osed to be the particles, they have been thought to be
the things from which both electrons and protons were built. Therefore,
everybody except Norman Brandon has supposed them the ultimate units of
creation, so that it would be useless to try to go any further...."

"Why, we were taught that they _are_ the ultimate units!" she protested.

"I know you were--but we really don't know anything, except what we
have learned empirically, even about our driving forces. What is called
the fourth-order particle is absolutely unknown, since nobody has been
able to detect it, to say nothing of determining its velocity or other
properties. It has been assumed to have the velocity of light only
because that hypothesis does not conflict with observational data. I'm
going to give you the generally accepted idea, since we have nothing
definite to offer in its place, but I warn you that that idea is very
probably wrong. There's a lot of deep stuff down there hasn't been dug
up yet. In fact, Brandon thinks that the product of conversion isn't
what we think it is, at all--that the actual fundamental unit and the
primary mechanism of the transformation lie somewhere below the fourth
order, and possibly even below the level of the ether--but we haven't
been able to find a point of attack yet that will let us get in
anywhere. However, I'm getting 'way ahead of our subject. To get back to
it, energy can be converted into something that acts like matter through
Roeser's Rays, and that is the empirical fact underlying the drive of
our space-ships, as well as that of almost all other vehicles on all
three planets. Power is generated by the great waterfalls of Tellus and
Venus--water's mighty scarce on Mars, of course, so most of our plants
there use fuel--and is transmitted on light beams, by means of powerful
fields of force to the receptors, wherever they may be. The individual
transmitting fields and receptors are really simply matched-frequency
units, each matching the electrical characteristics of some particular
and unique beam of force. This beam is composed of Roeser's Rays, in
their energy aspect. It took a long time to work out this tight-beam
transmission of power, but it was fairly simple after they got it."

He took out a voluminous notebook, at the sight of which Nadia smiled.

"A computer might forget to dress, but you'd never catch one without a
full magazine pencil and a lot of blank paper," he grinned in reply and
went on, wr