EXPERIMENT.
We may now consider the great advantages which we enjoy in examining
the combinations of phenomena when things are within our reach and
capable of being experimented on. We are said *to experiment* when we
bring substances together under various conditions of temperature,
pressure, electric disturbance, chemical action, &c., and then record
the changes observed. Our object in inductive investigation is to
ascertain exactly the group of circumstances or conditions which being
present, a certain other group of phenomena will follow. If we denote
by A the antecedent group, and by X subsequent phenomena, our object
will usually be to discover a law of the form A = AX, the meaning of
which is that where A is X will happen.
The circumstances which might be enumerated as present in the simplest
experiment are very numerous, in fact almost infinite. Rub two sticks
together and consider what would be an exhaustive statement of the
conditions. There are the form, hardness, organic structure, and all
the chemical qualities of the wood; the pressure and velocity of the
rubbing; the temperature, pressure, and all the chemical qualities of
the surrounding air; the proximity of the earth with its attractive
and electric powers; the temperature and other properties of the
persons producing motion; the radiation from the sun, and to and from
the sky; the electric excitement possibly existing in any overhanging
cloud; even the positions of the heavenly bodies must be mentioned.
On *à priori* grounds it is unsafe to assume that any one of these
circumstances is without effect, and it is only by experience that we
can single out those precise conditions from which the observed heat of
friction proceeds.
The great method of experiment consists in removing, one at a time,
each of those conditions which may be imagined to have an influence
on the result. Our object in the experiment of rubbing sticks is to
discover the exact circumstances under which heat appears. Now the
presence of air may be requisite; therefore prepare a vacuum, and
rub the sticks in every respect as before, except that it is done
*in vacuo*. If heat still appears we may say that air is not, in
the presence of the other circumstances, a requisite condition. The
conduction of heat from neighbouring bodies may be a condition. Prevent
this by making all the surrounding bodies ice cold, which is what Davy
aimed at in rubbing two pieces of ice together. If heat still appears
we have eliminated another condition, and so we may go on until it
becomes apparent that the expenditure of energy in the friction of two
bodies is the sole condition of the production of heat.
The great difficulty of experiment arises from the fact that we must
not assume the conditions to be independent. Previous to experiment we
have no right to say that the rubbing of two sticks will produce heat
in the same way when air is absent as before. We may have heat produced
in one way when air is present, and in another when air is absent.
The inquiry branches out into two lines, and we ought to try in both
cases whether cutting off a supply of heat by conduction prevents its
evolution in friction. The same branching out of the inquiry occurs
with regard to every circumstance which enters into the experiment.
Regarding only four circumstances, say A, B, C, D, we ought to test
not only the combinations ABCD, ABC*d*, AB*c*D, A*b*CD, *a*BCD, but
we ought really to go through the whole of the combinations given in
the fifth column of the Logical Alphabet. The effect of the absence
of each condition should be tried both in the presence and absence of
every other condition, and every selection of those conditions. Perfect
and exhaustive experimentation would, in short, consist in examining
natural phenomena in all their possible combinations and registering
all relations between conditions and results which are found capable
of existence. It would thus resemble the exclusion of contradictory
combinations carried out in the Indirect Method of Inference, except
that the exclusion of combinations is grounded not on prior logical
premises, but on *à posteriori* results of actual trial.
The reader will perceive, however, that such exhaustive investigation
is practically impossible, because the number of requisite experiments
would be immensely great. Four antecedents only would require sixteen
experiments; twelve antecedents would require 4096, and the number
increases as the powers of two. The result is that the experimenter
has to fall back upon his own tact and experience in selecting those
experiments which are most likely to yield him significant facts. It
is at this point that logical rules and forms begin to fail in giving
aid. The logical rule is--Try all possible combinations; but this being
impracticable, the experimentalist necessarily abandons strict logical
method, and trusts to his own insight. Analogy, as we shall see, gives
some assistance, and attention should be concentrated on those kinds
of conditions which have been found important in like cases. But we
are now entirely in the region of probability, and the experimenter,
while he is confidently pursuing what he thinks the right clue, may
be overlooking the one condition of importance. It is an impressive
lesson, for instance, that Newton pursued all his exquisite researches
on the spectrum unsuspicious of the fact that if he reduced the hole in
the shutter to a narrow slit, all the mysteries of the bright and dark
lines were within his grasp, provided of course that his prisms were
sufficiently good to define the rays. In like manner we know not what
slight alteration in the most familiar experiments may not open the way
to realms of new discovery.
Practical difficulties, also, encumber the progress of the physicist.
