“What properties are essential to a medium capable of transmitting wave motion? Roughly, we may say two: elasticity and inertia.”

[Seismology and plate tectonics, Cambridge, UK; New York, Cambridge University Press, 1990, http://books.google.com/books?id=tZRxPzwoChIC&pg=PA4] (pp. 4–5)
Seismology and Plate Tectonics (1990)

(p. 12)
Seismology and Plate Tectonics (1990)

Explosions generate elastic waves by an impulsive change in volume in the material. Small explosive charges are used in controlled-source seismic experiments in which the waves penetrate only a few kilometres into the earth.
[Seismology and plate tectonics, 1990, http://books.google.com/books?id=tZRxPzwoChIC&pg=PA12] (p. 12)
Seismology and Plate Tectonics (1990)

"Energy and Force" (Mar 28, 1873)

Source: Crystallizing Public Opinion (1923), p. 171

Address to the Society for Psychical Research (1897)
Context: These rays, as generated in the vacuum tube, are not homogeneous, but consist of bundles of different wave-lengths, analogous to what would be differences of colour could we see them as light. Some pass easily through flesh, but are partially arrested by bone, while others pass with almost equal facility through bone and flesh.
It seems to me that in these rays we may have a possible mode of transmitting intelligence which, with a few reasonable postulates, may supply a key to much that is obscure in psychical research. Let it be assumed that these rays, or rays even of higher frequency, can pass into the brain and act on some nervous center there. Let it be conceived that the brain contains a center which uses these rays as the vocal chords use sound vibrations (both being under the command of intelligence), and sends them out, with the velocity of light, to impinge on the receiving ganglion of another brain. In this way some, at least, of the phenomena of telepathy, and the transmission of intelligence from one sensitive to another through long distances, seem to come into the domain of law and can be grasped. A sensitive may be one who possesses the telepathic transmitting or receiving ganglion in an advanced state of development, or who, by constant practice, is rendered more sensitive to these high-frequency waves. Experience seems to show that the receiving and the transmitting ganglions are not equally developed; one may be active, while the other, like the pineal eye in man, may be only vestigial. By such an hypothesis no physical laws are violated; neither is it necessary to invoke what is commonly called the supernatural.

The Ether of Space https://books.google.com/books?id=ycgEAAAAYAAJ&pg=PA26, p. 26
The Ether of Space (1909)

Source: Dealing with Complexity (1988), p. 247.