the sun is situated. As this motion is through space, and not through a resisting medium like the air, the retarding forces which diminish the motion of bodies near the earth do not affect them, and hence they move with undiminished speed. This speed, however, varies with their distance from the sun, and the following rule, discovered by Kepler, shows the relation that exists between the speed and the distance:-The straight line drawn from the planet to the sun always describes equal areas in equal times. This law partly depends on another, which teaches us that the attraction of any body for another diminishes with the square of the distance. If, for instance, we remove a body to double the distance, the attraction is, if to three times the distance, it is only, and so on. This is an experimental law, though by analogy with light we can easily see why it should be so. If we take a piece of board, and having cut out of it a piece a foot square, hold the board at any distance from a bright light, and place a screen behind it at twice the distance from the light, the illuminated space on the screen will measure 2 feet each way, or 4 feet in all. The light is thus spread over four times the area, and therefore the illumination at any point is only one-fourth as great. Similarly, if the distance of the screen from the light be three times as great as that of the board, a space of 9 square feet will be illuminated, and each part will have one-ninth of the brilliancy. From this we see that when a planet is in the part of its orbit most remote from the sun, it is attracted less powerfully, and therefore its velocity must be less than when nearer the sun, or else it would fly out of its path. THE PENDULUM. We must now notice this very important instrument, so valuable to us, not only as a regulating power for clocks, but also for calculating the force of gravity and its variations in different places. A simple pendulum is one all the weight of which is collected at a single point. Such a one can, of course, only exist theoretically; but we may obtain a near approach to it by suspending a small ball of some heavy substance, as lead or platinum, by a very fine string. A common pendulum is called compound, for the weight is divided throughout it, and it may therefore be considered as a number of simple pendulums connected together, so that all swing at the same rate. All are familiar with its action, but many do not know why it is used as a regulator. When a pendulum hangs freely, all its oscillations, if not of wide extent, occupy exactly the same time. If the pendulum be made to swing in a cycloidal curve, instead of an arc of a circle, then from whatever part of the arc it falls it always takes exactly the same time. This remarkable property is called the isochronism of the pendulum, this term being derived from two Greek words, meaning "equal" and "time." Galileo was the first to discover this law, and it is said his attention was called to it by observing a chandelier in a cathedral. By some cause it had been set swinging, and he noticed that however long the arc, it appeared to swing in exactly the same time. He accordingly tried some experiments on his return home, and found that such was the case. In Fig. 106 let o represent the point of suspension. When oc is vertical the force of gravity is exactly overcome by the tension of the cord, and thus the pendulum serves as a plumb-line, for if c be raised above the lowest point it will swing backwards and forwards till it settles at that point. Now raise c to A. The same two forces act upon it, namely, tension along A O and the force of gravity acting vertically downwards along A X. Produce o A to Y, and draw A z a tangent to the Z arc. We can now resolve the force of gravity into two, acting along A Y and a Z. The former of these will be overcome by the tension of the string, the other part acting along a z will cause c the pendulum to move towards c. On arriving there it will have acquired a velocity which Fig. 106. will carry it on over an arc nearly equal to CA, and thus it will continue to oscillate till its motion is stopped by the resistance it meets with. now draw a line through Y parallel to A Z, Y X will repre sent the portion of gravity which produces motion in the The time of oscillation increases in the same ratio as the square root of the length of the pendulum. If we take three pendulums whose lengths are in the proportion of 1, 4, and 9-say, for example, 6 inches, 2 feet, and 4 feet 6 inches, respectively-we shall find that while the long one makes one vibration, that two feet long will make two, and the shortest, three. In the latitude of London, a pendulum to beat seconds must have a length of 39.13 inches; at the equator, the length must only be 39.01 inches. In the compound pendulum all parts must swing at exactly the same rate; but by what we have