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Aboukir, Egypt ......... .. 81° 21' N, 80 8.B.
THE object of physics, or natural philosophy, is the study of all phenomena which material substances present, except those which relate to changes of internal composition; the latter come under the domain of chemistry. For example, selecting the metal iron as a subject of contemplation, we may study its specific gravity, its degree of hardness, its property of welding, of being drawn out into wire, and rolled or beaten into plates; all these phenomena depend upon the physical properties of the metal, and the study of such phenomena comes under the domain of physics, or natural philosophy, sometimes called mechanical philosophy. But iron is endowed with another set of qualities. It is capable of being dissolved in certain acids, and rendered invisible as iron, although its presence may be recognised by various tests. All this department of study belongs to chemistry. We have stated that matter (or material bodies) admits of being studied under two aspects: but what is matter? It is necessary to arrive at some understanding as to this question before proceeding farther. Perhaps the best definition of matter is comprehended in the expression, whatever falls or is capable of falling under the immediate cognisance of tho Sen SeS. At this time, there are sixty-three known elementary or sirople bodies; that is to say, bodies out of which chemical analysis has not succeeded in extracting more than one species of matter. Nevertheless the number sixty-three is by no means to be regarded as the permanent representative of simple bodies. Possibly their number may hereafter be increased or diminished, according as new siniple bodies may be discovered, or those with which chemists are at present acquainted may be proved to be made up of simple constituents. Bodies, Atoms, Molecules.—Every definite or limited amount of matter is termed a body or mass, and the properties of such bodies or masses show that the matter of which they are composed is not continuous, but is made up of elements, as it were, infinitely small; so small that they are incapable of physical or mechanical division, and not in actual contact, but in near proximity; the distances between them being maintained by reciprocal repulsions, known under the name of molecular forces. These minute elements of bodies are termed atoms, and groups of atoms are termed molecules, Lof which latter, a body or mass is only an aggregated collection. Mass.-The term mass of a body is applied to the amount of matter which it contains. The absolute mass of a body cannot be determined, but its relative mass, considered with regard to the mass of some other body taken as unity, can be readily arrived at. 4 Physical Conditions or States in which Bodies exist.--These states are three, each being well characterized and readily distinguishable from the others. 1. The solid state. This condition is manifested at ordinary temperatures by wood, stone, snd metals. It is characterized by an entire adherence of molecules amongst themselves, so that they only admit of separation by the exercise of a certain degree of force, varying for different solids, and for the same under different circumstances. It is a direct consequence of this molecular adherence, that solid bodies retain their original forms. 2. The liquid state. Of which we are furnished with examples in water, alcohol, and oils. The distinctive character of liquids is an adherence of so feeble a degree between their molecules, that the latter slide upon and pass each other with extreme facility, in consequence of which it results that liquid bodies do not affect any external form of their own, but invariably assume that of the containing vessel. 3. The gaseous state. Of this we have examples
in the air, and a great number of other bodies, to which the
general appellation gas or ačriform fluid is applied. In gases the mobility of the molecules is still greater than in liquids; but the special characteristic of gases is their unceasing tendency to expand into a greater volume; a characteristic expressed by the term expansibility, and which will hereafter be demonstrated experimentally. The general term fluid is applied both to liquids and to gases. The greater number of simple bodies, and many compound ones, are capable of presenting themselves successively under the three forms of solid, liquid, and gaseous, according to the variations of temperature to which they are exposed. Of this successive change, water affords a well-known example. Hereafter, when we farther advance into the regions of natural philosophy, it will be found that the three states of solid, liquid and gaseous, depend chiefly on variations of molecular attraction and repulsion.
On Physical Phenomena.-Every change which the state of a body may undergo without involving alteration of composition is a physical phenomenon. The falling of a body, the sound produced by such falling, the freezing of water, all are physical phenomena.
Laws and Physieal Theories.—The term physical law is applied to designate the constant relation which exists between any particular phenomenon and its cause. For example, in demon= strating the fact that a given volume of gas becomes one-half, one-third, one-fourth, &c., its original size, according as it is exposed to a degree of pressure, twice, three times, &c., we iliustrate the well-known physical law which is expressed by saying that the volumes of gases are in an inverse ratio to the pressures under which they exist. A physical theory is the collection of laws relating to the same class of phenomena. Thus we speak of the theory of light, the theory of electricity. Nevertheless this expression also applies, though in a more restricted sense, to the explication of certain particular phendmena. In this latter sense, we speak of the theory of dew, the theory of mirage, &c.
