The opposite tendency is deathwards. Protoplasm drops extraneous groups of atoms; retires into itself; loses irritability; settles down to rest. The molecules of proteins exhibit a property which appears to pertain in some degree to living matter also. When their relation to the water in which they are dissolved and the electrolytes which it contains is disturbed, they appear to go out of solution, they coagulate. This disturbance is brought about in all proteins by heat; in some it is the result of altering the amount of salt in the water in which they are dissolved. Coagulation in protoplasm is the prelude to death; but it would appear that a step in this downward path is taken whenever an impulse is conducted. Coagulation is due to the clustering of the molecules of a protein. When protoplasm drops electrolytes and water its molecules cluster in some degree, regaining their independence and reattaching their accessory groups of atoms as the cause which drove them to make for safety passes by. Our conception—not of life, but of “the physical basis of life,” may be very wide of the mark. The account given above is intended as little more than a hint of the lines along which thought is travelling at the present time. The reader must not regard it as a serious attempt to present in detail the views of any of the workers who are endeavouring to apply the results of recent discoveries in molecular physics to the solution of problems in the chemistry of living matter. There can, however, be little doubt but that we are on the eve of further advances which will secure data upon which it will be legitimate to construct hypotheses. At present it would be unreasonable to do more than indicate the direction from which it may be hoped that light will shine. A stimulus is a change of circumstance rather than a transient disturbance. When an electric current is thrown into it, protoplasm dissociates—parts with something. It instantly reassociates. The continued passage of the electric current does not maintain it in a dissociated condition. When the current is cut off, the sudden change again acts as a stimulus. Within limits, the efficiency of an electric stimulus varies as its suddenness. Similarly with all other stimuli to which protoplasm responds: crushing, burning, chemical decomposition, are effective at the moment of their occurrence. When they generate a succession of responses, it is because they continue to produce changes in the protoplasm. Their continued action does not, under ordinary circumstances, prolong the response.