40 Results of Chemical Attraction. carbonic oxide gas, or two of carbon unite with four of hydrogen, producing olefiant gas. 98. All this proportionality and definiteness seems to point to some ultimate definiteness of character in matter, of which this is only the outcome. Such is the atomic theory or hypothesis, which asser's that there are ultimate particles or atoms, unchangeable by any known force, which have an individuality of size, weight, and chemical power for each simple substance, and that the union between two elementary bodies is one among the atoms, whereby they group themselves in pairs, triplets, &c., according to circumstances. Assuming this, we have at once a clear conception of the cause to which the facts above mentioned are due; without this assumption we are left in mist and mystery. 99. Atomic attraction plays in the universe a most important and most potent part, its energy being as intense as its sphere of action is limited. How very soon will a few drops of acid destroy the tenacity of an iron wire, which a weight of perhaps a ton could not tear in pieces! What an irresistible (heating) power is called into action by the union of oxygen and hydrogen, as is seen in the oxy-hydrogen blowpipe, which will burn or break up the most intractable substances! The electric pulse that beats through the 2000 miles of Atlantic cable between the old world and the new, is but another manifestation of this power. Heat is, doubtless, the direct product of atomic attraction; but heat is power. Whence comes the moving power of our coal fields, our engines, and our factories, but directly or indirectly from this atomic attraction? To this we owe all combustion and heat on our globe which does not come to us through space from the sun. 100. Attraction, then, pervades creation from centre to circumference. As gravitation, it is the muscle and tendon of the universe by which its mass is held together, and its huge limbs are wielded : As cohesion and adhesion, it determines the multitude of physical features of its different parts: As interatomic action, it is the final source to which we trace all material changes. Some would attempt to ascribe the three varieties of attraction to one common origin, or to reduce them to different forms of the same force, as there are some who would have the different kinds of substances to be but variations of one fundamental material. But these generalisations are yet far from being established. SECTION II.-THE PHENOMENA OR MOTIONS OF THE UNIVERSE. ANALYSIS OF THE SECTION. The material universe from its minutest to its grandest masses is in ceaseless motion, and to the intelligent study of its phenomena, the investigation of the general laws of motion is a fitting preliminary. VELOCITY is the rate of motion, and this may be, first, UNIFORM, as that of the earth round its own axis, which is therefore taken as our standard of TIME, the co-existence of an event with a whole or a twenty-fourth part of a turn of the earth being denominated a duration of a day or of an hour Most commonly, however, we find velocity VARIABLE, and either (1) ACCELERATED, i.e. gradually increasing, as in the case of falling stones, water, &c., or (2) RETARDED, i.e. gradually diminishing, as in the case of bodies thrown upwards, and in the case of all artificial motions which, owing to the effect of friction, tend sooner or later to cease, the grand impossibility being a perpetual motion. Several motions may exist simultaneously in the same body, as, when a stone is thrown obliquely, it has a horizontal and at the same time a downward motion, owing to the attraction or pulling of the earth. The resulting or resultant motion is found in such a case by the rule known as the PARALLELOGRAM OF VELOCITIES; and the resultant of several simultaneous motions by a simple extension of this rule. Lat matter in motion is FORCE, and the measure of force is the MOMENTUM or quantity of motion, which depends on the quantity of matter moved and on the velocity conjointly. Thus a force acting on a body being equivalent to a definite motion of the body, the composition and resolution of forces are identical with the composition and resolution of velocities. Forces, as velocities, may be uniform, accelerated, or retarded; the most important example of accelerating force being gravity, or the attraction of the earth; the uniformity of this acceleration, which is the same for all bodies, light as well as heavy, being very clearly shown by means of Attwood's machine. A comprehensive summary of the laws of force or moving matter is given in NEWTON'S LAWS OF MOTION; the first of which—viz. that a body free from external influence cannot of itself change its state either of rest or of motion-corresponds to the law popularly known as the Law of INERTIA : the second justifies or is the foundation of the rule given as the PARALLELO⚫ GRAM OF FORCES, and is to the effect that the direction and amount of change of motion is a measure of the external force or influence acting on a body. The third, which is commonly quoted in the words, "action and reaction are equal and opposite," corresponds to the modern doctrine of ENERGY, which asserts that the creation of force, energy, or moving power is not to be found in nature, and that every motion is but the result, outcome, or transformation of an equivalent amount of energy already existing in some form or other: all the various forms of energy in the universe being mutually connected and interchangeable according to definite though but partially discovered laws, a fact which is usually referred to under the name of the CONSERVATION OF ENERGY OR FORCE. "Motion and force." 101. Motion is the changing of place among bodies, their positions being compared with each other or with some fixed position. Force is whatever produces or is capable of producing any kind of material change. These three phenomena-matter, motion, force-are constantly associated in our experience, so that we do not know of the existence of either apart from the others. Motion is everywhere. The rising and the setting sun-the rolling river-the moving winds-all animal existence-the quiver of the air that we call sound—the rays of heat and light that in very truth beat upon our earth—exemplify its universal reign. To trace the relations of these various motions and changes, and to assign them, where possible, to a common origin is of prime importance, as it enables us, to a great extent, to foretell the future and to adapt our actions accordingly. "Motion." 102. Motion may be swift, as that of the lightning; slow, as that of the sun-dial shadow; straight (or rectilinear), as that of a body dropped from a height; bent (or curvilinear), as that of a body thrown obliquely; regular (or uniform), as that of the hands of a clock; irregular (or variable), as the motions of the wind, of animals, and as most familiar motions; accelerated (or gradually increasing), as the motion of a falling stone ; retarded (or gradually diminishing), as in the case of a stone thrown up, and in the case of most visible motions. 103. Velocity is the term used to denote merely the rate of motion, no matter how fast or slow. We measure or estimate it by Velocity of Moving Bodies. 43 the space passed over in any time-such as an hour, or a minute, or a second, according to the degree of nicety we wish to express. Only we suppose that during this time the velocity remains unchanged. A flash of lightning or a meteor may dart across the sky, and may last but a small part of a second. When we say that it had a velocity of a thousand miles a minute, we simply mean that had it travelled at the same rate for one complete minute it would have passed over a thousand miles. A passenger in a railway carriage, saying that the train is now at thirty miles an hour, understands not that he will actually travel thirty miles during the next hour, but that he would do so were the speed to continue what it is for an hour. Probably the speed is at no two successive moments precisely the same; it has gradually increased from rest to its present rate, and it may gradually get less and less till reduced to zero again at the next station. 104. A motion is fast or slow only by comparison with some common or well known velocity, such, for example, as any of the following (the unit of time being an hour, and of distance a mile) :— If the unit of time is taken at a single second, then we have the means of comparing a large number of velocities of various kinds, premising that the spaces traversed by light and electricity are indicated in miles in place of feet. * It would thus require only two seconds to traverse the whole circum. ference of the earth at the Equator. Mean rate of the earth's centre in its orbit round the sun. Sound traversing solid bodies Mean velocity of air from explosion of gunpowder Feet in one second 205,920 I 14,000 101,061 11,280 5,000 Sound traversing water. 4,480 A point at the surface of the earth under the A point at the earth's surface, latitude of London 950 697 Waves in a heavy swell of the open South Atlan- 130 A storm (also a tidal wave in the British Channel) 95 80 73 42 |