only necessary to place the balloon near a gasometer, and fill it by means of a connecting-pipe.

In fig. 90 is represented the mode of filling a balloon with pure hydrogen. On the right of the figure is shown a series Fig. 91.

of casks, which contain iron filings, water, and sulphuric acid, substances necessary for the preparation of the hydrogen. From each cask, the gas is conveyed to a central cask, open at bottom, and immersed in a butt full of water. The gas, after passing through this water, is conveyed into the balloon by a long canvas pipe, fixed at one end to the central cask, and at the other to the bottom.

In order to facilitate the filling of the balloon, two masts are erected, having at their top pulleys traversed by a rope, which passes through a ring fixed at the top of the valve. By this means, the balloon being at first raised about a yard above the ground, the gas is admitted; then, in proportion as the balloon is filled, it is raised a little higher, and it is allowed to expand more and more, until it frees itself from this apparatus. It is now necessary to oppose the force with which it begins to ascend. For this purpose, a number of men are employed to hold it down by means of cords fixed to the netting. When the balloon is completely filled, it is then necessary to remove the pipe which conveyed the gas, and to attach the car to the net-work. These different preparatory operations require at least two hours. The aeronaut is then seated in the car, and at a given signal, the cords are loosed, and the balloon ascends with a velocity in proportion to its lightness as compared with the air which it displaces.

It is important to observe that a balloon should not be completely inflated; for the atmospheric pressure diminishing in proportion to the height of the ascent, the interior gas expands in consequence of its elastic force, and tends to make the balloon burst. It is sufficient that the force of ascent; that is,

the excess of the weight of the air displaced above the whole weight of the apparatus, be about ten pounds. It is to be observed that this force remains constant so long as the balloon is not completely inflated by the expansion of the interior gas. For, if the atmospheric pressure be reduced to one-half, the gas in the balloon, according to Mariotte's law, is increased to double its volume. Whence it follows, that the volume of air displaced is itself doubled, and its density is reduced to one-half; therefore its weight, and consequently its upward pressure or buoyancy are still the same. But as soon as the balloon is completely inflated, if it continue to rise, the force of ascent diminishes; for the volume of air displaced remaining the same, the density diminishes. Accordingly, the balloon will ere long reach a point where the upward pressure Consequently, the balloon can only take then a horizontal direction, being carried by the currents of air which exist in the atmosphere.

is zero.

The indications of the barometer are the most certain means by which the æronaut knows when he is ascending and when he is descending. In the former case, the column of mercury falls; in the latter, it rises. By the assistance of the same instrument, he is enabled to ascertain the height which he has reached. A long streamer fixed to the car, fig. 91, also indicates, by the position which it takes above or below the car, whether he is ascending or descending. When the aeronaut wishes to descend, he draws the cord which opens the valve placed at the upper part of the balloon; the hydrogen mixes with the exterior air, and the balloon descends. On the contrary, in order to slacken his descent when it is too rapid, or to re-ascend if placed in a perilous situation, the aeronaut empties bags full of sand, a sufficient quantity of which had been placed in the car for this purpose. Thus lightened, the balloon rises again, in order to descend in a more suitable place. The descent is facilitated by suspending an anchor to the car by means of a long cord. When this anchor has taken hold of a proper obstacle on the ground, the car and balloon are lowered by gently drawing the cord.

Balloons have not as yet received any important applications. At the battle of Fleurus, in 1794, a balloon, retained by a cord, was employed to discover the movements of the enemy, which were made known to the army by signals made by an observer seated in the car. Several ascents have also been undertaken with the view of making meteorological observations in the higher regions of the atmosphere. But balloons will only become of real utility when the power of directing them has been attained. The trials hitherto made for this purpose have completely failed. At present, we can only rise in the atmosphere until we meet a current of air which will carry us in the direction answerable to the end we have in view.

