ntage. He was in intimate relations by letter with many other
distinguished inventors and investigators besides Peregrinus and was a
source of incentive and encouragement to them all.

The more one knows of Aquinas the more surprise there is at his
anticipation of many modern scientific ideas. At the conclusion of a
course on cosmology delivered at the University of Paris he said that
"nothing at all would ever be reduced to nothingness" (_nihil omnino in
nihilum redigetur_). He was teaching the doctrine that man could not
destroy matter and God would not annihilate it. In other words, he was
teaching the indestructibility of matter even more emphatically than we
do. He saw the many changes that take place in material substances
around us, but he taught that these were only changes of form and not
substantial changes and that the same amount of matter always remained
in the world. At the same time he was teaching that the forms in matter
by which he meant the combinations of energies which distinguish the
various kinds of matter are not destroyed. In other words, he was
anticipating not vaguely, but very clearly and definitely, the
conservation of energy. His teaching with regard to the composition of
matter was very like that now held by physicists. He declared that
matter was composed of two principles, prime matter and form. By _forma_
he meant the dynamic element in matter, while by _materia prima_ he
meant the underlying substratum of material, the same in every
substance, but differentiated by the dynamics of matter.

It used to be the custom to make fun of these medieval scientists for
believing in the transmutation of metals. It may be said that all three
of these greatest teachers did not hold the doctrine of the
transmutation of metals in the exaggerated way in which it appealed to
many of their contemporaries. The theory of matter and form, however,
gave a philosophical basis for the idea that one kind of matter might be
changed into another. We no longer think that notion absurd. Sir William
Ramsay has actually succeeded in changing one element into another and
radium and helium are seen changing into each other, until now we are
quite ready to think of transmutation placidly. The Philosopher's Stone
used to seem a great absurdity until our recent experience with radium,
which is to some extent at least the philosopher's stone, since it
brings about the change of certain supposed elements into others. A
dis