Dr. Carpenter has shown that it is by a series of forces acting upon matter that man conveys his ideas to man, the sonorous undulations of the atmosphere being the medium between the two. On one side the will, or power of mind, acts upon the nerves, nerve-force acts upon the muscles of speech, and these muscles, while in the act of speaking, produce sonorous undulations in the atmosphere. On the other side, these undulations are communicated by the mechanism of the ear to the auditory nerves, exciting nerve-force, and nerve-force acts upon the mind of the hearer. ‘Thus the consciousness of the speaker acts upon the consciousness of the hearer by a well-connected series of powers.’ Nerve-force generates, directly or indirectly, light, heat, chemical power, and electricity. When the optic nerve is pressed in the dark, a luminous ring is seen round the eye, and a blow on the face excites a flash of light. Nervous excitement, by accelerating respiration, increases the chemical combination of the oxygen of the air with the carbon of the blood, and thus produces animal heat. But the development of electricity by nervous and muscular force, is one of the most unexpected and singular results of physiological research. MM. Matteucci and Du Bois Reymond have proved that the intensity of the nervous and muscular forces is at a maximum when the muscles are contracted; and that if each arm of a man be put in contact with a wire of a galvanometer so as to form an electric circuit, an instantaneous deviation of the needle will take place, now in one direction and now in the other, according as he contracts his right arm or his left. The electricity thus evolved, when conveyed to the needle through several miles’ length of coiled insulated wire, will cause a deflection amounting to sixty or seventy degrees, according to the strength of the man—that is, according to his muscular and nervous force; the amount of the electricity being exactly in proportion to the amount of muscular force. It appears that the electric currents in the nerves are eight or ten times stronger than those in the muscles. M. Helmholtz found that the time required to contract a muscle, together with the time required to relax it again, is not more than the third of a second, and is a constant quantity, for the compensation of energy prevails also in organic nature. He also found that the motion or velocity of the electric current in a man is at the rate of 200 feet in a second. The electric equivalent, as determined by M. Helmholtz, is equal to the electricity produced in a voltaic battery by the seven millionth part of a milligramme of zinc consumed in the ten-thousandth part of a second, a milligramme being the 0·015432 part of a grain.