i. Evidence afforded by the outward facts of nature.
The general problem of the universe, as hitherto presented by the phenomena which Tyndall, in this house, called "the outward facts of nature," demands that matter, besides being continuous in time and occupying space to exclusion of other matter, shall have such physical properties as admit:
(i) of conditions in space which allow of motions of matter, like those of the earth and planets round the sun, at velocities of upwards of 20 miles a second, with scarcely any diminution after thousands of years; i.e. they must admit of a perfect vacuum of matter, such as would be obtained by a perfect air-pump;
(2) they must also allow of the transmission of light, such as is being transmitted through these windows, to be reflected or absorbed by the opposite wall;
(3) of the gravitation of matter, as when I drop this ball;
(4) of the (limited) cohesion of matter, on which the strength of our structures depend. Thus I can break this stick of sealing-wax, and when I warm the ends and bring them together, when cold it is as strong as before;
(5) of the elasticity of matter, as shown by the continued vibration of this spring;
(6) of the limited friction of matter, as is shown by the weight resting on the inclined plane until the inclination reaches a certain angle, when it slides down at an accelerating rate;
(7) of the viscosity of matter, as is shown by putting oil on the inclined plane, when the weight slides down slowly and at a steady rate;
(8) of the electric and magnetic properties of matter, shown by the absence of any affinity of the stick of sealing-wax for the paper until it is rubbed by silk, when it at once picks up the paper;
(9) of the freedoms and mutual constraints of the molecules of matter, shown by the uniform pressure of the air in this room;
(10) of the combination and dissociations of molecules, as shown respectively by any combustion and any electrolytic decompositions.
2. Phenomena not hitherto explained.
That the physical properties demanded for the mechanical explanation of the ten phenomena illustrated, as well as others, exist, is certain.
But it is equally certain, that, hitherto, they had not been found, in spite of all attempts.
3. Theories of the transmission of light.
In place of explanations there have been the theories of Huygens and Newton, two hundred years ago, put forward as explaining, in some measure, the transmission of light; and again, the modification of Huygens' theory, by Dr Young, a hundred years ago, which latter up to the present time has carried all before it.
Thus for the last hundred years the idea of the structure of the universe, or the luminiferous ether, which has prevailed, is that of space occupied by an incompressible elastic jelly yielding to tangential stress, having a density which is all but indefinitely small.
And so the idea which has alone prevailed as to the structure of the universe is such as approximates to empty space.
There have been other ideas not so much as to the structure of the universe, which have been strongly held; these will however come in at a later stage, while our attention is turned to the ideas, as to the structure, which follow as the results of an exhaustive research "On the sub-mechanics of the universe."
4. The granular structure of the universe.
This research has occupied 20 years, and is just now completed.
It has revealed the prime cause