The Parachute.-The object of the parachute (from the French, a guard from falling) is to enable the aeronaut to leave his balloon, by giving him the means of slackening the velocity of his descent. This apparatus is composed of a large circular sail, fig. 92, of about five or six yards in diameter, which, by the effect of the resistance of the air, expands and forms a huge umbrella which slowly descends to the ground. On its edges are fastened cords, which support a car, in which the aeronaut is seated. In the centre of the parachute, there is an opening for the escape of the air which is compressed by the effect of the descent; without this, the air would produce oscillations on the parachute, which would be communicated to, the car and render the position of the aeronaut perilous. In fig. 91 is shown, on the side of the balloon, a parachute folded and attached to the netting, by means of a cord passing over a pulley and fixed to the car. By loosening this cord, the parachute is placed in the power of the aeronaut. M. J. Garneri was the first who descended in a parachute; but M. Blanchard appears to have been the inventor.

Weight required to raise a Balloon.-In order to calculate the weight required to raise a balloon of given dimensions, when it is supposed to be perfectly spherical, the following formula is employed: v=D, which represents geometrically the volume of a sphere, whose diameter is D, 7 being the ratio of the circumference to the diameter, or 31416 nearly. Thus, if a balloon of thirty-six feet in diameter were completely filled with hydrogen, its volume would be about 24,430 cubic feet. But in general, the balloon, when it begins to ascend, is only about half filled, whence its volume may be assumed at

12,215 cubic feet; and such is the volume of displaced air at the first moment of its ascent. According to calculations formerly shown, this quantity of air weighs about 9914 lbs. or nearly nine cwt., and this is the upward pressure which tends Fig. 92.

to raise the balloon. But in order to calculate the real force of the ascent, we must subtract from this pressure the weight of the hydrogen in the balloon, and of the globe of which it is made, with its appendages. Now, the weight of hydrogen is about part of the weight of air; whence, the weight of the gas in the balloon is about 9911-14-71 lbs., nearly. Adding to this weight that of the globe and its appendages, formerly reckoned at about three cwt., we have upwards of 3 cwt., say four cwt., for the weight to be subtracted from the nine cwt. just mentioned; this leaves a remainder of about five cwt. for the force of the ascent. But we have seen that it is sufficient for the force of ascent to be about 10 lbs.; whence, there is a little less than the weight of five cwt. remaining for the additional weight which a balloon may safely carry into the atmosphere.

LESSONS IN CHEMISTRY.-No. XIX. THE subject of our present lesson shall be the metal silver; not only so interesting for its commercial value, but as regards its striking chemical qualities.

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There are not many metals which admit of being traced through a long list of combinations, and again obtained in the metallic form, so easily as silver. Its chemical physiognomy is, in point of fact, exceedingly well marked, as we shall presently see. It is always well to begin the chemical examination of a substance, by choosing the same in a pure condition, unmixed with any accessory that might veil its properties or obscure the result. therefore recommend, as the source for obtaining a silver specimen, a few grains, say eighteen or twenty, of the salt called nitrate of silver. This substance occurs in commerce under two forms: either as sticks something like slate-pencil, only whiter, or as crystals. The latter will be somewhat the purer of the two; but the former, known popularly as "lunar caustic," will answer very

well.

Let the student then take about eighteen or twenty grains of lunar caustic, or rather more of crystallised nitrate of silver, and effect a solution of the substance in about half a pint of distilled water. The solution takes place with great facility, and

may be readily accomplished in a Florence flask,-all the more rapidly under the influence of a gentle heat. The solution will be perfectly colourless and transparent; not the slightest amount of milkiness will be perceptible. I can fancy many a reader poring over his solution at this moment, and imagining the writer of these lessons to have erred. Some, in looking at a milky opalescent solution, will be ready to think that the assurance of "perfect clearness" is altogether untrue. If the water be quite pure, the solution will be absolutely transparent; but inasmuch as nitrate of silver is a most delicate test for certain classes of impurities, it is more than probable that many students may get a turbid solution.

