rally to depend upon the molecule; thus, the power of certain substances to cause the plane of polarization of a ray of light to rotate, is exactly the same whatever be its degree of density, or the diluteness of the solution in which it is contained. Taken as a whole, the physical properties of substances and their quantitative laws, present a problem of infinite complexity, and centuries must elapse before any moderately complete generalizations on the subject become possible.

Uniform Properties of all Matter.

Some laws are held to be true of all matter in the universe absolutely, without exception, no instance to the contrary having ever been noticed. This is the case with the laws of motion, as laid down by Galileo and Newton. It is also conspicuously true of the law of universal gravitation. The rise of modern physical science may perhaps be considered as beginning at the time when Galileo showed, in opposition to the Aristotelians, that matter is equally affected by gravity, irrespective of its form, magnitude, or texture. All objects fall with equal rapidity, when disturbing causes, such as the resistance of the air, are removed or allowed for. That which was rudely demonstrated by Galileo from the leaning tower of Pisa, was proved by Newton to a high degree of approximation, in an experiment which has already been referred to (vol. ii. p. 55).

Newton formed two pendulums of as nearly as possible similar outward shape, by taking two equal round wooden boxes, and suspending them by equal threads, eleven feet long. The motion of each pendulum was therefore equally subject to the resistance of the air. He filled one box with wood, and in the centre of oscillation of the

other placed an equal weight of gold. The pendulums were then equal in weight and in size; and, on setting them simultaneously in motion, Newton found that they vibrated for a great length of time with exactly equal vibrations. He tried the same experiment with silver, lead, glass, sand, common salt, water, and wheat, instead of gold, and ascertained that the rapidity of motion of his pendulum was exactly the same whatever was the kind of matter inside them h. He considered that a difference of a thousandth part would have been apparent. The reader must observe that the pendulums were made of equal weight only in order that they might suffer equal retardation from the air. The meaning of the experiment is that all the substances manifest exactly equal acceleration from the force of gravity, and that therefore the inertia or resistance of matter to force, which is the only independent measure of mass in our possession, is always proportional to gravity.

These experiments of Newton were considered conclusive up to very recent times, when certain discordances. between the theory and observations of the movements of planets led Nicolai, in 1826, to suggest that the equal gravitation of different kinds of matter might not be absolutely exact. It is perfectly philosophical and desirable thus to call in question, from time to time, some of the best accepted laws. On this occasion Bessel carefully repeated the experiments of Newton with pendulums composed of ivory, glass, marble, quartz, meteoric stones, &c., but was unable to detect the least difference. This conclusion is also confirmed by the ultimate agreement of all the calculations of physical astronomy based upon it. Thus, whether the mass of Jupiter be calculated from the motion of its own satellites, from the effect upon the small

h Principia,' bk. III. Prop. VI. Motte's translation, vol. ii. p. 220.

i

planets, Vesta, Juno, &c., or from the perturbation Encke's Comet, the results are closely accordant, showing that precisely the same law of gravity applies to the most different bodies which we can observe. The granty or weight of a body, again, appears to be entirely independent of its other physical conditions, being totally unaffected by any alteration in the temperature, density. electric or magnetic condition, or other physical properties of the substance.

One almost paradoxical result of the law of equal gravitation is the theorem of Torricelli, to the effect that all liquids of whatever density fall or flow with equal rapidity. If there be two equal cisterns respectively ed with mercury and water, the mercury, though thirteen times as heavy, would flow from an aperture neither more rapidly nor more slowly than the water, and the same would be true of ether, alcohol, or any other liquids, allowance being made for the resistance of the air, and the differing viscosities of the liquids.

In its exact equality and its perfect independence of every circumstance, except mass and distance, the force of gravity stands apart from all the other forces and phenomena of nature, and has not yet been brought into any relation with them except through the general principle of the conservation of energy. Magnetic attraction, as temarked by Newton, follows a wholly different law as depending upon the chemical quality and molecular structre of each particular substance.

We must remember that in saying all matter gravitates, we exclude from the term matter the basis of lightundulations which is almost infinitely more extensive in amount, and obeys in many other respects the laws of mechanics. This adamantine basis of undulations appears, so far as can be ascertained, to be perfectly uniform in its properties when existing in space unoccupied by matter.

Light and heat are conveyed by it with equal velocity in all directions, and in all parts of space so far as observation informs us. But the presence of gravitating matter modifies the density and mechanical properties of the so-called ether in a way which is yet quite unexplained.

Leaving gravity, it is somewhat difficult to discover other laws which are equally true of all matter. Boerhaave was considered to have established that all bodies expand by heat, but not only is the expansion very different in different substances, but we now know positive exceptions. Many liquids and a few solids contract by heat at certain temperatures. There are indeed other

relations of heat to matter which seem to be universal and uniform; thus all substances begin to give off rays of heat or light at the same temperature, according to the law of Draper; and gases will not be an exception if sufficiently condensed, as in the experiments of Frankland. Grove considers it to be universally true that all bodies in combining produce heat; all solids, with the doubtful exception of sulphur and selenium, in becoming liquid, and all liquids in becoming gases, absorb a certain quantity of heat; but the quantities of heat absorbed vary with the chemical qualities of the matter. On the other hand, Carnot's Thermodynamic Law is held to be exactly true of all matter without distinction; it expresses the fact that the amount of mechanical energy which might be theoretically obtained from a certain amount of heat energy depends only upon the temperatures between which a substance is made to change, so that whether an engine be worked by water, air, alcohol, ammonia, or any other substance, the result would theoretically be the same, if the boiler and condenser were employed at similar temperatures.

Variable Properties of Matter.

I have enumerated some of the few properties of matter, which are manifested in exactly the same manner by all substances, whatever be their differences of chemical or physical constitution. But by far the greater number of qualities vary in degree; substances are more or less dense, more or less transparent, more or less compressible, more or less magnetic, and so on. One very common result of the progress of science is to show that qualities supposed to be entirely absent from many substances are present only in so low a degree of intensity that the means of detection were insufficient. Newton believed that most bodies were not affected by the magnet at all; Faraday and Tyndall have rendered it very doubtful whether any substance whatever is wholly non-magnetic, including under that term diamagnetic properties. We are rapidly learning to believe that there are no substances absolutely opaque, or non-conducting, non-electric, non-elastic, non-viscous, non-compressible, insoluble, infusible, or non-volatile. All tends to become a matter of degree, or sometimes of direction. There may be some substances oppositely affected to others, as ferro-magnetic substances are oppositely affected to diamagnetics, or as substances which contract by heat are opposed to those which expand; but the tendency is certainly for every affection of one kind of matter to be represented by something similar in other kinds. On this account one of Newton's rules of philosophizing seems quite to lose all validity; he said, 'Those qualities of bodies which are not capable of being heightened and remitted, and which are found in all bodies on which experiment can be made, must be considered as universal qualities of all bodies.' As far as I can see, the contrary is more probable, namely,