If we are to form a conception of the cause of the irritability of living matter, we must have a mental picture of the physical conditions which distinguish life from death. All matter is in a state of motion. It consists of separate molecules, each moving in its orbit with vast rapidity. A molecule is a cluster of atoms. The dimensions of its orbit depend upon the number and weight of the atoms in its cluster. If we could watch the dance of the molecules of proteins and other substances into which protoplasm breaks up on dying, we should see each separate cluster executing the figure appropriate to its mass, indifferent to the movements of neighbouring groups. But if living protoplasm were of the company, the scene would be one of vastly greater animation; for now it is the ambition of our dancers to form a single group. To this they can never attain. There is a physical limit to the number of dancers who can hold together while the music carries them in wide sweeps backwards and forwards across the floor. At every gust of wind which bursts through an open doorway a group breaks, to clasp hands again as the wind subsides. Protoplasm is always on the verge of instability; always snatching at additional atoms which it draws within its ring; always shaking off other groups of atoms because the ring is too large to hold together. Touch it, and it falls into simpler combinations. Kill it, and it becomes a mixture of organic and inorganic compounds which we know and can name. But as long as it is alive—as long as it is protoplasm, that is to say—integration and disintegration are occurring. Simultaneous complication and simplification _is_ life. The protoplasm-molecule, if we dare to think of it as a molecule, in the sense in which a chemist uses the term, is always changing. It is its variability which makes stimulation possible. Irritability is a tendency to dissociation under the influence of an external force, with reassociation when the force ceases to act.
The molecules which protoplasm gathers into itself may be classified under the headings oxygen, foods, water, and inorganic salts. It is the two latter which most affect its state, conferring upon it the capacity of exhibiting the phenomena of life. Water and the ions of salts dissolved in water, electrolytes, are linked to the other elements of its groups. Striving to find room for more molecules of water and more ions, protoplasm expands. It becomes more mobile and more irritable; for irritability and mobility vary as the number of these extraneous groups of atoms which protoplasm is in a position to let drop. As an impulse travels through it they lose their hold, recovering it as they pass the impulse on. This progress towards expansion is the lifeward tendency; the quickening of activity which leads also to the incorporation of additional atoms of nitrogen-containing substances, and consequent growth.