Should this be the case in the present instance, heed it not. The occurrence will serve to mark a fact, without interfering with the current of our experiments. You have only to wait awhile, and the turbidity will settle, leaving a clear solution above, well adapted for our purposes. Having followed out the preceding directions, it is evident that a solution of nitrate of silver in water will have been obtained. We will proceed to investigate its chemical characters presently; meantime, let it be well impressed upon the mind that the solution is colourless: hence it follows that any solution which is not colourless, must contain some other substance besides nitrate of silver. We may generalise still more, and say that all silver solutions are colourless. Strictly true this assertion is not, I am aware; but it is, nevertheless, so nearly true, as to warrant its being considered by the student as a universal fact. Accepting the proposition as absolute, we may then make the further assertion, that, though a metallic coloured solution may contain silver, it must contain some metal in addition to silver.

The appreciation of these broad qualities-these general characteristics, are of the highest importance in chemistry: several metals being recognisable at once, by noticing the colour of their solution. That the reader may at once see the force of this remark, let him dissolve a small silver coin in some pure aquafortis, diluted with about an equal volume of water, for the purpose of moderating the violence of the action which ensues. The experiment is best conducted in a Florence flask, which may be placed in hot sand on a grate hob, in order that the injurious fumes which escape may be carried up the chimney.

When the operation of solution has been effected, remark well the tint of the resulting fluid. The experimenter has employed a silver coin, I have assumed, dissolved it in an acid, i. e. aqueous nitric acid or aquafortis. Having regard to the substances used, therefore, it would seem that a solution of nitrate of silver should result. Nevertheless the solution is no longer colourless but blue, and if the student evaporates it, blue crystals will appear. It follows, therefore, that if there be any truth in what I have stated, the silver coin must have contained something in addition to silver. Now supposing the colouring agent to be metallic, and it must be so-by "construction," as geometers say-in other words, it must be so, because we have only used a metallic coin, then it follows, firstly, that the coin was not of pure silver, but an alloy. Secondly, that the alloying substance was a metal yielding a blue nitric acid solution. Now I am only aware of two metals which are capable of yielding such a blue solution. These metals are copper and nickel; and most people know, I presume, that copper is the metal used for alloying our silver coins. Pure silver would be altogether too soft for the purpose, as the reader will not fail to see when he shall have developed a little of that metal from its liquid combination.

Put away this cupreous silver solution, duly labelled. To expatiate on it here would be so far out of order, that we are discussing the properties of silver, not copper. It will, neverthe less, come under our notice when we treat of the latter metal; indeed even before, for I shall put the student in possession of an easy means by which all the silver may be separated, and the copper left behind.

Returning now to our solution of nitrate of silver, let the student question it thus:

(1) What is its nature?

To arrive at an answer to this question, drop a little of your strong solution, say twenty or thirty drops, into a wine-glass; fill up the wine-glass with distilled water, and test with hydrosulphuric acid solution. We get a well-pronounced black precipitate, on observing which we immediately deduce the following truths. (1) The solution contains as its base, a metal. (2) A calcigenous metal (vide Lesson p. 39). (3) Neither zinc, arsenic,

antimony, cadmium, nor tin, in the state of persalt; because the precipitate would either have been white or yellow. (4) Nor iron, manganese, nickel, cobalt, or uranium, because hydrosulphuric acid without ammonia, does not precipitate them. Consider, then, the nature of these deductions, and see into what a corner we are driving metal, even by the evidence of one single witness.

Let us now try another witness, namely, ferrocyanide of potassium; and once for all let the student remember that hydrosulphuric acid, hydrosulphate of ammonia, and ferrocyanide of potassium, are the three witnesses always first cited in a court of chemical inquiry, supposing the substance ander question to be in the state of liquidity and totally unknown. Whatever evidence is to follow, theirs comes first; all three, if we want them, or two or one as the evidence may require. As regards the case now under consideration, the reader will not fail to see that hydrosulphate of ammonia could only afford positive testimony, given already negatively by hydrosulphuric acid. Now, in many chemical examinations, negative testimony is as valuable as positive. It is so in the present instance. Let us now proceed to use the third test, ferrocyanide of potassium (yellow prussiate of potash), in solution of course. For this purpose, add a few drops of the strong nitric acid solution to a little distilled water, and test with prussiate of potash. We now get a whitish sort of precipitate.