seen, those nearer the point of suspension have a tendency to swing more rapidly, and thus to accelerate the motion of those below, while those at the extreme end exert just the contrary influence. Now there is evidently some point where the particles are as much retarded by those below as accelerated by those above, and this point must move at the same rate as if it were free. We might, in fact, have all the weight collected at this spot without altering in any degree the rate of oscillation. This point is called the "centre of oscillation," and when we speak of a pendulum of any length-as e.g., 39·13 incheswe measure from this point to that of suspension. This centre of oscillation is always below the centre of gravity. However much we alter the weight of the pendulum, provided we make no difference in the position of this point, the time of oscillation remains exactly the same. The rate of vibration, then, is not at all affected by the nature or weight of the pendulum, but depends alone upon its length. We see, thus, the way in which we can regulate the speed; we can either raise the bob by means of a small nut, as is usually done, or we can have a smaller weight sliding on the rod, and raise or lower this. In either case, the effect produced is the same-the position of the centre of oscillation is moved, and thus the length altered. When the pendulum rod is made of metal, as it usually is, it varies in length with the alterations of temperature, being lengthened by heat and contracted by cold, and thus a source of irregularity is introduced which would be very objectionable. This difficulty is met by what is called the compensation pendulum. In one form of this the bob consists of a cup of mercury. When the rod lengthens by the heat and lowers the centre of oscillation, the mercury expands and rises, and its bulk is so arranged that this expansion raises the centre in exactly the same degree as the expansion of the rod depresses it. In large church clocks the pendulum rod is frequently made of wood, and thus this difficulty is avoided. Another compensating pendulum is composed of parallel bars of brass and zinc, so arranged that by their joint alterations in length the position of the bob remains unaltered. The balance wheel of a watch acts on the same principle as the pendulum, its vibrations being isochronous. When a pendulum is made to swing by itself it soon comes to rest; and even if every care be taken to remove the air and reduce the friction, it will not continue in motion more than about twenty-four hours. A maintaining force is therefore needed, when it is employed as a measurer of time. This is supplied by If we the spring or weight of the clock. The pendulum rod works in a fork which is attached to the anchor and pallets. These catch in the teeth of the escapement wheel, and allow it at each oscillation to move forward half a tooth, and then again stop it. The motion of the escapement wheel is thus at each stoppage transferred to the pendulum, and keeps it in vibration. A train of wheels connects this escapement with the hands. We have thus acquired a general acquaintance with the more important facts of Mechanics. The subject is far from exhausted, but we must leave you to follow it up in works specially devoted to it. Our attention will now be turned to the other branches of Natural Philosophy, all of which are of great interest and importance. The next branch we shall take up is Hydrostatics, a scienco which has been claimed as a branch of mechanics, but is more accurately considered as a separate science. William. Wherein does it lie ? Thomas. No, it would be wrong; the fleet has sailed" is correct English. The true rule in this matter is this: Nouns of multitude require their verb to be in the plural when the mind dwells on the individual objects which they comprise; but when those objects are presented or contemplated as a whole, then the verb must be in the singular. In the phrase "the majority of us," the idea of plurality is made prominent, you of necessity think of several persons, therefore your verb must be in the plural; but in the phrase "the fleet has sailed," you conceive cf the component parts as forming a whole, several elements coalesce into one, unity is the predominant feeling, and conse quently you must employ a singular verb. I give you another instance: "The imprisonment of us is wrong." What say you William. Oh, no, that would be ridiculous. Thomas. And yet I heard a man, the other day, say we is; Thomas. You have used a plural verb where you should have nay, I am not sure that you yourself-speaking, for example, of used a singular one. William. But "conversations" is in the plural. Thomas. It is. That word, however, is not the subject to the verb of the sentence; it comes immediately before the verb, and so has led you to put the verb into the plural, by a kind of latent attraction, against the influence of which I must put you on your guard. William. What, then, is the subject? Thomas. "Substance" is the subject, or what in common grammars is called the nominative case, and the sentence should have stood thus: "The substance of our two conversa 66 tions is pretty clear to me now." 