The comparison of the human soul with physical ether as a qualitatively similar idea has assumed a more concrete shape in recent times through the great progress of optics and electricity (especially in the last decade); for these sciences have taught us a good deal about the energy of ether, and enabled us to formulate certain conclusions as to the material character of this all-pervading agency. As I intend to describe these important discoveries later on (in chap. xii.), I shall do no more at present than briefly point out that they render the notion of an "etheric soul" absolutely untenable. Such an etheric soul--that is a psychic substance--which is similar to physical ether, and which, like ether, passes between the ponderable elements of the living protoplasm or the molecules of the brain, cannot possibly account for the individual life of the soul. Neither the mystic notions of that kind which were warmly discussed about the middle of the century, nor the attempts of modern "Neovitalists" to put their mystical "vital force" on a line with physical ether, call for refutation any longer. Much more widespread, and still much respected, is the view which ascribes a gaseous nature to the substance of the soul. The comparison of human breath with the wind is a very old one; they were originally considered to be identical, and were both given the same name. The _anemos_ and _psyche_ of the Greeks, and the _anima_ and _spiritus_ of the Romans, were originally all names for "a breath of wind"; they were transferred from this to the breath of man. After a time this "living breath" was identified with the "vital force," and finally it came to be regarded as the soul itself, or, in a narrower sense, as its highest manifestation, the "spirit." From that the imagination went on to derive the mystic notion of individual "spirits"; these, also, are still usually conceived as "aëriform beings"--though they are credited with the physiological functions of an organism, and they have been photographed in certain well-known spiritist circles.