It is often impossible to alter one condition without altering others
at the same time; and thus we may not get the pure effect of the
condition in question. Some conditions may be absolutely incapable of
alteration; others may be with great difficulty, or only in a certain
degree, removable. A very treacherous source of error is the existence
of unknown conditions, which of course we cannot remove except by
accident. These difficulties we will shortly consider in succession.
It is beautiful to observe how the alteration of a single circumstance
sometimes conclusively explains a phenomenon. An instance is found in
Faraday’s investigation of the behaviour of Lycopodium spores scattered
on a vibrating plate. It was observed that these minute spores
collected together at the points of greatest motion, whereas sand and
all heavy particles collected at the nodes, where the motion was least.
It happily occurred to Faraday to try the experiment in the exhausted
receiver of an air-pump, and it was then found that the light powder
behaved exactly like heavy powder. A conclusive proof was thus obtained
that the presence of air was the condition of importance, doubtless
because it was thrown into eddies by the motion of the plate, and
carried the Lycopodium to the points of greatest agitation. Sand was
too heavy to be carried by the air.
*Exclusion of Indifferent Circumstances.*
From what has been already said it will be apparent that the detection
and exclusion of indifferent circumstances is a work of importance,
because it allows the concentration of attention upon circumstances
which contain the principal condition. Many beautiful instances may be
given where all the most obvious antecedents have been shown to have no
part in the production of a phenomenon. A person might suppose that the
peculiar colours of mother-of-pearl were due to the chemical qualities
of the substance. Much trouble might have been spent in following out
that notion by comparing the chemical qualities of various iridescent
substances. But Brewster accidentally took an impression from a piece
of mother-of-pearl in a cement of resin and bees’-wax, and finding
the colours repeated upon the surface of the wax, he proceeded to
take other impressions in balsam, fusible metal, lead, gum arabic,
isinglass, &c., and always found the iridescent colours the same. He
thus proved that the chemical nature of the substance is a matter of
indifference, and that the form of the surface is the real condition
of such colours.[319] Nearly the same may be said of the colours
exhibited by thin plates and films. The rings and lines of colour will
be nearly the same in character whatever may be the nature of the
substance; nay, a void space, such as a crack in glass, would produce
them even though the air were withdrawn by an air-pump. The conditions
are simply the existence of two reflecting surfaces separated by a very
small space, though it should be added that the refractive index of the
intervening substance has some influence on the exact nature of the
colour produced.
[319] *Treatise on Optics*, by Brewster, Cab. Cyclo. p. 117.
When a ray of light passes close to the edge of an opaque body, a
portion of the light appears to be bent towards it, and produces
coloured fringes within the shadow of the body. Newton attributed
this inflexion of light to the attraction of the opaque body for the
supposed particles of light, although he was aware that the nature
of the surrounding medium, whether air or other pellucid substance,
exercised no apparent influence on the phenomena. Gravesande proved,
however, that the character of the fringes is exactly the same, whether
the body be dense or rare, compound or elementary. A wire produces
exactly the same fringes as a hair of the same thickness. Even the
form of the obstructing edge was subsequently shown to be a matter of
indifference by Fresnel, and the interference spectrum, or the spectrum
seen when light passes through a fine grating, is absolutely the same
whatever be the form or chemical nature of the bars making the grating.
Thus it appears that the stoppage of a portion of a beam of light is
the sole necessary condition for the diffraction or inflexion of light,
and the phenomenon is shown to bear no analogy the refraction of light,
in which the form and nature of the substance are all important.
It is interesting to observe how carefully Newton, in his researches
on the spectrum, ascertained the indifference of many circumstances by
actual trial. He says:[320] “Now the different magnitude of the hole
in the window-shut, and different thickness of the prism where the
rays passed through it, and different inclinations of the prism to the
horizon, made no sensible changes in the length of the image. Neither
did the different matter of the prisms make any: for in a vessel made
of polished plates of glass cemented together in the shape of a prism,
and filled with water, there is the like success of the experiment
according to the quantity of the refraction.” But in the latter
statement, as I shall afterwards remark (p. 432), Newton assumed an
indifference which does not exist, and fell into an unfortunate mistake.
[320] *Opticks*, 3rd. ed. p. 25.
In the science of sound it is shown that the pitch of a sound depends
solely upon the number of impulses in a second, and the material
exciting those impulses is a matter of indifference. Whatever fluid,
air or water, gas or liquid, be forced into the Siren, the sound
produced is the same; and the material of which an organ-pipe is
constructed does not at all affect the pitch of its sound. In the
science of statical electricity it is an important principle that
the nature of the interior of a conducting body is a matter of no
importance. The electrical charge is confined to the conducting
surface, and the interior remains in a neutral state. A hollow copper
sphere takes exactly the same charge as a solid sphere of the same
metal.
