ROBERT BOYLE (1627-1691)
Some of Robert Boyle's views as to the possible structure of
atmospheric air will be considered a little farther on in this
chapter, but for the moment we will take up the consideration of
some of his experiments upon that as well as other gases. Boyle
was always much interested in alchemy, and carried on extensive
experiments in attempting to accomplish the transmutation of
metals; but he did not confine himself to these experiments,
devoting himself to researches in all the fields of natural
philosophy.
He was associated at
scientists, including Wallis and Wren, who held meetings and made
experiments together, these gatherings being the beginning, as
mentioned a moment ago, of what finally became the Royal Society.
It was during this residence at Oxford that many of his valuable
researches upon air were made, and during this time be invented
his air-pump, now exhibited in the Royal Society rooms at
Burlington House.[1]
His experiments to prove the atmospheric pressure are most
interesting and conclusive. "Having three small, round glass
bubbles, blown at the flame of a lam 242i83c p, about the size of
hazel-nuts," he says, "each of them with a short, slender stem,
by means whereof they were so exactly poised in water that a very
small change of weight would make them either emerge or sink; at
a time when the atmosphere was of convenient weight, I put them
into a wide-mouthed glass of common water, and leaving them in a
quiet place, where they were frequently in my eye, I observed
that sometimes they would be at the top of the water, and remain
there for several days, or perhaps weeks, together, and sometimes
fall to the bottom, and after having continued there for some
time rise again. And sometimes they would rise or fall as the air
was hot or cold."[2]
It was in the course of these experiments that the observations
made by Boyle led to the invention of his "statical barometer,"
the mercurial barometer having been invented, as we have seen, by
Torricelli, in 1643. In describing this invention he says:
"Making choice of a large, thin, and light glass bubble, blown at
the flame of a lamp, I counterpoised it with a metallic weight,
in a pair of scales that were suspended in a frame, that would
turn with the thirtieth part of a 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 numerous stories
he heard concerning the growth of metals in previously exhausted
mines, he believed that the air was responsible for producing
this growth--in which he undoubtedly believed. The story of a
tin-miner that, in his own time, after a lapse of only
twenty-five years, a heap, of earth previously exhausted of its
ore became again even more richly impregnated than before by
lying exposed to the air, seems to have been believed by the
philosopher.
As Boyle was an alchemist, and undoubtedly believed in the
alchemic theory that metals have "spirits" and various other
qualities that do not exist, it is not surprising that he was
credulous in the matter of beliefs concerning peculiar phenomena
exhibited by them. Furthermore, he undoubtedly fell into the
error common to "specialists," or persons working for long
periods of time on one subject--the error of over-enthusiasm in
his subject. He had discovered so many remarkable qualities in
the air that it is not surprising to find that he attributed to
it many more that he could not demonstrate.
Boyle's work upon colors, although probably of less importance
than his experiments and deductions upon air, show that he was in
the van as far as the science of his day was concerned. As he
points out, the schools of his time generally taught that "color
is a penetrating quality, reaching to the innermost part of the
substance," and, as an example of this, sealing-wax was cited,
which could be broken into minute bits, each particle retaining
the same color as its fellows or the original mass. To refute
this theory, and to show instances to the contrary, Boyle, among
other things, shows that various colors--blue, red, yellow--may
be produced upon tempered steel, and yet the metal within "a
hair's-breadth of its surface" have none of these colors.
Therefore, he was led to believe that color, in opaque bodies at
least, is superficial.
"But before we descend to a more particular consideration of our
subject," he says, " 'tis proper to observe that colors may be
regarded either as a quality residing in bodies to modify light
after a particular manner, or else as light itself so modified as
to strike upon the organs of sight, and cause the sensation we
call color; and that this latter is the more proper acceptation
of the word color will appear hereafter. And indeed it is the
light itself, which after a certain manner, either mixed with
shades or other-wise, strikes our eyes and immediately produces
that motion in the organ which gives us the color of an
object."[6]
In examining smooth and rough surfaces to determine the cause of
their color, he made use of the microscope, and pointed out the
very obvious example of the difference in color of a rough and a
polished piece of the same block of stone. He used some striking
illustrations of the effect of light and the position of the eye
upon colors. "Thus the color of plush or velvet will appear
various if you stroke part of it one way and part another, the
posture of the particular threads in regard to the light, or the
eye, being thereby varied. And 'tis observable that in a field of
ripe corn, blown upon by the wind, there will appear waves of a
color different from that of the rest of the corn, because the
wind, by depressing some of the ears more than others, causes one
to reflect more light from the lateral and strawy parts than
another."[7] His work upon color, however, as upon light, was
entirely overshadowed by the work of his great fellow-countryman
Newton.
Boyle's work on electricity was a continuation of Gilbert's, to
which he added several new facts. He added several substances to
Gilbert's list of "electrics," experimented on smooth and rough
surfaces in exciting of electricity, and made the important
discovery that amber retained its attractive virtue after the
friction that excited it bad ceased. "For the attrition having
caused an intestine motion in its parts," he says, "the heat
thereby excited ought not to cease as soon as ever the rubbing is
over, but to continue capable of emitting effluvia for some time
afterwards, longer or shorter according to the goodness of the
electric and the degree of the commotion made; all which, joined
together, may sometimes make the effect considerable; and by this
means, on a warm day, I, with a certain body not bigger than a
pea, but very vigorously attractive, moved a steel needle, freely
poised, about three minutes after I had left off rubbing it."[8]
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