mass to the particle, and here the same set of questions again occur. A particle as truly as a mass occupies space, and we wish to know if the force proceeds from the surface of the particle or from its interior.

150. Then again we likewise wish to know how this force is communicated between one particle and another? Now before we can solve these questions we must have some definite conception of the nature of a particle and of the constitution of the surrounding medium. Sir W. Thomson, as we have seen, has attempted to advance towards the nature of an atom or particle in his supposition that atoms are vortex-rings generated out of a perfect fluid filling all space. While, however, this conception accounts for some of the properties of an atom it does not easily account for gravitation, and hence he adopts in addition the hypothesis of ultra-mundane corpuscles, which he supposes to be only a finer form of vortices.

151. There is, however, one objection to the precise form of vortex-ring hypothesis introduced by Thomson which from our point of view is very strong. The act by which the atom was produced must necessarily by this hypothesis have been an act of creation (Art. 133) in time, that is to say, an act impressed upon the universe from without, and it must therefore have denoted a breach of continuity (Art. 85); for if the invisible universe be nothing but a perfect fluid, can we imagine it capable of originating such a development in virtue of its own inherent properties, and without some external act implying a breach of continuity?-we think not. In the production of the atom from a perfect fluid we are driven at once to the uncondi tioned to the Great First Cause; it is, in fine, an act of creation and not of development. But from our point

of view (Art. 86) creation belongs to eternity and development to time, and we are therefore induced to modify the hypothesis so. as to make it consistent with this view. We cannot, in fact, if we agree to hold at the same time the principle of unbroken continuity and the vortex-ring theory of formation of the visible universe, regard the invisible universe as an absolutely perfect fluid.

152. This way of regarding the invisible universe is strengthened by the fact that the hypothesis which seems most likely to account for gravitation presumes the existence of ultra-mundane corpuscles, and the observations of Struve upon the extinction of star-light tend (whatever they are worth) towards the same conclusion, since the absorption of light is more compatible with a corpuscular constitution than with that of a perfect fluid. But if the visible universe be developed from an invisible which is not a perfect fluid, then the argument deduced by Sir W. Thomson in favour of the eternity of ordinary matter disappears, since this eternity depends upon the perfect fluidity of the invisible. In fine, if we suppose the material universe to be composed of a series of vortex-rings developed from an invisible universe which is not a perfect fluid, it will be ephemeral, just as the smoke-ring which we develop from air, or that which we develop from water, is ephemeral, the only difference being in duration, these lasting only for a few seconds, and the others it may be for billions of years.

153. Thus, in our last chapter, we came to the conclusion that the available energy of the visible universe will ultimátely be appropriated by the invisible, and we may now perhaps imagine, at least as a possibility, that the separate existence of the visible universe will share the same fate,

so that we shall have no huge useless inert mass existing in after ages to remind the passer-by of a form of energy and a species of matter that is long since out of date and functionally effete. Why should not the universe bury its dead out of sight?

"All nature is but art, unknown to thee;

All chance, direction, which thou canst not see,

All discord, harmony not understood;

All partial evil, universal good;

And spite of pride, in erring reason's spite,

One truth is clear, whatever is, is right.”—POPE.

154. IN Chapters III. and IV. we have dwelt upon the laws of energy and the ultimate constitution of matter; in other words, we have discussed those laws according to which the machine called the visible universe works, as well as the probable nature of that material of which it is composed. We have in this process (Arts. 86, 151) come to the conclusion that the visible universe has been developed out of the invisible. Once developed, it has its own laws of action which we may discover,-laws which at present appear to be invariably followed, as far at least as our strictly scientific experience can inform us.

In fine, the visible universe is that which we are in a

position to observe, gaining an insight into its present method of working, and trying also to reply to that very interesting question, Has it always worked in its present manner, or has there ever been any apparent break?

Let us, therefore, take this visible universe after its production, and endeavour to become acquainted with the course of its development. What did it do? Was it entirely left to itself, and to what may be termed the natural laws impressed upon it when it was produced?

In replying to these questions, let us, for the sake of convenience, consider development under the three following heads, viz., (a) Chemical or Stuff Development, (8) Globe Development, (y) Life Development.

155. To begin with chemical or stuff development, we come at once to a very interesting and important question. Assuming that the atoms of the present universe were developed from the invisible, were different kinds of atoms thus developed, or were they all of one kind?

To this question the chemist of last century would have replied:-Undoubtedly there were many kinds of primeval atoms, and then would follow a list of all these various substances which he was unable to decompose.

The chemist of thirty or forty years later would still reply to the question in the same way, but he would probably have a different list of primeval elements less formidable in number.

If the chemist of forty, years ago were asked, he would have furnished a list of perhaps fifty simple substances; but then, probably, the minimum would have been reached; for ask the chemist of to-day, and he will furnish a list of sixty-three elements.

156. But while the number of undecomposed bodies is