Some of Faraday’s most elegant and successful researches were devoted
to the exclusion of conditions which previous experimenters had
thought essential for the production of electrical phenomena. Davy
asserted that no known fluids, except such as contain water, could be
made the medium of connexion between the poles of a battery; and some
chemists believed that water was an essential agent in electro-chemical
decomposition. Faraday gave abundant experiments to show that other
fluids allowed of electrolysis, and he attributed the erroneous opinion
to the very general use of water as a solvent, and its presence in most
natural bodies.[321] It was, in fact, upon the weakest kind of negative
evidence that the opinion had been founded.
Many experimenters attributed peculiar powers to the poles of a
battery, likening them to magnets, which, by their attractive powers,
tear apart the elements of a substance. By a beautiful series of
experiments,[322] Faraday proved conclusively that, on the contrary,
the substance of the poles is of no importance, being merely the path
through which the electric force reaches the liquid acted upon. Poles
of water, charcoal, and many diverse substances, even air itself,
produced similar results; if the chemical nature of the pole entered at
all into the question, it was as a disturbing agent.
[321] *Experimental Researches in Electricity*, vol. i. pp. 133, 134.
[322] Ibid. vol i. pp. 127, 162, &c.
It is an essential part of the theory of gravitation that the proximity
of other attracting particles is without effect upon the attraction
existing between any two molecules. Two pound weights weigh as much
together as they do separately. Every pair of molecules in the world
have, as it were, a private communication, apart from their relations
to all other molecules. Another undoubted result of experience pointed
out by Newton[323] is that the weight of a body does not in the least
depend upon its form or texture. It may be added that the temperature,
electric condition, pressure, state of motion, chemical qualities,
and all other circumstances concerning matter, except its mass, are
indifferent as regards its gravitating power.
[323] *Principia*, bk. iii. Prop. vi. Corollary i.
As natural science progresses, physicists gain a kind of insight
and tact in judging what qualities of a substance are likely to be
concerned in any class of phenomena. The physical astronomer treats
matter in one point of view, the chemist in another, and the students
of physical optics, sound, mechanics, electricity, &c., make a fair
division of the qualities among them. But errors will arise if too
much confidence be placed in this independence of various kinds of
phenomena, so that it is desirable from time to time, especially
when any unexplained discrepancies come into notice, to question the
indifference which is assumed to exist, and to test its real existence
by appropriate experiments.
*Simplification of Experiments.*
One of the most requisite precautions in experimentation is to
vary only one circumstance at a time, and to maintain all other
circumstances rigidly unchanged. There are two distinct reasons for
this rule, the first and most obvious being that if we vary two
conditions at a time, and find some effect, we cannot tell whether
the effect is due to one or the other condition, or to both jointly.
A second reason is that if no effect ensues we cannot safely conclude
that either of them is indifferent; for the one may have neutralised
the effect of the other. In our symbolic logic AB ꖌ A*b* was shown to
be identical with A (p. 97), so that B denotes a circumstance which is
indifferently present or absent. But if B always goes together with
another antecedent C, we cannot show the same independence, for ABC ꖌ
A*bc* is not identical with A and none of our logical processes enables
us to reduce it to A.
If we want to prove that oxygen is necessary to life, we must not
put a rabbit into a vessel from which the oxygen has been exhausted
by a burning candle. We should then have not only an absence of
oxygen, but an addition of carbonic acid, which may have been the
destructive agent. For a similar reason Lavoisier avoided the use of
atmospheric air in experiments on combustion, because air was not a
simple substance, and the presence of nitrogen might impede or even
alter the effect of oxygen. As Lavoisier remarks,[324] “In performing
experiments, it is a necessary principle, which ought never to be
deviated from, that they be simplified as much as possible, and that
every circumstance capable of rendering their results complicated be
carefully removed.” It has also been well said by Cuvier[325] that
the method of physical inquiry consists in isolating bodies, reducing
them to their utmost simplicity, and bringing each of their properties
separately into action, either mentally or by experiment.
[324] Lavoisier’s *Chemistry*, translated by Kerr, p. 103.
[325] Cuvier’s *Animal Kingdom*, introduction, pp. 1, 2.
The electro-magnet has been of the utmost service in the investigation
of the magnetic properties of matter, by allowing of the production
or removal of a most powerful magnetic force without disturbing any
of the other arrangements of the experiment. Many of Faraday’s most
valuable experiments would have been impossible had it been necessary
to introduce a heavy permanent magnet, which could not be suddenly
moved without shaking the whole apparatus, disturbing the air,
producing currents by changes of temperature, &c. The electro-magnet
is perfectly under control, and its influence can be brought into
action, reversed, or stopped by merely touching a button. Thus Faraday
was enabled to prove the rotation of the plane of circularly polarised
light by the fact that certain light ceased to be visible when the
electric current of the magnet was cut off, and re-appeared when the
current was made. “These phenomena,” he says, “could be reversed at
pleasure, and at any instant of time, and upon any occasion, showing a
perfect dependence of cause and effect.”[326]
[326] *Experimental Researches in Electricity*, vol. iii. p. 4.
