[36] A string steeped in a solution of the sulphate of quinine, and illuminated by the violet rays of the electric lamp, exhibits brilliant fluorescence. When the fork to which it is attached vibrates, the string divides itself into a series of spindles, and separated from each other by more intensely luminous nodes, emitting a light of the most delicate greenish-blue. [37] The subject of musical intervals will be treated in a subsequent lecture. [38] “This quality of sound, sometimes called its register, color, or timbre.”—Thomas Young, “Essay on Music.” [39] “Lehre von den Tonempfindungen,” p. 135. [40] The action of such a string is substantially the same as that of the siren. The string renders intermittent the current of air. Its action also resembles that of a _reed_. See Lecture V. [41] Chladni also observed this compounding of vibrations, and executed a series of experiments, which, in their developed form, are those of the kaleidophone. The composition of vibrations will be studied at some length in a subsequent lecture. [42] I copy this figure from Sir C. Wheatstone’s memoir; the nodes, however, ought to be nearer the ends, and the free terminal portions of the dotted lines ought not to be bent upward or downward. The nodal lines in the next two figures are also drawn too far from the edge of the plates. [43] Under the shoulder of the Wetterhorn I found in 1867 a pool of clear water into which a driblet fell from a brow of overhanging limestone rock. The rebounding water-drops, when they fell back, rolled in myriads over the surface. Almost any fountain, the spray of which falls into a basin, will exhibit the same effect. [44] This experiment succeeds almost equally well with a glass tube. [45] This experiment is more easily executed with hydrogen than with coal-gas. [46] Only an extremely small fraction of the fork’s motion is, however, converted into sound. The remainder is expended in overcoming the internal friction of its own particles. In other words, nearly the whole of the motion is converted into heat. [47] The clear illustrations of organ-pipes and reeds introduced here, and at page 226, have been substantially copied from the excellent work of Helmholtz. Pipes opening with hinges, so as to show their inner parts, were shown in the lecture. [48] I owe it to this eminent artist to direct attention to his experiments communicated to the Royal Society in May, 1855, and recorded in the “Philosophical Magazine” for 1855, vol. x., page 218. [49] The velocity in glass varies with the quality; the result of each experiment has therefore reference only to the particular kind of glass employed in the experiment. [50] This experiment was first made with a hydrogen-flame by Sir C. Wheatstone. [51] A gas-jet, for example, can be ignited five inches above the tip of a visible gas-flame, where platinum-leaf shows no redness.