the 26th, Capricornus; on the 28th, Aqua-sulphuric acid to render it a better conrius and Pisces on the 30th. LECTURE V.-WATER. I Now proceed to speak more particularly of the composition of water. There are two modes in which the composition of a substance may be determined, viz. 1. By Analysis, or separation; 2. By Synthesis, or putting together. The former is universally applicable, the latter only in particular cases. In the present instance, both methods may be applied, thereby furnishing the most complete evidence that can be obtained respecting the composition of a substance. 1. Composition of water determined by Analysis-When an electric current is passed through water, that substance is completely decomposed, and its elements separated in the gaseous form. Fig. 13 represents a shallow cylindrical glass vessel, through the bottom of which pass two platinum wires, terminating in plates of the same metal; over these are inserted two glass tubes (A B.) The vessel and tubes are filled with pure water, acidulated with con ductor of electricity. If the wires be now nected with the poles of a voltaic battery, bubbles of gas will rise from the surface of the platinum plates; the gas in the tube over the negative plate, or that connected with the zinc end of the battery, occupying twice the volume of that in the other tube. On examining these gases, the former will be found to burn with the peculiar flame of hydrogen, and the latter will rekindle a slip of wood with the end red-hot, Fig. 13. thereby exhibiting the characteristic property of oxygen. Hence it appears that water is composed of two volumes of hydrogen, united with one volume of oxygen; and as hydrogen is sixteen times lighter than oxygen, the corresponding proportion by weight must be one part of hydrogen to eight parts of oxygen. The following method gives the composition by weight directly. Water, in the state either of liquid or of vapour, is decomposed by red-hot iron, the oxygen combining with the iron, and the hydrogen being set free. If the water be boiled in a flask, and the vapour made to pass through a porcelain tube or a gun-barrel containing turnings of iron, and which is heated to redness in a small charcoal fire, the iron turnings will be oxidized, and the hydrogen, as it passes off, may be collected over water in the pneumatic trough. By certain contrivances, which you are not yet quite prepared to understand, we may ensure the collection of the whole of the hydrogen, and measure its volume exactly, and thence, knowing the specific gravity of the gas, we may calculate its weight. The quantity of oxygen separated from this hydrogen is equal to the weight which the iron has gained by oxidation, and may be determined by weighing the tube before and after the experiment. The results of this method of analysis agree with the former, in assigning to water the proportion of eight parts of oxygen, to one part of hydrogen. 2. By Synthesis. You have seen that a mixture of oxygen and hydrogen explodes when raised to a certain temperature. The combination may likewise be induced by passing an electric spark through the mixture; and this method has the advantage of enabling us to observe with accuracy the proportions in which the gases combine. For this purpose, we require an instrument called the detonating eudiometer (or air-measurer). The most convenient form is that of a strong glass tube (Fig. 14) bent in the form of the letter U; closed at A, open at C, and having two platinum wires inserted through the glass, at a short distance from A, and separated by a small interval within the tube. The arm A B is marked with a scale 'of equal parts, as into cubic inches, and tenths of a cubic inch. A U B Fig. 14. D To use this instrument, you fill it with mercury, and then, closing the end, C B, with the finger, you invert it in mercury contained in a trough similar to the watertrough, but much smaller (it should hold about twenty pounds of mercury). You next introduce into the arm, C B, a quantity of oxygen prepared from chlorate of potassium (see p. 59), which should be previously dried by heating it till it just begins to melt; then, closing the end C with the finger, you turn the tube in such a manner as to cause the gas to pass into the arm A B, and displace the mercury. The next step is to bring the mercury in the two arms to the same level, which may be most conveniently done by thrusting a stick into the arm C B, so as to cause some of the liquid to run out. The levelling of the mercury is necessary to give the true volume of the gas; if it stood higher at E than at D, the gas would be compressed; if lower, it would be expanded. The arm BC is now to be once more filled with mercury-inverted in the trough-and hydrogen introduced into it in quantity as nearly as possible equal to twice the volume of the oxygen. You then transfer it to the other arm, level the mercury as before, and read off by the scale the total quantity of gas introduced. The hydrogen must not be passed directly from the generating vessel into the tube, because it would be wet; it must first be dried, by causing it to pass through a tube filled with some substance which absorbs moisture very rapidly; as, for instance, with fragments of pumicestone, soaked in oil of vitrol. We will suppose the quantity of oxygen to be two cubic inches, and the whole quantity of gas six-and-a-half inches; this gives four-anda-half cubic inches for the hydrogen. The end C is now to be firmly closed with the finger, and an electric spark passed through the mixture, by connecting the wire (a) with the outside coating of a charged Leyden jar, and