'Substance," I repeat, is the subject. What is clear? You do not mean that the " versations are clear ? con William. No; for there are some things in them that I do not quite comprehend; but they are clear on the whole. Thomas. Yes, your language expressed your meaning correctly, although your grammar is at fault. This I have often observed in persons of defective education. Right in their logic, and having a good command of words, they are unable to put them together correctly, and so lose a large part of the advantage they ought to derive from their efforts at self-culture. Observe, now, "conversations" is dependent on the preposition "of." In the ordinary phraseology, it is governed by that preposition; and being governed by it, is in what is called the objective case-it cannot be the nominative, or the subject to the ensuing verb. In fact, the word "conversations" is a part of the compound subject of the sentence, as you may see exhibited thus:: the potatoes you might have had to-day for dinner-did not say, "they is good." What think you? William. It is not impossible; these things are very perplexing. Thomas. Yes, at first they are troublesome; but study and practice will remove all difficulties. They have done so in my case, why not in yours? William. Well, I am not going to yield. Thomas. Certainly not. Bonaparte is reported to have said that the French had not such a word as "impossible" in their language. However this may be, you, as an Englishman, will not, I am sure, easily admit the idea into your mind, or the thing itself into your conduct. 'Impossible?" No, nothing that is good and honest is impossible. What man has done, man may do. Now I must put you to rights in regard to this verb is and are; it is a word against which many, very many, persons sin grievously. Study this form : This surely is not very complicated, yet it contains all you need know in order to speak and write correctly, so far as this point is concerned. Take care, then, not to separate the pronouns from the proper forms of the verb. Take care not to mix together verbs and pronouns that should be kept apart. Do not take the first person I, and put it before the third person is. In other terms, 1 and i. must go together; 1 and iii. must Take another instance: "The majority of us are stonemasons." not be combined. You must say we were (1 and i.), and not they Is that correct? William. No. Thomas. I beg your pardon, it is quite correct. Thomas. Because the word "majority" is what is called a noun of multitude a noun, that is, which being singular in form, is plural in signification. In a majority, you know, there must be more than one. Now nouns of this kind, as they imply more than one, are constructed according to their sense, and not according to their form. Consequently, "majority" requires its verb to be in the plural. William. Then it would be right to say, "The fleet have Bailed," for a fleet consists of many ships. was (iii. and 1). Before I conclude, let me impress it on your mind that you will never speak grammatically, or, at any rate, never be sure that you speak grammatically, unless you take the trouble to make yourself familiar with the terms and the laws of grammar. Many, finding the study somewhat difficult, after a little while give it up in a sort of confident spirit, thinking such drudgery beneath them, and fancying they can do all that is necessary by a sort of nondescript grammatical feeling. This is silly. Accurate knowledge is not obtained by genius, or inspiration, or any other fancied short cut to science. If you would know, you must condescend to learn, and all true learning demands, as it well rewards, diligent and constant labour. William. Well, I do not know that I am in that danger; I never thought myself a "genius," and as for "inspiration," that belongs to a subject too sacred for me to venture on-a subject on which I had rather worship than speculate, much less be over-confident. Thomas. Those are wise words; the man who is without reverence will be a small man to the end of his days. LESSONS IN CHEMISTRY.-XI. CARBON, SYMBOL, C; ATOMIC WEIGHT, 12. No solid plays a more prominent part in the economy of nature than carbon, as it forms well nigh the whole of the wood of vegetation. It is a very remarkable substance, for it appears in three perfectly distinct states-the diamond, graphite, and charcoal. 