Platinum, especially as found naturally in impure form.

Keely
"The nearest approach to molecular uniformity in metallic masses is in the wire drawn for commercial uses, gold and platina being the nearest to freedom from differentiation. But even these wires, when tested by a certain condition of the first order of intensified molecular vibration for a transferring medium between centers of neutrality, I find to be entirely inadequate for the transfer of concordant unition, as between one and the other, on account of nodal interferences. We can appreciate the difficulty of converting such a medium to a uniform molecular link, by knowing that it can be accomplished only after removing all nodal interferences, by inducing between the nodal waves a condition in which they become subservient to the inter-sympathetic vibratory molecular link of such structure or wire." [Snell Manuscript - The Book, GRADUATION OF MACHINES, page 5]

"As far as my [Keely] researches have gone, I find that there is but one condition approaching reliability, and that is in computing the intermittent periodic disturbances along a nodal vibratory transmitter the nodes of gold, silver and platina a fixed number placed at such different distances along its line, as to take up and equalize (by a certain order of vibratory transmissions) the chord masses of the nodal interferences between the triple metals of which the nodes are composed, and also the acoustic introductory impulse of whatever chord is set. This will determine the rate of their accelerated molecular oscillation, so induced beyond their normal standard, and give us some definite figures in the computing of vibration, thousands of billions of times more than those of light. [Keely, Snell Manuscript - the book, ELECTROMAGNETIC RADIATION - Snell]