THEORIES OF MATTER.
Nally theories have been advanced to account for the properties of matter; some deny an ob jective reality to matter, others affirm it. It. is possible to show that all observed phenomena in nature may be predicted from certain general mathematical equations, the quantities in which are not necessarily connected with the percep tions of man; and that our mode of interpreting these quantities in terms of matter is not the only possible one. Again, there was a theory of matter, due to in which all actions of matter, as revealed by our senses, are attrib uted to 'force-centres,' which act on each other according to different laws for different dis tances. This theory fails to explain inertia.
All theories which affirm the objective reality of matter consider any portion of it as made up of 'molecules' and 'atoms,' meaning by molecule the smallest portion of the given kind of matter \Illicit retains the properties of the whole (e.g. a of copper, of water), and by atom r one of the fragments of a molecule which at the present time with our present knowledge we can not break up into smaller parts. There are many theories of this kind. which ditTer in the way they regard molecules and atoms; but they all agree in one respect, they consider both the molecules and atoms to be in motion. On the idea that molecules are in motion it is possible to explain the main ditTerences between solids, liquids, and gases and the principal features of diffusion, osmosis. evaporation, dissociation, heat-conduction, fluid pressure, viscosity. etc., and in particular to deduce the most important properties of a gas. Such theories as this are called 'kinetic theories.' On any kinetic theory the molecules of a gas are conceived to he in motion in paths long compared with their own size, the average length of path being wiled the 'mean free path.' It is possible by identifying certain actual physical quantities, such as pres sure. viscosity, diffusion, with these quantities as predicted by mathematical treatment of the simple kinetic theory of gases, to arrive at an idea as to the order of magnitude of the num ber of molecules in one cubic centimeter, and the length of the mean free path at different pres sures, etc. The approximate number of mole cules in I cubic centimeter at atmospheric pres sure is 6 X 10", the mean distance apart of two molecules is about 2.6 X centimeters, the mean free path is about 1 X 10' centimeters, and the volume actually filled by the molecules in 1 cubic centimeter is cubic centimeter. if the pressure is diminished, these quantities all change.
If the pressure is reduced to .001 centimeter of mercury, the mean free path becomes about I centimeter. A space so exhausted of matter as
this has special physical and is called a 'Crookes's, vacuum' o• the 'fourth state of matter,' In a liquid the molecules are supposed to he moving about, having encounters with each other, rebounding, etc., yet having practically no free path.
In a solid the molecules are supposed to be held more o• less in fixed positions. about which they may vibrate, thus forming an elastic con figuration which can be strained or even perma nently deformed.
In the cases of all liquids, and solids—while the molecules are moving about, the atoms in the nmlecule are supposed to he making immensely rapid vibrations, which pro duce the ether waves manifested by thermal, lumill01.1.•?, and chemical effects when they are ab sorbed. (See RADIATioN.) These kinetic ideas of molecules and atoms can be used to form a concrete picture of nearly all the phenomena and properties of matter.
The question remains, What is the 'atom'? one idea was that an atom is a perfectly elastic sphere, which is obviously incompatible with facts; but the theory which at present is under discussion and not disproved is that atoms are vortices (q.v.) in a perfect fluid. The simplest type of vortex is like a smoke-ring; but there are many more complicated forms, which can be shown to he stable. A vortex once formed in a perfect fluid will maintain its identity as it moves about, not being a wave-motion passing through the fluid, but always consisting of the same portion of the fluid; vortices are elastic; they can 'combine,' o• come together, and form a single system. Thus, if atoms are simply vor tices of ether moving freely through the ether, many of the properties of matter may be ex plained. A still more recent theory of matter is based upon the fact that an electric charge in motion has an inertia quite apart from that of the matter which carries the elmrge. Consult a series of palters, "Electrons," by Sir Oliver Lodge, in the Electrician (London, 1902-03).
BIBLIOGRAPHY. Meyer, The Kinetic Theory of Bibliography. Meyer, The Kinetic Theory of gases (Breslau, 1S77, Eng. trans., London, 189A) ; Holman, Matter•, Energy, Force, aml Work (New York, 1898) ; Risteen, Molecules and the Molecular Theory (Boston, 1,995) ; well, Matter and Motion (2d ed., New York, 1892) ; Tait, Properties of Matter (3d ed., Lon don and New' York, 1894) ; Poynting and Thom son, Properties of Matter (London and Philadel phia, 1901) ; Lehmann, Molekular Physik (Leip zig, 1888-89) : .1daxwell, Theory of Heat (Lon don. 1897) ; Kelvin, Popular Lectures and Ad dresses, vol. i. (New York, 1891) ; Kimball, The Physical Properties of Gases ( Boston, 1890).