grain. Both the
frame and the balance were then placed near a good barometer, whence
I might learn the present weight of the atmosphere; when, though the
scales were unable to show all the variations that appeared in the
mercurial barometer, yet they gave notice of those that altered the
height of the mercury half a quarter of an inch."(3) A fairly sensitive
barometer, after all. This statical barometer suggested several useful
applications to the fertile imagination of its inventor, among others
the measuring of mountain-peaks, as with the mercurial barometer, the
rarefication of the air at the top giving a definite ratio to the more
condensed air in the valley.

Another of his experiments was made to discover the atmospheric pressure
to the square inch. After considerable difficulty he determined that the
relative weight of a cubic inch of water and mercury was about one to
fourteen, and computing from other known weights he determined that
"when a column of quicksilver thirty inches high is sustained in the
barometer, as it frequently happens, a column of air that presses upon
an inch square near the surface of the earth must weigh about fifteen
avoirdupois pounds."(4) As the pressure of air at the sea-level is now
estimated at 14.7304 pounds to the square inch, it will be seen that
Boyle's calculation was not far wrong.

From his numerous experiments upon the air, Boyle was led to believe
that there were many "latent qualities" due to substances contained in
it that science had as yet been unable to fathom, believing that there
is "not a more heterogeneous body in the world." He believed that
contagious diseases were carried by the air, and suggested that
eruptions of the earth, such as those made by earthquakes, might send
up "venomous exhalations" that produced diseases. He suggested also that
the air might play an important part in some processes of calcination,
which, as we shall see, was proved to be true by Lavoisier late in the
eighteenth century. Boyle's notions of the exact chemical action in
these phenomena were of course vague and indefinite, but he had observed
that some part was played by the air, and he was right in supposing that
the air "may have a great share in varying the salts obtainable from
calcined vitriol."(5)

Although he was himself such a painstaking observer of facts, he had
the fault of his age of placing too much faith in hear-say evidence of
untrained observers. Thus, from the numer