213. And now let us try to balance the egg upon its longer axis. Probably, a sufficient amount of care will enable us to achieve this also. But the operation is a difficult one, and requires great delicacy of touch, and even after we have succeeded we do not know how long our success may last. The slightest impulse from without, the merest breath of air, may be sufficient to overturn the egg, which is now most evidently in unstable equilibrium. If the egg be thus balanced at the very edge of the table, it is quite probable that in a few minutes it may topple over upon the floor; it is what we may call _an even chance_ whether it will do so, or merely fall upon the table. Not that mere chance has anything to do with it, or that its movements are without a cause, but we mean that its movements are decided by some external impulse so exceedingly small as to be utterly beyond our powers of observation. In fact, before making the trial we have carefully removed everything like a current of air, or want of level, or external impulse of any kind, so that when the egg falls we are completely unable to assign the origin of the impulse that has caused it to do so. 214. Now, if the egg happens to fall over the table upon the floor, there is a somewhat considerable transmutation of energy; for the energy of position of the egg, due to the height which it occupied on the table, has all at once been changed into energy of motion, in the first place, and into heat in the second, when the egg comes into contact with the floor. If, however, the egg happens to fall upon the table, the transmutation of energy is comparatively small. It thus appears 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. _Chemical Instability._ 215. We thus see that a body, or system, in unstable equilibrium may become subject to a very considerable transmutation of energy, arising out of a very small cause, or antecedent. In the case now mentioned, the force is that of gravitation, the arrangement being one of visible mechanical instability. But we may have a substance, or system, in which the force at work is not gravity, but chemical affinity, and the substance, or system, may, under certain peculiar conditions, become _chemically unstable_. When a substance is chemically unstable, it means that the slightest impulse of any kind may determine a chemical change, just as in the case of the egg the slightest impulse from without occasioned a mechanical displacement.