Physical Agents.-As causes of the phenomena which bodies present, philosophers admit the existence of physical agents or natural forces, by the operation of which all matter is governed. These agents are universal attraction, caloric or heat, light; magnetism and electricity. Mere physical agents only manifest themselves to us by their effects, their ultimate nature being completely unknown. In the present state of science, the question still remains undetermined, whether the physical agents are to be regarded as properties inherent in matter, or whether they are in themselves subtle material bodies, impalpable, pervading all nature, and the effects of which are the result of movements impressed upon their mass. The latter hypothesis is that most generally admitted; but being admitted, next follows the important question,-‘Are these kinds of matter distinct amongst themselves, or are we to refer them to one and the same source?” This latter opinion appears to gain ascendency in proportion as the boundaries of natural philosophy become expanded. Under the assumption that the physical agents are subtle forms of matter, devoid of all apprecis able weight when tested by balances of the highest sensibility, they have been termed imponderable fluids; hence arises the distinction between ponderable matter, or matter properly so called, and imponderable matter, or imponderable physical agents.
come within the spnere of our cognisance. These properties are distinguished into general and special. The former are those which belong to all bodies, of whatever kind and in whatever state they may be examined. The properties necessary to be considered at this time, are impenetrability, extension, divisibility, porosity, compressibility, elasticity, mobility and inertia. Special properties are such as are observed in certain bodies, or under certain physical conditions. Of this kind are solidity, fluidity, tenacity, ductility, malleability, hardness, transparency, colour, &c. For the present we shall only be concerned with the general properties of matter already mentioned; but it is proper to remark that impenetrability and extension, are not so much to be regarded in the light of general properties of matter as the essential attributes of matter itself, and which serve to define it. Furthermore we may here remark, that the terms divisibility, porosity, compressibility, and elasticity only apply to bodies regarded as made up of aggregated molecules; they are inapplicable to atoms. Impenetrability.—This is the property by virtue of which no two material elements can simultaneously occupy the same point in space. This property, strictly speaking, only applies to atoms. In a great number of cases bodies appear to be susceptible of penetration. For example, there exist certain alloys, of which the volume is less than the joint volume of the metals entering into their composition. Again, on mixing water with oil of vitriol or with alcohol, the mixture contracts in volume. Such phenomena do not represent actual penetration. The appearance is solely referable to the fact, that the materials of which the acting bodies are composed are not in actual contact. Certain intervals exist between them, and these intervals are susceptible of being occupied by other matters, as will be demonstrated further on, when we come to treat of porosity. Extension.—This is the property which every material body possesses of occupying a limited and definite portion of space. A multiplicity of instruments has been constructed, Yaving for their object the measuring of space. Amongst these the vernier and the micrometric screw are very important; we will therefore proceed to their consideration. The Vernier is so called from the name of its inventor, a French mathematician, who died in 1637. This instrument enters into the construction of numerous kinds of apparatus used in the study of the physical sciences, such, for example, as barometers, cathetometers, goniometers, &c. It is composed of two engraved rules, the larger of which AB (fig. 1), is fixed and divided into equal parts. The smaller rule is moveable, and to this in strict language the term vernier is alone applicable. To graduate the vernier, the process is as follows. First of all it is cut to such a length as corresponds with nine divisions of the large or fixed rule. It is then divided into ten equal parts, from which arrangement it follows that every division of the rule a d is smaller than a division of the rule A B by One-tenth.
The vernier being thus constructed as already described, let ins explain the manner of its application. Suppose it was desired to measure the length of an object M. N. We place it as represented in the figure upon the great rule, the long axis of which corresponds with that of the body to be measured, and we find that its length equals four units plus a certain fraction. To value the amount of this fraction is the object of the vernier. This is accomplished by sliding the vernier along the length of the fixed rule, until the end of the vernier corresponds with the end M N of the object to be measured. This adjustment being made, we next seek for the point of coincidence between the divisions of the two rules. In the
diagram this correspondence occurs at the eighth division or the vernier, counting from the point N. This coincidence shows that the fraction to be measured is equal to eight-tenths.
In other words, the divisions on the vernier being smaller than. than those on the fixed rule by one-tenth, it follows that if we begin to count at the point of coincidence, and proceed in the direction from right to left, each successive degree on the ver
nier falls in arrear of the corresponding degree on the fixed rule by one-tenth. Hence it follows, that in the case under
consideration from the extremity N of the vernier, to the fourth
division on the fixed rule, the intervening space is eight-tenths,
and we arrive at the final conclusion that the length of the
object M N to be measured, is equal to four of the divisions of A B
plus eight-tenths. Consequently if the divisions on the great or
fixed rule are hundredths of inches the length of M N will be
obtained almost exactly correct to one-thousandth of an inch.
Were it desired to be still more accurate, to obtain the length
correct to the two or three thousandth part of an inch, it would
then be necessary to divide AB into hundredths of an inch, to cut
off the vernier rule until its length should be equal to nineteen
or twenty-nine divisions of the great rule, as the case might be,
and finally to divide the vernier into twenty or thirty equal parts.