1. The diamond is found in alluvial débris-that is, in water-worn deposits of gravel. It is presumed that the gem was formed by crystallisation when the rock was in a fluid state, and when in after ages it became broken in pieces by the action of water and other geological forces, the hard diamond was delivered from its matrix, and mixed with the débris. The chief diamond mines are those of Golconda and Bundelcund, in India, Borneo, and Brazil. When found, the stone has the appearance of a piece of white glass; occasionally there is an approach to the crystal form of the octahedron. It is the hardest of all known bodies, and is capable of dispersing light-that is, of breaking up white light into its coloured component rays-in a greater degree than any other body except chromate of lead. To exhibit this property to its full advantage, the gem must be cut. This was once an operation of the greatest difficulty, and could only be executed by the Dutch diamond-cutters, who fastened two stones in cement, and then rubbed them against each other until a facet was produced. Now, diamond-cutting is much less laborious. The stone is fixed, as before, in a metallic cement, and pressed upon a disc of steel about eight inches in diameter, which revolves horizontally with a great velocity. As with all crystals, there are certain directions in which the diamond is more readily cut. It is the skill of the cutter to place the stone upon the disc in the right position. The steel, were this not done, would itself be cut, instead of making any impression on the diamond. The secret of the disc being enabled to wear down the hard mineral is, that the minute interstices of the metal become filled with dust from the diamond. This is in many instances applied to the plate mixed with olive oil; but when a disc has been some time in work, it is sufficiently impregnated with the dust not to need this addition. In the Brazil mines is found a dark brown carbonaceous matter, in small pieces, which is as hard, if not harder, than the diamond itself; and it commands as high a price on account of its use in forwarding the cutting of the stones. The most important use of the diamond is the cutting of glass. This is effected by a natural face of the crystal. If the edge be formed by the intersection of two artificial faces, the cut produced on the glass is not a true cut, but only a scratch with rugged edges. The natural faces of the diamond are frequently curved. The diamond may be heated intensely in an atmosphere of any gas except oxygen, but if it be suspended in a cage of platinum wire, and heated to a bright redness, and then plunged in a jar of that gas, it burns with a steady red light, producing carbonic acid gas (CO2). It was reserved for Sir H. Davy to show that this gas was the sole product of the combustion of the diamond, though the fact that it was combustible was known in 1694 to the philosophers at Florence. The combustion, however, is not complete, as there always remains an ash, which is generally in the form of a cellular network-the skeleton, as it were, of the gem, and which consists of silica and the oxide of iron. With this exception the diamond is pure carbon. When submitted to the most intense of heats, that of the voltaic arch, the diamond loses its transparency, begins to swell, and is converted into a black mass resembling coke, the amorphous form of carbon. In this state it is a good conductor of electricity, a property it does not. possess in its transparent condition. 2. Graphite or Plumbago, erroneously called blacklead, is all but pure carbon, slight traces of iron generally being present. It is a crystallised body, belonging to the third or Rhombohedral system. The distinction between this and the first or regular system, in which the diamond crystallises, will be explained in a future lesson on crystallisation. It occurs in veins, always in rocks of the earliest formations. The most celebrated mine is that of Borrowdale, in Cumberland. Here it is found in "nests in trap traversing clay slates. It is a good conducter of electricity, and is as difficult to burn as the diamond. It is chiefly used for manufacturing lead pencils. Being very friable, it leaves its particles on paper when passed across it. The particles themselves, however, are extremely hard, and soon wear out the saws with which the graphite is cut. Graphite is used for lubricating machinery, and also for making crucibles. For this purpose it is mixed with fire-clay. These crucibles are not so liable to crack as those made of clay only. 3. The third form of carbon has no appearance of crystallisation. It is therefore said to be "amorphous," or without form. This state is shown in coal, charcoal, soot, etc. Of the composition of coal we shall treat in the next lesson. Charcoal is got from the "destructive distillation" of wood-that is, the wood is heated in vessels of iron, closed so that no air can cause the carbon in the wood to burn. Of course there is a pipe to carry off the gases liberated by the heat. If blood or bones be submitted to