If now we press our inquiry further, seeking some explanation of the ultimate nature or structure of matter, that is, the common physical characteristics of matter in all its forms, whether air, water, or solid mass, science has no certain answer to give. There is no theory of the ultimate structure of matter which has secured general acceptance. On the contrary, there is the acknowledgment that the complexity of the problem is so great as completely to baffle the present resources of science. There have been discussions, and careful investigations as to the _divisibility of matter_, and it has been generally admitted on rational grounds, that there must be in all matter particles or atoms so minute as to be quite beyond the range of the microscope. This has led to the acceptance of an _atomic theory_ as in one form or another applicable to the structure of matter, _belief_ in such particles or molecules being a natural result of scientific procedure. I say _belief_, for the existence of such ultimate atoms is not established on experimental evidence, and yet is generally acknowledged; for it is clearly enough recognized that there is a region of faith for science, as for theology, just as there must be for all ordinary exercise of human intelligence. Besides the actual divisibility of matter, we have in the same connection to consider its _compressibility_, for the recognized facts as to compression of iron, for example, or of any metal, seem to imply that there are certain particles related to each other, which can be pressed in upon each other, or brought into nearer proximity. There is, however, a clear limit to compressibility, as there is to divisibility of matter. Even if this be granted, however, we are still without a scientific account of the ultimate structure of matter. This is still a perplexity to be handed on to future workers. There may, indeed, seem to be promise of aid in the analysis of different forms of matter, as in the reduction of water to its constituent gases by the action of a galvanic battery; but such processes, however rich in suggestiveness, are insufficient to advance the main inquiry. It is oftentimes in this very class of experiments, that science at once manifests its power, and discovers the limits which encircle and restrain its efforts. It can decompose, what it can not recompose, thus leaving difficulties as perplexing as before. And besides, even when by analysis the ultimate parts or chemical constituents, of compound substances have been discovered, science is unable to demonstrate that the constituent elements are ultimately composed of distinct atoms, as for example that oxygen and hydrogen are so constituted. We are thus without a science of the ultimate nature of matter. There is, indeed, the suggestion of Sir William Thomson that matter of all kinds may be regarded as of a common nature, only variously compounded, filling space in a fluid state, and that its compressibility can be accounted for on the supposition that its ultimate forms are vortex rings capable of compression and expansion like an india-rubber ball; but this can not be regarded otherwise than as a bold conjecture, beset with a host of difficulties both physical and mathematical which neither Thomson nor any of his fellow-workers in physical science, professes to have yet grappled with.