bringing the knob of the jar to the wire (b.) The whole body of gas will then be seen to inflame; and, on removing the finger from C, the mercury will be forced, by the pressure of the external air, into the arm B A, which it will nearly fill, leaving only half a cubic inch of gas at A, which, on examination, will be found to be hydrogen. The two cubic inches of oxygen have, therefore, taken up four cubic inches of hydrogen; that is to say-the gases have combined in the proportion of one volume of oxygen to two volumes of hydrogen. If they had been mixed exactly in that proportion, they would have disappeared altogether. The detonating eudiometer is a very useful instrument in the examination of gases. It is tolerably safe; for, independently of the glass being very thick, the column of air between D and C (when C is closed by the finger), acts as an elastic cushion, to break the force of the explosion. Still, these instruments do sometimes burst; it is best, therefore, to guard against danger, by wrapping the instrument well round with a cloth, leaving only the top exposed, and setting it upright in a glass jar, to catch the mercury, if spilled. The method just described, though it gives a very near approximation to the truth, is yet not susceptible of that minute accuracy which is to be desired in a point of such fundamental importance as the composition of water. The following is that which is most relied on : Many metallic oxides, which are not reduced to the metallic state by heat alone, 9 nevertheless, give up their oxygen when heated in contact with hydrogen. Such is the case with the oxide of copper; the decomposition of which may be made the means of accurately determining the composition of water. For this purpose, pure oxide of copper is heated in a glass bulb, by means of a spirit-lamp, and hydrogen gas, previously dried, by passing through a tube containing fragments of pumicestone, soaked in oil of vitrol (a liquid which absorbs water with great avidity), is passed over it. The oxide then becomes red-hot, gives up its oxygen, and is reduced to the metallic state. The oxygen combines with the hydrogen, and forms water, which passes off in vapour, and is condensed in a small receiver, connected, airtight, with the bulb. A second drying-tube is likewise attached to this receiver, to absorb any vapour of water that may escape condensation in the receiver. The bulb containing the oxide of copper is weighed before and after the experiment; so, likewise, are the receiver and the drying-tube, attached to it. The loss of weight of the oxide of copper gives the quantity of oxygen; and the increase of weight of the receiver and drying-tube shows the quantity of water formed; the difference is the hydrogen. A great number of experiments of this kind, made with every possible precaution to ensure accuracy, have shown that the proportion of hydrogen to oxygen in water is as 1: 8.009-a proportion which is so nearly that of 1: 8, that the difference may fairly be attributed to unavoidable errors of experiment. I have been thus particular in describing the various methods of determining the composition of water, partly because the knowledge of its composition is one of the most important points in chemical science, and partly because the methods themselves are highly instructive, and involve considerations to which we shall often have to recur. I do not expect that you can repeat all the experiments that I have described; for to make them with accuracy, takes a great deal of time, and requires attention to a variety of details which you are not yet prepared to understand. But it is important, even at the commencement of your chemical studies, to know something of the manner in which such experiments are made; for chemistry is a science of quantity. The reduction of oxide of copper by hydrogen involves a point of interest which we must not omit to notice. Water passed in the state of vapour over red-hot iron, is decomposed (see page 148), the metal being oxidized, and the hygroden set free. The same effect is produced by passing vapour of water over red-hot copper, the metal in either case overcoming the force which unites the oxygen to the hydrogen, and taking the former to itself; a result which seems to show that oxygen has a greater inclination to combine with iron or copper than with hydrogen. And yet we find that when hydrogen gas is passed over oxide of copper (or oxide of iron) at a red heat, the hydrogen abstracts the oxygen from its combination with the metal, and unites with it to form water. Hence we may conclude that the chemical relations of a substance are greatly modified by the circumstances under which it is placed. In the older works on chemistry, you will find it stated that red-hot iron decomposes water, because the attraction or affinity of iron for oxygen is stronger than that of hydrogen for the latter element; and tables are drawn up, exhibiting the comparative degress of affinity between one substance and a number others; as, for example, between oxygen and various metals, between sulphuric acid and the different bases, &c. But the facts just stated, and many others of similar character, plainly indicate that no such orders of affinity really exist; for we find one experiment tending to show that the affinity of iron for oxygen is greater than that of hydrogen, while another would lead us to conclude that it is less. It is clear, then, that chemical compo sition and decomposition are affected by circumstances independent of the mere absolute strength of affinity that may exist between elements. What these circumstances are, we shall have occasion to notice as different cases come before us. In the present instance, the result appears to be affected by the diffusive tendency of gases already spoken of; for, in the one case, when water is decomposed by a metal, the hydrogen, as soon as it is liberated, tends to diffuse itself through the surrounding atmosphere of aqueous vapour; and, in the contrary case, the vapour of water finds an atmosphere of hydrogen, through which it can diffuse itself as fast as it is formed. MATHEMATICAL QUESTIONS, SOLUTIONS, &c. EDITED BY W. J. REYNOLDS, ESQ., B.A. Solutions to Questions on Page 62. 