At one time, rather more than fifty years ago, the germs of life were considered to be an exception on the terrestrial globe; but Helmholtz discovered them even in rocky masses; and he proved to Liebig that putrefaction was not a simple chemical reaction, but was due to the action of a living organism. M. Pasteur was one of the first to profit by this lesson. Each definite science has its own special sphere in which it is occupied only with itself; Helmholtz, a physician and musician, worked entirely in connection with his own science only; without reference to the conclusion that comparative science might draw from his labours, he gave himself up to the study of the rapidity of the transmission of nervous impressions, and dogmatically laid down his thesis in his book _The Physiological Theory of Music_, which is perhaps the most important of his works; at least it is the one of which I have made the most use. In nature we never hear simple sounds; nothing but a fusion of noises reaches us. Helmholtz, however, succeeded in distinguishing a fundamental sound in a mass of others; but it is quickly amalgamated with two or three other sounds which are higher and feebler than itself, as distant echoes. Helmholtz became convinced that music is composed of single sounds accompanied by others of a decreasing intensity, and he demonstrated by calculation that the number of vibrations of these secondary sounds called harmonics are greater than those of the fundamental sounds; and the differences of the grouping of harmonic sounds determines the difference of timbre. In this way Helmholtz discovered the cause of musical timbre, and was able to explain the reason, hitherto unknown, of the sound of a flute differing from that of a hautbois, or of a woman’s voice from that of a man’s. There are two marvellous things in music; timbre and rhythm. By rhythm is understood the number of a group of corresponding vibrations recurring in a second. Rhythm may be defined as a recurring movement, composed of unequal parts; the beat of a pulse, in which each pulsation can be separately distinguished, will serve as an example. Rhythm may be found everywhere, in poetry equally with music; and it is this which imparts its chief charm. The beauty of the rhythmic prose of the Hebrew Nābhī naturally attracted the multitude independently of the subject matter of their words; and the rhythmic language of Renan’s translation of the book of Job enables us perfectly to grasp and appreciate the special charm incidental to rhythm. Music is provocative of nervous effects, at times of great intensity; beneficent to the greater number of persons, but to others quite the reverse; in his infancy Mozart almost fainted on hearing the sound of the trumpet.