"You mentioned the word chlorin," said Mr. Thornton. "That is
another element?"

"Yes, that is a very common element. Ordinary table salt is sodium
chlorid: NaCl. Sodium is called natrium in Latin, and Na is the
symbol used in English to be in harmony with all other languages,
for practically all use the same chemical symbols. Sodium and
potassium are very similar elements in some respects, and in the
free state they are very peculiar, apparently taking fire when
thrown into water. Chlorin in the free state is a poisonous gas.
Thus the change in properties is well illustrated when these two
dangerous elements, sodium and chlorin, unite to form the harmless
compound which we call common salt.

"It is a shame," continued Percy, "that agricultural science has so
long been burdened with such a term as 'phosphoric acid,' which
serves to complicate and confuse what should be made the simplest
subject to every American farmer and landowner. As agriculture is
the fundamental support of America and of all her other great
industries, so the fertility of the soil is the absolute support of
every form of agriculture. Now, if there is any one factor that can
be the most important, where so many are positively essential, then
the most important factor in the problem of adopting and maintaining
permanent systems of profitable agriculture on American soils is the
element phosphorus.

"Phosphorus in very appreciable amount is positively necessary for
the growth of every organism. It is an absolutely essential
constituent of the nucleus of every living cell, whether plant or