[8] The rapidity with which an impression is transmitted through the nerves, as first determined by Helmholtz, and confirmed by Du Bois-Reymond, is 93 feet a second. [9] And long previously by Robert Boyle. [10] A very effective instrument, presented to the Royal Institution by Mr. Warren De La Rue. [11] By directing the beam of an electric lamp on glass bulbs filled with a mixture of equal volumes of chlorine and hydrogen, I have caused the bulbs to explode in vacuo and in air. The difference, though not so striking as I at first expected, was perfectly distinct. [12] It may be that the gas fails to throw the vocal chords into sufficiently strong vibration. The _laryngoscope_ might decide this question. [13] Poisson, “Mécanique,” vol. ii., p. 707. [14] To converge the pulse upon the flame, the tube was caused to end in a cone. [15] It is recorded that a bell placed on an eminence in Heligoland failed, on account of its distance, to be heard in the town. A parabolic reflector placed behind the bell, so as to reflect the sound-waves in the direction of the long, sloping street, caused the strokes of the bell to be distinctly heard at all times. This observation needs verification. [16] “Encyclopædia Metropolitana,” art. “Sound.” [17] Placing himself close to the upper part of the wall of the London Colosseum, a circular building one hundred and thirty feet in diameter, Mr. Wheatstone found a word pronounced to be repeated a great many times. A single exclamation appeared like a peal of laughter, while the tearing of a piece of paper was like the patter of hail. [18] “Poggendorff’s Annalen,” vol. lxxxv., p. 378; “Philosophical Magazine,” vol. v., p. 73. [19] Thin India-rubber balloons also form excellent sound lenses. [20] For the sake of simplicity, the wave is shown broken at _o′_ and its two halves straight. The surface of the wave, however, is really a curve, with its concavity turned in the direction of its propagation. [21] See “Heat as a Mode of Motion,” chap. iii. [22] In fact, the prompt abstraction of the motion of heat from the condensation, and its prompt communication to the rarefaction by the contiguous luminiferous ether, would prevent the former from ever rising so high, or the latter from ever falling so low, in temperature as it would do if the power of radiation was absent. [23] “Heat a Mode of Motion,” chap. x. [24] According to Burmeister, through the injection and ejection of air into and from the cavity of the chest. [25] On July 27, 1681, “Mr. Hooke showed an experiment of making musical and other sounds by the help of teeth of brass wheels; which teeth were made of equal bigness for musical sounds, but of unequal for vocal sounds.”—Birch’s “History of the Royal Society,” p. 96, published in 1757.