this process, the result is animal charcoal. Wood charcoal is also made by cutting the wood into logs, and arranging them on end, then covering the whole with sods, and setting fire to the heap at some central point. A portion of the wood is consumed, but the heat thus produced converts the remainder into charcoal. Charcoal is a bad conductor of heat and electricity. It is very porous, in virtue of which, like spongy platinum, it absorbs gases, the quantity varying with the nature of the gas. Thus, boxwood charcoal absorbs of This power is also shown in what are termed the antiseptic properties of charcoal-that is, the power it has of removing offensive smells. If putrefying meat or fish be packed in charcoal, all smell is removed; for the gases which cause the unpleasant effluvia are absorbed by the charcoal; and while in this state the oxygen, previously in its pores, attacks and changes the various gases, or oxidises the volatile organic matter. The process of putrefaction, however, is not arrested, but rather increased. This is often resorted to by unprincipled butchers and fishmongers who present tainted meat or fish for sale, which escapes detection for the moment by its inodorousness. Yet it possesses all the deleterious properties of unwholesome food. Charcoal, especially animal charcoal, clears coloured liquids which are passed through it. This may be well illustrated by shaking a little of it with a few ounces of port wine in a bottle, and then filtering the mixture; the liquid which passes the filter will be colourless. Sugar is clarified by means of charred bullock's blood. After being in use for some time the charcoal is found to lose its decolorising power; but this it regains upon being calcined at a low red heat. These properties render charcoal invaluable in the construction of filters; not only does it "aërate" and decolorise the water, but by oxidising any animal matter it may contain, does much to render it innoxious. Lampblack is prepared by burning turpentine or resin with a limited supply of air, and condensing the smoke; when mixed with linseed oil and soap, it forms "printers' ink." The oxides of carbon are: atom of carbon. In each instance the gas contains a little carbonic acid, and therefore to obtain it pure, it must be collected over water containing potash, which retains the carbonic acid to form potassium carbonate. Another method of preparing it is to digest pulverised ferrocyanide of potassium with seven or eight times its weight of sulphuric acid. Properties.-The gas is colourless, tasteless, inodorous, poisonA taper introduced into it is extinguished; but the gas burns where it meets the oxygen with a blue flame, forming carbonic acid, thus ous. Fig. 36. CO+OCO, 2+1= 2. The numerals refer to the volumes-that is, the carbonic acid formed is of the same volume as the carbonic oxide burnt. This may be proved by mixing in the eudiometer carbonic oxide, and half its volume of oxygen; after the spark has passed, nothing but carbonic acid will remain, whose volume is that of the carbonic oxide. This gas has a great affinity for oxygen, and therefore is a powerful reducing agent; in iron-smelting furnaces the reduction of the ore is chiefly due to its action. A solution of cuprous chloride (Cu, Cl,), in hydrochloric acid absorbs carbonic oxide, as also does melted potassium. Carbonic Acid (symbol, CO,; atomic weight, 44; density, 22).When limestone or chalk, which are both carbonates of lime (CaOCO,), are heated in a kiln, the heat drives off the gas (CO,), leaving the lime (CaO). The gas, being half as heavy again as air, collects in the fire-pit, and in any hollows close by the kiln; and here many a wanderer has slept his last sleep, poisoned by the gas. It is also the chief product of fermentation, and collects at the bottom of vats; its presence is ascertained by lowering a lighted candle, which will be extinguished if the gas be present. The escape of this gas causes the effervescence of wines, the froths of porter, ale, etc., and makes bread "rise." It is best prepared by pouring dilute hydrochloric acid on chalk, or, what is better, on pieces of marble (Fig. 36). On account of its density it may be collected by displacement, and even may be poured from one jar into another. The reaction is thus expressed : is better not to use sulphuric acid, as the fragments of marble are then coated with calcium sulphate, which is insoluble, and thus the action is retarded. The gas is unable to support life, and when breathed, spasm of the glottis prevents its entrance into the lungs. If its presence in air exceed four per cent., it acts as a narcotic poison. It is exhaled from the lungs, which are organs composed of a membrane some 160 square yards in surface, which has the property of absorbing the oxygen from the air inhaled, and thus bringing it in contact with the venous blood beneath. This