The conveyance of _sound_ is, perhaps, the most extraordinary service which gutta-percha tubes have yet rendered. There are two qualities required in a speaking tube; first, that it shall concentrate a large amount of sound into a small space; and secondly, that it shall not stifle the acoustic vibrations within the tube itself. Any material will answer equally well, so far as the first-named quality is concerned, for it requires simply a trumpet-shaped mouth at one end, and a very small orifice at the other; but gutta-percha possesses rare qualities in respect to the second kind of service. Whether it is the smoothness of the texture, or the peculiar kind and degree of elasticity, or the relation of the substance to heat or electricity—whatever may be the cause, a tube of gutta-percha preserves sonorous vibrations with a surprising degree of clearness and equability; and the modes in which this quality are brought into useful requisition are also very numerous. There is, for example, the _long ear-trumpet_, with a wide orifice at one end and a small one at the other; and there is the _portable ear-trumpet_, differing from the former only in bringing the speaker and the hearer closer together, by a ‘French-horn’ system of twisting in the tube. There is the _ear-cornet_, so small and neat that one may be almost invisibly attached to or near each ear. There is the _paraboloid trumpet_, in which the sound is echoed from a large concave receiver before it enters the tube. There is the trumpet with a long flexible tube, or with several tubes, so that several persons round a table can communicate in turn with the user. In short, there have been almost as many useful variations of the principle as there are variations in the social inconveniences of those who require such aid.