, which is intended to give a passage to the air of the pipe MN to the or fice o; 3rd, an elastic lamina, i, which is

pipe м N, and that a current of air is admitted into the latter by the foot P; the tongue will then be acted on by the air, and by this action, being bent inwardly, will give a passage to the air, which will escape by the orifice o. But the tongue, in consequence of its elasticity, will return immediately to its first position; it will thus form a series of oscillations which will alternately open and close the channel, and cause the current of air to act intermittingly; hence arise the sonorous waves producing a sound whose height increases with the velocity of the current.

Membraneous Tongues.-The effects of membraneous tongues are not yet well ascertained; but they are interesting, as connected with the theory of the human voice. The following are the principal results of the researches of M. Muller, the physiologist, on this subject:

1st. If a narrow strip or ribbon of caoutchouc be stretched over a ring about of an inch in diameter, and if by means of a pipe of small diameter we blow on one of the edges of the ribbon in a direction oblique to its surface, the ribbon will be made to vibrate and a sound will be emitted..

2nd. If a strip of caoutchouc, of about of an inch in breadth, be stretched over a ring or a wooden frame, and if on each side of this elastic strip we fix a rigid plate made of card or of wood, so as to admit of a very straight and narrow slit between the plates and the caoutchouc, this strip will be made to vibrate by surrounding the ring or frame with the lips and blowing through it, or by blowing through a porte-vent or pipe, at one end of which the apparatus is placed.

3rd. If one-half of the orifice of a very short pipe be covered with an elastic membrane, and the other half with a rigid plate, leaving a narrow slit between them for the passage of the air, the membrane will be put into a state of vibration by the same means as in the preceding case.

4th. If the orifice of a very short pipe be covered with two elastic membranes instead of one elastic membrane and a rigid plate, leaving the narrow slit between them as before, the same effect will be produced by employing the same means. This arrangement is most like that of the glottis, or chink of the larynx, which is the upper part of the windpipe.

LESSONS IN CHEMISTRY.-No. XXVII. The Metal Lead.-The physical characteristics of this very useful metal are so well known, that they scarcely require description. Soft, blueish white, possessing considerable specific gravity, very fusible, laminable, ductile, easily tarnishing on exposure to air,-lead, when obtained free from combination, could scarcely be confounded with any other metal. Not only is lead very plentifully distributed throughout nature, but it is a very useful metal, as the slightest reflection will demonstrate to the student. Its combinations, however, are for the most part poisonous, the amount of their power being dependent on their degree of solubility; and as lead is employed for thousands of purposes involving intimate contact with articles of food and drink, it behoves us to be able to appreciate the conditions under which this metal, so useful a servant, may become an

insidious enemy.

Chemical Qualities of Lead Combinations.-Combinations of this metal, as of every other, admit of classing into the soluble and the insoluble. For the purposes of our experiments we shall require a soluble solution, and none is better for our purposes than a solution of acetate of lead, commonly called "sugar of lead." It consists of protoxide of lead united with acetic acid. Take about as much as will lie heaped upon a fourpenny piece of crystallised sugar of lead, and dissolve it in about a wine-glass full of common water; if pump or well-water, all the better. Try to dissolve it, I should have said; because, as the operator will find, perfect solution cannot be effected, but a white powder will remain. So far, then, as concerns the solution we were desirous of making, our operation is a failure. I meant it to be a failure, in order to demonstrate a fact. The white appearance developed is, however, not without its own significance, and we shall revert to it by-and-by; meantime preserve the whitened liquor in a properly-labelled corked bottle. Repeating now the experiment with the substitution of pure distilled water for impure or common water, a solution of absolute transparency will result, if the water employed be totally free from impurity; gradually, however, it will be found that mere contact with atmospheric air gives rise to a certain turbidity, and this for a reason we shall discover presently.

Having prepared our solution, let us now deal with it analytically. Let the reader assume it to be unknown; let him, for the purpose of argument, call it x. What is the ? Such is the question we require to solve.

By this time the student will know intuitively, so to speak, the series of questions we have to demand of our unknown x. Is it a metal? Yes or no. If a metal, what class of metal? what section of the class? and lastly, what metal?

I need scarcely state that you will begin your testing operation by the employment of hydrosulphuric acid in aqueous solution. It yields a black precipitate, thereby teaching us that the solution we have to deal with not only contains a metal, but a metal of the calcigenous class; teaches us, moreover, that the metal is neither iron, manganese, uranium, cobalt, or nickel, all of which require hydrosulphate of ammonia to effect their precipitation: teaches us that the metal is neither zinc, nor arsenic, nor antimony, cadmium, or tin in per-combination; since, were it either of these, the precipitate could not be black. Gradually, then, we have narrowed the list of bodies to which our a might belong. The analyst might now, in the ordinary course of testing, employ an aqueous solution of ferrocyanide of potassium (prussiate of potash) as a test; under which circumstances he would obtain a whitish precipitate-the teaching conveyed by which would be still less than that already imparted by the hydrosulphuric acid. The operator would now have recourse to "random tests;" likely enough he would at once have recourse to a soluble chloride, say chloride of sodium, or common salt. Supposing a solution of this substance employed-the stronger the better, saturated by preference-he would most likely obtain from a solution of acetate of lead of the strength described, a white precipitate. I say most likely, because in proportion as the temperature of the lead-containing solution is higher, so is the tendency less of the white precipitate to fall. Its precipitation, however, may be assured by adding a certain amount of alcohol (rectified spirit of wine).

On collecting a little of this white precipitate (say a few grains), placing it in a little flask, adding pure water, and

boiling, it will be found to dissolve completely. In other words, the white substance obtained by the addition of chloride of sodium to acetate of lead is the chloride of lead, which is so far from being totally insoluble in water (even cold), that we were obliged to add alcohol in order to insure its absolute precipitation; and is so perfectly soluble in hot water that we have been enabled to convert the former white powder into a colourless solution. This precipitate, moreover, is neither whitened nor turned black by the agency of ammonia. Let us now draw these facts into one focus and appreciate their significance. In the first place, we have already seen that there only exist two metals the chlorides of which possess the slightest claim to the quality of insolubility: of these the protochloride of mercury (calomel) and the chloride of silver may be regarded as abso lutely insoluble (in water); the third, which is chloride of lead, is not absolutely insoluble. Clearly, then, it is chloride of lead we are dealing with, and consequently the metal whose solution we are investigating must be lead. The action of ammonia still further demonstrates that it can neither be chloride of silver nor of mercury. However, the chemist would scarcely think it safe, in any instance, to rely implicitly on the indications of one line of testing. He would scarcely then be warranted in allowing our unknown x to escape without further questioning. Effect of Sulphuric Acid and soluble Sulphates on Salts of Lead.Let me premise, in reference to these tests, that the sulphuric acid to be employed as a test in this instance should be for convenience diluted, the dilution being in the ratio of about one of acid and nineteen of water (by measure). As to the soluble sulphates, the experimentalist may either employ sulphate of soda, much used in veterinary practice under the name of "Glauber's salt," or else sulphate of magnesia, better known as Epsom salts." Taking a little of the solution of x, add to it a portion of dilute sulphuric acid, or any soluble sulphate. Remark the heavy white precipitate which deposits. Remark, too, how insoluble this precipitate is in water-even hot: for if a portion be boiled in pure water, the clear liquid decanted, and tested with hydrosulphuric acid, not the slightest blackening results; an evidence so palpable as to require no comment. Remark, too, that this same white precipitate is scarcely soluble, if soluble at all, in nitric acid, even though boiling.

66