MATTER. 190 MATTER. all our knowledge of the former comes to U3 through sensation. In so far as matter is con- ceived as the basis of the reality of the physical world, the term designates the object-matter of all physical science. It is defined in physical treatises wholly by its properties. PROPERTIES OF IIATTEB. From the standpoint of physics, the properties of matter may be classified as inertia, weight, and various cliaracteristics of size, shape, and molecular connections. When one changes tlie niotion of a piece of matter in any way one is conscious of a definite sensation, the intensity of which depends upon two things — the suddemiess of the change and the (|Uantity of the matter, using this word "quantity" in a general sense. This sensation, being assciciatcd willi matter, is said to be due to a definite property of matter, which is called its "inertia" (f|.v.). .gain, if a portion of matter is held in the hand and so kept from falling toward the earth, there is a definite sensation which is attributed to a property of matter called 'weight.' It can be shown that if our senses were delicate enough they would ex- perience a similar sensation when any two pieces of matter, e.g. two bullets, were held a small distance apart. See Gr.wit.vtio.v. There are a great man.v properties common to all kinds of matter, but to dillerent degrees; while other properties are eonlincd to certaiij forms of matter, e.g. solids or liquids or gases. A solid has a definite shape and size of its own, which can. however, be changed by the applica- tion of certain forces. Some solids, e.g. copper, have (luililili/ (q.v. ) and can be drawn out into wires; some have maUrnhiHly (q.v.), and can be hammered out into thin sheets; .some have porosity (q.v.), and allow various other portions of matter to pass through them; some are 'glazed' and are nearly im|M'rvious to other por- ti<ins of matter; some are hard, others soft; some are brittle, others tough; some are plastic, like putty, etc. A li(/iiitl is such a form of matter that if left to itself in air (or in any gas or other liquid with which it docs not mix I it forms a spherical drop, or, if contained in a hollow solid here on the surface of the earth, it takes the shape of the vessel, keeping a constant volume. A liquid has then certain molecular properties in its sur- face which makes it contract as far as possible. See Capillarity. .• fins is such a form of matter that, heing contained within any closed vessel, it distributes itself uniformly throughout the space open to it; thus having neither a shape nor a size of its o»vii. (See flA.SEs. Cenerai. Properties of.) Gases and liquids are called /liiiils because they can flow; they yield to any force, however snutll, which is acting in sich a direction as to make one layer move over the other. ( See IlvhROSTATir.s.) Some bodies behave as liquids to feeble but long-eoiitinucd forces, but as solids to intense and sudden forces; shoemaker's wax will flow .so as to fill a tumbler if time is given, but it may lie broken by n sudden blow, just like a picee of glass. .ll forms of matter arc divisible into smaller parts. (See following paragraphs on Thmrint of Matter.) They are aNo more or less 'idastic'; that is, if the shape or size of a solid is deformed slightly by a small force, or if the volume of a fluid is so changed, they will return to their previous conditions more or less perfectly when the deforming force is removed: tliis jiroves the existence of internal molecular forces of restitu- tion. (See Elasticity.) Whenever the shape of a solid is changod^not the shajie of the whole solid necessaiily, but the shape of the little cubical jiortions out of which the body niay be imagined constructed — there is always to some degree a slipping of the layers of matter over each other, and corresponding internal or molecular friction. Similarly if currents are produced in lluids, there is more or less friction between the layers, which is attributed to a property called riscositi/ (q.v.). In the case of liquids there is a superficial viscosity also, which is made mani- fest when a body tloating in the surface is moved. A properly common to all forms of matter is that of "diUusion"; if two portions of dili'erent kinds of matter arc brought closely togetlier — 'iu contact" — it is believed that there is always a liassage across the bounding surface of molecules of the two kinds of matter. Sometimes this pas- sage can Ix! actually observed, e.g. in the case of any two ga.ses, two such liquids as water and alcohol, two such .solids as lead and gold. Since matter as such has so many properties: inertia, weight, size, elasticity, etc., two portions of matter may have some properties in connnon and not others. Therefoie if two portions of matter arc to be defined as equal, or to have ci|ual quantities, it is necessary to select some basis of comparison. By definition, two portions of matter are said to have equal quantities — or equal "masses' — if they have the same inertia; the experimental test being imagined somewhat as follows: Subject one body to the propulsive action of a couqircssed spring, measuix' its veloc- ity along a smooth horizontal table; compress the same spring to the same amount as before, allow it by its expansion to set in motion a second body, and measure its velocity; if these two velocities are the same, the two bodies have the same inertia. Xewton, and later Bessel, proved tluit the ac- celeration of a falling body toward the earth at any one place on the earth's surface is a constant for all kinds and amounts of matter. (Sec tJRAViTATlo.N.) tall it (/. The weight of a body of mass III is j<i;/," and so if two bodies have the same mass, as defined above, they also have the same weight, and conversely. Consequently the mass of a body is always in practice mc;isnred by comparing its weight with that of .a coml)ina- tion of standards. A standard body is chosen, a gram; other bodies of the same mass are made; others whose masses are fractions or null- tiplcs of that of the standard; etc. Such a set of bodies is called a 'set of weights.' It is believed that matter as such is indc- struetil)le; that is. however it changes its form or whatever reactions it undergoes, a |Mirtion of matter preserves its mass unaltered. Tliis idea, which is entirely in accorii with all experiments and observations, is called the principle of the conservation of matter. It is perfectly possible that the irrifihl of a body changes as its tempera- ture, or one of its other properties, is altered, but there is no experimental evidence in favor of such an idea. For full discussion of proi>ertieS of matter, the render niav consult Tait, Proper- ties of Matter (F.dinburgh. 1S8.5).
