But the term "energy" itself, as used in definite sense by the physicist, rather involves a modern idea and is itself a generalisation. Things as distinct from each other as light, heat, sound, rotation, vibration, elastic strain, gravitative separation, electric currents, and chemical affinity, have all to be generalised under the same heading, in order to make the law true. Until "heat" was included in the list of energies, the statement could not be made; and, a short time ago, it was sometimes discussed whether "life" should or should not be included in the category of energy. I should give the answer decidedly No, but some might be inclined to say Yes; and this is sufficient as an example to show that the categories of energy are not necessarily exhausted; that new forms may be discovered; and that if new forms exist, until they are discovered, the law of conservation of energy as now stated may in some cases be strictly untrue; just as it would be untrue, though partially and usefully true, in the theory of machines, if heat were unknown or ignored. To jump, therefore, from a generalisation such as this, and to say, as Professor Haeckel does on page 5, that the following cosmological theorems have already been "amply demonstrated," is to leap across a considerable chasm:— "1. The universe, or the cosmos, is eternal, infinite, and illimitable. "2. Its substance, with its two attributes (matter and energy), fills infinite space, and is in eternal motion. "3. This motion runs on through infinite time as an unbroken development, with a periodic change from life to death, from evolution to devolution. "4. The innumerable bodies which are scattered about the space-filling ether all obey the same 'law of substance'; while the rotating masses slowly move towards their destruction and dissolution in one part of space, others are springing into new life and development in other quarters of the universe." Most of this, though in itself probable enough, must, when scientifically regarded, be rated as guess-work, being an overpressing of known fact into an exaggerated and over-comprehensive form of statement. Let it be understood that I am not objecting to his speculations, but only pointing out that they are speculations.