13. Let ABC be a triangle, and let AbC, CaB, and BcA, be segments of circles, respectively containing angles equal A But A, B, and C, are together equal to two right angles. Euclid, B. i. Prop. 32. Therefore COB and A are, together, equal to two right angles: whence the circle BaC being completed, must pass through O. Euclid, B. iii. Prop. 22. Q.E.D. 14. Let O, P, be the centres of the circles: Ab, AB, the lengths of the tangents to be drawn to them respectively. From A C draw AC at right angles to Ab, and A on it make Ac= radius of first circle, second; and join BC bc. B AC = radius of With centre Q, and radius bc, describe a circle; and with B Q R Р T centre P, and radius equal to BC describe another circle, intersecting the former in the points R and T. Then the points R and T will satisfy the conditions of the Problem. For from R draw RQ, a tangent to first circle at Q, and join OQ. Then, since Q is a right angle, and OQ and OR are, by construction, respectively equal to Ac and bc, the right-angled triangle bAc and RQO have one side and the hypothenuse of one triangle equal to one side and the hypothenuse of the other: consequently the remaining side of one equals the remaining side of the other, so Ab. Similarly, the tangent from R to second circle might be proved to be equal to AB; and the same reasonings will apply to the point T. that RQ = Remark. The possibility of this Problem is determined by the conditions that the circles described with centres O and P, and radii bc and BC respectively, must intersect; and it will be seen that in order to secure this, the lengths of AB ab must be such that their sum is greater, and their difference less, than OP, the distance between the centres of the circles. When the tangents to the circles are equal to the same given length, the line joining the points R and T bears many peculiar relations to the circles. The tangents drawn from any point of this line to the circles will be equal, as may be easily seen. When the given circles intersect, the line will coincide with their common chord. It has been called the radical axis of the circles; and its determination is of use in the construction of the celebrated problem :-To describe a circle to touch three given circles. We will give the solution of this problem shortly; and in the mean time invite such of our readers as take an interest in geometrical constructions, to direct their attention to this. QUESTIONS FOR SOLUTION. 20. A banker discounting a bill of £1,515 for 73 days by the common method, deducts three shillings more than he would have done, had he taken the true discount. Find the rate per cent. per annum at which the bill was discounted, and interpret the negative result. 21. In each of three straight lines, O P, O Q, 0 R, diverging from a point, O, take AC; cb, CB; be produced to meet in the points p, q, r, respectively, these points will range in one straight line. 23. A puncheon (84 gallons) of spirits was purchased at the rate of 13s. 6d. a gallon, to which was added some of an inferior quality which had cost 9s. 6d. a gallon; and 25 per cent. was gained on the outlay by selling the whole at 12s. 6d. a gallon. To what per centage of the whole quantity did the added spirit amount? 22. Wishing, the other day, to purchase a second-hand alarum clock, I requested a dealer to send some for inspection. I wound up three, and set them all to show the true time at 11 A.M. On inspecting them at 2 P.M. on the same day, I found that one showed 18 minutes past 2, another 5 minutes past two, and the third 14 minute to 2. Supposing these clocks to be kept wound up, and to go uniformly, what o'clock will it be at the instant when they all again indicate 11 o'clock? 23. A person counting the number of shillings he had, found that if they had been half-guineas he could have spent six shillings, and even then have been ten times as rich as he is now. Find the number of shillings. 24. On a certain railway whose termini are A and E, there is an intermediate station D, 123 miles distant from E, at which all up-trains stay five minutes: also at B, the nearest station to A, all uptrains slacken their speed, so that the distance BA is traversed at an average rate of 20 miles per hour. At the distance of 61 miles down the line is another station, C, visible from B. A person escaping from pursuit takes the 2 P. M. express uptrain from E, which travels at the rate of 45 miles per hour, including all stoppages except that at D. His followers arriving soon afterwards at E, order a special engine, and start in pursuit at 3 P.M. On coming to D, they find the express had left that station 44 minutes before their arrival; and after a delay of 3 minutes, they continue their journey at an additional speed of 5 miles per hour, and as they pass C they observe the express (due at R the terminus in 12 minutes from that time) just passing B. Find the length of the line, and the rate of the special any two points, a, A; bB; c, C; respectively, and prove that if the lines ba, BA; ac, train. |