Omitting to repeat such of the evidence yielded by this test as we happen to know already, what novelty does it communicate? What has it to say of its own specific knowledge? Why it tells us that, in addition to all the metals amongst which ours is not, it furthermore is not

Copper
Uranium

Molybdenum

Titanium ;

because either of these, similarly treated, would have yielded a mahogany brown colour. This fact I have not brought before the student hitherto; let it therefore be committed to memory at once, and never forgotten. It follows, then, that our unknown metal is at length hunted into an exceedingly narrow corner. If the student will only refer to a list of metals, and see the names of those of which the present is not, he will arrive at the conclusion that it must be one of a very few. At this point I will assume the operator to appeal to the evidence of another test, either hydrochloric acid (spirit of salt), or else common salt dissolved in water; practically, so far as relates to the present investigation, these tests are the same, and the student may use whichsoever he pleases.

Treated with either of these substances, our solution (assumed to be unknown) will throw down a dense white precipitate; hence we know at once that the metal we are hunting for is either silver or mercury; no other metals being capable of producing a similar effect.

Finally, the addition of a little hartshorn (liquor ammonia) causes the precipitate to dissolve and the whiteness totally to disappear; which characteristic result demonstrates the metal to he silver, nothing but silver.

CURIOSITY.

Its aim oft idle, lovely in its end,

We turn to look, then linger to befriend; The maid of Egypt thus was made to save A nation's future leader from the wave; New things to hear, when erst the Gentiles ran, Truth closed what Curiosity began. How many a noble art, now widely known, Owes its young impulse to this power alone; E'en in its slightest working, we may trace A deed that changed the fortunes of a race: Brace, banned and hun'ed on his native soil, With curious eye surveyed a spider's toil; Six times the little spider strove and failed; Six times the chief before his foes had quailed; "Once more," he cried, "in thine, my doom I read, Once more I dare the fight, if thou succeed; 'Twas done: the insect's fate he made his own: Once more the battle waged, and gained a throne.

Cependant il entrait encore quelque hésitation dans la compagnie,' et déjà deux fois le capitaine qui commandait avait donné l'ordre au tambour-maître de prendre deux tambours, de se mettre en avant, et de battre la charge.2 Celui-ci restait appuyé sur sa grande canne, hochant la tête et peu disposé à obéir. Pendant ce temps Bilboquet, à cheval sur son tambour et les yeux levés sur son chef, sifflait un air de fifre et battait le pas accéléré avec ses doigts. Enfin l'ordre venait d'être donné une troisième fois au tambour-maître, et il ne paraissait pas disposé à obéir, lorsque tout à coup, Bilboquet se relève, accroche son tambour à son côté, prend ses baguettes, et passant sous le nez du tambour-maître, il le toise avec orgueil, lui rend d'un seul mot toutes les injures qu'il avait sur le cœur, et luit dit :---Viens donc, grand poltron!

Le tambour-maître veut lever sa canne, mais déjà Bilboquet était à la tête des deux compagnies,10 battant la charge comme un enragé. Les soldats, à cet aspect, s'avancent après lui et courent vers la terrible batterie." Elle décharge d'un seul coup ses six pièces de canon, et desi rangs de nos braves voltigeurs s'abattent et ne se relèvent plus.12 La fumée, poussée par le vent, les enveloppe, le fracas du canon les étourdit; mais la fumée passe, le bruit cesse un instant, et ils voient debout, à vingt pas devant eux, l'intrépide Bilboquet battant la charge,13 et ils entendent son tambour, dont le bruit, tout faible qu'il soit,' semble narguer tous ces gros canons qui viennent de tirer. Les voltigeurs courent toujours, et toujours,15 devant eux le tambour et son terrible rlan rlan les appelle;" enfin une second décharge de la batterie éclate et perce d'une grèle de mitraille les débris acharnés des deux belles compagnies. 16 A ce moment, Bilboquet se retourne et voit qu'il reste à peine cinquante hommes des deux cents qui étaient partis, et aussitôt, commne transporté d'une fureur de vengeance, il redouble de fracas :15 on eût dit vingt tambours battant à la fois; jamais le tambour-maître n'avait si hardiment frappé une caisse. Les soldats s'élancent de nouveau et entrent dans la batteric, Bilboquet le premier, criant à tue-tête aux Russes:

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