Every one of these is within the jurisdiction of the laws of mechanics, even when the motions are so exalted in degree and dignity as to seem of other stock than their real parents. Or, to change the metaphor, the tortuous labyrinth of the whole series diverges by clear and continuous avenues from one simple highway, where the elementary laws of motion are visibly obeyed.
The consistences of matter, as well as its properties, illustrate in a remarkable manner the principle of continuity. Sir William Herschel long ago ventured on general grounds to predict that the solid, liquid, and gaseous states of matter would be found to shade off imperceptibly into each other. Twenty years afterward, the labors of Prof. Andrews, of Belfast, proved the great astronomer right. By the most ingenious appliances, he detained for convenient inspection processes of transition from gas to liquid, which, in their ordinary progress, coalesce so abruptly as to seem instantaneous. In some familiar cases we can perceive changes of the same kind going on; as, for example, in the melting of wax we can follow the alteration from brittle hardness to plasticity, and thence to viscosity and liquefaction. From facts such as these, here very briefly indicated, has arisen the conviction that all matter can assume any of the three consistences. Faraday liquefied, by cold and great pressure, several of what had been called permanent gases, and improvements in the means of producing pressure and cold will doubtless enable us in the future to liquefy the remainder. Although the greatest heat we can bring to bear on carbon does not fuse it, still the tendency of our knowledge is to induce us to believe that coal in burning for a brief instant, too short for observation, exists in the liquid state. A second of time is divisible into millionths quite as perfectly as a geological cycle.
The thread of continuity has, in a variety of cases, been established in the laboratory. No two physical facts would seem to stand more decidedly apart than chemical union and mechanical admixture, yet we find them inextricably joined when we add sulphuric acid and water together. In all possible percentages do these liquids chemically combine, and this at variance with the generally-obeyed law of definite proportions. The same departure from the usual rule also obtains among other complex unions, and corroborates what first principles affirm—namely, that chemical forces are but intense and involved mechanical ones.
In the progress of science there has been much speculation as to the method by which light, electricity, and gravitation, are propagated through space. It is the old question again, "Can matter act where it is not?" Newton found the idea inconceivable, and imagined an ether as the vehicle of motions between the suns and planets of the universe. This position has been criticised by Mill, who says that inconceivableness is no test of truth, and who asserts, with a lack of his usual caution, that scarcely any living thinker of eminence now