blue blood owes its colour to the presence of carbon. The oxygen combines with this carbon, forming carbonic acid, which is exhaled. This may easily be proved by blowing through a glass tube into some lime water, Fig. 37. or barytic water. The calcium, or barium carbonate, is formed, which renders the water milky. This chemical action is not only carried on in the lungs, but over the whole body through the pores of the skin. If the hand be introduced into a jar of oxygen standing over water, in a short time that gas will be found to be converted into carbonic acid. Hence from this simple experiment will be seen the imperative necessity of cleanliness. This chemical action is the source of animal heat, and is exactly that which goes on more vigorously in a coal fire. In this case the oxygen entering in at the bottom of the grate combines with the coal, forming carbonic acid. As this passes upward through the fire, it takes another atom of carbon, forming, as we have seen, carbonic oxide, which burns again into carbonic acid when it reaches the top of the fire. The blue, flickering flame seen over a cinder fire is carbonic oxide burning into the higher oxide. To exhibit the presence of carbon in carbonic acid, it is only necessary to pass the gas over a piece of heated potassium. The arrangement is shown in Fig. 37. The metal deprives the gas of its oxygen, and the liberated carbon is deposited on the interior of the bulb. When submitted to a pressure of 35-4 atmospheres, the gas becomes a liquid. This is the means by which it is accom plished. A and c (Fig. 38) are two wrought-iron vessels. A mixture of water and sodium bicarbonate is introduced into A, and the tube b, filled with sulphuric acid, is placed upright in the generator," A. The top is now fixed, and the tap, s, turned. The vessel is inverted, being placed on stands for that purpose, and the acid is emptied into the solution of sodium bicarbonate. Sodium sulphate is formed, and a vast quantity of carbonic acid gas is liberated. By means of the pipe, d, the generator is connected with the "cordenser," c, which is packed in ice. So great is the pressure of the gas, that when both the taps are turned the liquid carbonic acid distils over. When the operation is complete, the two vessels are disconnected. A little of Fig. 38. the liquid can be received on a piece of wool by turning M, the pressure of the gas in c forcing the liquid up the tube. So rapid is the evaporation, that the liquid on the wool is frozen. When mixed with ether, a paste is formed, which possesses an extremely low temperature, by which mercury is at once solidified. Carbonic acid, though a very weak acid, forms a numerous class of "carbonates." All these effervesce with dilute nitric CaOCO, + 2HCl = CaCl, + H,O + CO,, calcium chloride, water, and carbonic acid being the result. It acid. SHOW TRACHEAL SYSTEM. L. PRIVET HAWK MOTH (SPHINX LIGUSTRI): a, CATERPILLAR; b, PUPA; c, IMAGO. II. COMMON WASP (VESPA,: a, LARVA; b, PUPA; c, IMAGO. III. UNDER-SIDE OF HEAD OF BED-BUG (CIMEX LECTULARIUS), WITH LOWER LIP REMOVED (MUCH MAGNIFIED). IV. BEETLE WITH DORSAL INTEGUMENTS REMOVED TO SHOW VISCERA. V. BEE. VI. PARASITICAL INSECT SEEN BY TRANSMITTED LIGHT, AND HIGHLY MAGNIFIED TO Refs. to Nos. in Figs.-III. 1, labrum, or upper lip; 2, 2, roots of the mandibles; 3, 3, roots of the maxilla; 4, mandibles and maxillæ combined into a piercer; 5, antennæ; 6, 6, eyes. IV. 1, œsophagus; 2, gizzard; 3, stomach; 4, entrance of the secreting organs; 5, sinall intestine; 6, large intestine; 7, ovaries; 8, spermotheca; 9, accessory glands; 10, common cloaca. V. 1, œsophagus; 2, crop; 3, stomach; 4, entrance of the secreting organs; 5, small intestine; 6, large intestine; 7, common cloaca; 8, ganglionic chain. possesses forms whose simplicity of organism and general inferiority of structure make the comparison of them with the highest members of the other groups, with any idea of rivalry, absurd. Each class, too, culminates in organisms whose varied parts and elaborateness of detail seem to place them at an elevation beyond which it seems impossible to mount. In many respects, as in their respiratory and circulatory systems, the spiders seem to show an advance upon the Insecta; while their larger size and the greater complexity of their nutritive organs and fluctuating manner, sometimes appearing to be degraded or altogether altering their type, as we proceed from one order to another; but the perfection of the external investment, and its better adaptation to the most efficient kind of locomotion, is seen in every upward step we make in our classification. And it is in the class Insecta we find such marvellous finish and efficiency in this part of the organism as to fill not only the naturalist, but even the casual observer, with wonder. The strength and beauty of the elegant body and sculptured limbs- 7 |