We cannot imagine a piece of dough exhibiting any of these reactions, or the like of them; it can only move passively under the action of some one or other of the recognised physical forces, and that only in direct _quantitative_ relation to the work that such forces can effect; in other words, the dough can have work done on it, but it cannot do work. The Amoeba or leucocyte on the contrary does work. It moves under the various circumstances by the transformation of some of its internal energy from the "potential" into the "kinetic" state, the condition corresponding with this being essentially a liberation of heat or work, either by the breaking down of its internal substances, or by the combination of some of them with oxygen.[11] Such of these changes as involve the excretion of carbonic acid are termed "respiratory." This liberation of energy is the "response" to an action of itself inadequate to produce it; and has been compared not inaptly to the discharge of a cannon, where foot-tons of energy are liberated in consequence of the pull of a few inch-grains on the trigger, or to an indefinitely small push which makes electric contact: the energy set free is that which was stored up in the charge. This capacity for liberating energy stored up within, in response to a relatively small impulse from without, is termed "irritability"; the external impulse is termed the "stimulus." The responsive act has been termed "contractility," because it so often means an obvious contraction, but is better termed {9}"motility "; and irritability evinced by motility is characteristic of all living beings save when in the temporary condition of "rest." Again, in the case of the cannon, the gunner after its discharge has to replenish it for future action with a fresh cartridge; the Amoeba or leucocyte can replenish itself—it "feeds." When it comes in contact with a fragment of suitable material, it enwraps it by its pseudopodia (Fig. 3), and its edges coalesce where they touch on the far side as completely as we can join up the edges of dough round the apple in a dumpling. It dissolves all that can be dissolved—_i.e._ it "digests" it, and then absorbs the dissolved material into its substance, both to replace what it has lost by its previous activity and to supply fuel for future liberation of energy; this process is termed "nutrition," and is another characteristic of living beings. [Illustration: FIG. 3.—_Amoeba_ devouring a plant cell; four successive stages of ingestion. (From Verworn.)]