But when such minute divisions as these have to be observed,
and the exact line of coincidence between the degrees of the ver
nier and the fixed rule accurately read off, the aid of a lens is
absolutely necessary. The vernier is not invariably a linear
measure, as we have already described it; very frequently graduated circular arcs are supplied with verniers, which are
then usually engraved in such a manner that fractions of a
degree are read off in minutes and seconds. It may be proper
here to remark that the vernier is also occasionally termed a
nonius, and still more frequently in mathematical books of a
past era, the nonius vernier. It derives this name from Nunez,
a Portuguese mathematician, who is considered by some to
have been its inventor. This, however, is not the case. The
instrument of Nunez, although designed for accomplishing a
similar purpose with the vernier, differed from it in some im
portant respects, and was far less efficient.
The Micrometric Screw and Dividing Machines. The term micrometric is applied to that variety of screw employed for measuring with precision the extension of length and breadth. It follows, from the very nature of a screw, that when it is well and accurately made, its pitch, or the interval existing between any two successive threads, must be everywhere throughout its length the same. From this it follows, that if a screw be rotated in a fixed mut, the former will advance a certain equal distance for each revolution, the rate of advance being proportionate to the degree of obliquity of the screw-thread. It follows, moreover, that for every fraction of a turn, say rāoth, it only advances the 1%gth of the length of an interval between any two threads. Consequently if this interval be equal to a hundreth of an inch, and if at the handle extremity of the screw there is attached a wheel or circle graduated into 400 divisions, and turning with the screw, then on turning the graduated wheel through only one division, the screw itself will be caused to advance to the extent of one 400th of an inch.
Dividing machines, as they are termed, depend on the application of this principle. Fig. 2 represents a dividing machine, intended for the division of straight lines. It is gomposed of a long screw, the thread of which ought to be perfectly regular, working through a fixed metallic plate, and its handle part attached to a fixed metallic circle A. Adjacent to this graduated wheel is attached a fixed index B, by means of which every fraction of a turn made by the wheel, and consequently the screw itself, may be easily discriminated. The nut E, through which the screw plays, is attached to an iron rule cI), which moves with the nut by a motion parallel to the axis of the screw. It is upon this rule which is fixed the object m n intended to be divided. Lastly, the table is supplied with two brass grooves perpendicular to D C, and upon which moves the slide-rest K, armed with the steel gravero.
The machine being arranged according to the description just given, two different cases may present themselves. Either the rule m n has to be divided into equal parts of a determinate length—for example, four hundredths of an inch—or it may have to be graduated into a given number of equal parts. Under the first conditions, the course of the screw, or its length from thread to thread, being equal to one hundredth of an inch, the
operator turns the circle A through one-fourth of an entire revoHution, engraves a mark on the rule, then turns the wheel through another fourth of a revolution, engraves another mark, and so proceeds until the operation is completed. Under the second conditions, let us suppose the division of the rule m n into eighty equal parts to be the problem , for solution. The operator now commences by causing the screw to turn in the direction from right to left, as relates to our diagram, until the extremity m exactly coincides with the point of the graver; then reversing the direction of rotation, and causing the wheel to move from left to right, in relation to the diagram until the other extremity n of the rule corresponds with the point of the graver. The operator counts the number of turns,
and the value of the fraction of a turn, if such exist, gone through by the graduated wheel in causing the rule c D to advance from one extremity of the object m n to the other. Then, dividing the total number of revolutions by 80, the quotient indicates the space along which the screw E must advance for each oth of on n. It only now remains to engrave a mark on m n at the cessation of each partial revolution of the wheel.
Divisibility.—This is the property which all bodies possess of being susceptible of division into distinct parts. Numerous examples might be cited illustrative of the extreme divisibility of matter. Thus one grain of musk is sufficient to evolve during many years the peculiar odorous particles of that
substance in an apartment the air of which is frequently renewed. Another example of the extreme divisibility of matter, even when organised, is furnished by the globules of the blood. Blood is made up of red globules, floating in a liquid termed serum. In man, these globules are spheroidal, and their diameter only amounts to about the ‘0003th part of an inch. Nevertheless, the particle of blood capable of being taken up. on the point of a needle contains nearly 1,000,000 of such globules. But, what is more wonderful still, certain animals, exist so amazingly small, that they can only be seen by the aid of a microscope of high power. They move about as large animals do; they are nourished; they possess organs; how
immeasurably small must those particles be of which such animals are composed The divisibility of any kind of matter having been pushed so far that its particles are altogether imperceptible, even by the aid of the most powerful microscope, experiments can no longer determine whether such matter be finitely or infinitely divisible. Nevertheless, the stability of chemical properties belonging to each kind of matter, the invariability of relation subsisting between the weights of combining elements, and other important considerations, point to a belief in a finite limit to material divisibility. Circumstances of this kind have led philosophers to assume that bodies are constituted of material elements not susceptible of division, and to which, therefore, the term atoms is applied.