Again, in Chapters III., IV., and V. we have sufficiently discussed the visible universe and its potentialities. We have seen that although at present it contains the essential requisites for organised existence, yet, in the remote future, a time will necessarily arrive when, through a degradation of the Energy of this universe, or at least of one part of it, that variety of motion which is essential to life will be unattainable. Immortality is, therefore, impossible or hardly possible in such a universe; but even allowing all this to be the case, it is at least conceivable that man may be at death drafted off into some superior rank of being connected with the present universe, and thence ultimately removed into a new order of things when the present universe shall have become effete. Let us now, therefore, very briefly discuss the question as to the possibility of intelligences superior to man existing in the present visible universe. And, in order to commence this inquiry, let us analyse with some minuteness the physical source of that peculiarity which the present universe possesses, in virtue of which it affords living beings the means of a varied existence. Whence is all this power derived? How comes it about that a living being possesses that abruptness and spontaneity of action which peculiarly characterise it? In fine, let us consider the exact position of life in the present physical universe. 178. Now, in the first place, it is well known that equilibrium may be of two kinds, stable and unstable, and if we take an egg balanced on its end at the edge of a table as an example of mechanical instability, we shall see that it ‘depends upon some external impulse so infinitesimally small as to elude our observation whether the egg shall fall upon the floor and give rise to a comparatively large transmutation of energy, or whether it shall fall upon the table and give rise to a transmutation comparatively small.’[53] But, just as there are other forces besides gravity, so there are other varieties of instability besides that which we treat of in mechanics. We may, for instance, have molecular instability, such as characterises water cooled below the freezing point, or a supersaturated solution of Glauber’s salt, where the advent of the smallest possible crystal of ice or of Glauber’s salt is sufficient to bring about a marked molecular change in the liquid, which immediately becomes thick with deposited crystals; or again, we may have chemical instability in which the slightest impulse of any kind may determine a chemical change, just as in mechanical instability the slightest possible impulse may determine a mechanical change. Thus fulminating silver or nitroglycerine are familiar examples of chemical instability in which the slightest blow or the smallest spark may be sufficient to bring about an instantaneous and violent generation of heated gas.