It was Newton’s omission to obtain the solar spectrum under the
simplest conditions which prevented him from discovering the dark
lines. Using a broad beam of light which had passed through a round
hole or a triangular slit, he obtained a brilliant spectrum, but one
in which many different coloured rays overlapped each other. In the
recent history of the science of the spectrum, one main difficulty has
consisted in the mixture of the lines of several different substances,
which are usually to be found in the light of any flame or spark. It
is seldom possible to obtain the light of any element in a perfectly
simple manner. Angström greatly advanced this branch of science by
examining the light of the electric spark when formed between poles of
various metals, and in the presence of various gases. By varying the
pole alone, or the gaseous medium alone, he was able to discriminate
correctly between the lines due to the metal and those due to the
surrounding gas.[327]
[327] *Philosophical Magazine*, 4th Series, vol. ix. p. 327.
*Failure in the Simplification of Experiments.*
In some cases it seems to be impossible to carry out the rule of
varying one circumstance at a time. When we attempt to obtain two
instances or two forms of experiment in which a single circumstance
shall be present in one case and absent in another, it may be found
that this single circumstance entails others. Benjamin Franklin’s
experiment concerning the comparative absorbing powers of different
colours is well known. “I took,” he says, “a number of little square
pieces of broadcloth from a tailor’s pattern card, of various colours.
They were black, deep blue, lighter blue, green, purple, red, yellow,
white, and other colours and shades of colour. I laid them all out upon
the snow on a bright sunshiny morning. In a few hours the black, being
most warmed by the sun, was sunk so low as to be below the stroke of
the sun’s rays; the dark blue was almost as low; the lighter blue not
quite so much as the dark; the other colours less as they were lighter.
The white remained on the surface of the snow, not having entered it at
all.” This is a very elegant and apparently simple experiment; but when
Leslie had completed his series of researches upon the nature of heat,
he came to the conclusion that the colour of a surface has very little
effect upon the radiating power, the mechanical nature of the surface
appearing to be more influential. He remarks[328] that “the question
is incapable of being positively resolved, since no substance can be
made to assume different colours without at the same time changing its
internal structure.” Recent investigation has shown that the subject
is one of considerable complication, because the absorptive power of a
surface may be different according to the character of the rays which
fall upon it; but there can be no doubt as to the acuteness with which
Leslie points out the difficulty. In Well’s investigations concerning
the nature of dew, we have, again, very complicated conditions. If we
expose plates of various material, such as rough iron, glass, polished
metal, to the midnight sky, they will be dewed in various degrees; but
since these plates differ both in the nature of the surface and the
conducting power of the material, it would not be plain whether one
or both circumstances were of importance. We avoid this difficulty by
exposing the same material polished or varnished, so as to present
different conditions of surface;[329] and again by exposing different
substances with the same kind of surface.
[328] *Inquiry into the Nature of Heat*, p. 95.
[329] Herschel, *Preliminary Discourse*, p. 161.
When we are quite unable to isolate circumstances we must resort to
the procedure described by Mill under the name of the Joint Method
of Agreement and Difference. We must collect as many instances as
possible in which a given circumstance produces a given result, and
as many as possible in which the absence of the circumstance is
followed by the absence of the result. To adduce his example, we
cannot experiment upon the cause of double refraction in Iceland spar,
because we cannot alter its crystalline condition without altering
it altogether, nor can we find substances exactly like calc spar in
every circumstance except one. We resort therefore to the method
of comparing together all known substances which have the property
of doubly-refracting light, and we find that they agree in being
crystalline.[330] This indeed is nothing but an ordinary process of
perfect or probable induction, already partially described, and to
be further discussed under Classification. It may be added that the
subject does admit of perfect experimental treatment, since glass,
when compressed in one direction, becomes capable of doubly-refracting
light, and as there is probably no alteration in the glass but change
of elasticity, we learn that the power of double refraction is probably
due to a difference of elasticity in different directions.
[330] *System of Logic*, bk. iii. chap. viii. § 4, 5th ed. vol. i.
p. 433.
*Removal of Usual Conditions.*
One of the great objects of experiment is to enable us to judge
of the behaviour of substances under conditions widely different
from those which prevail upon the surface of the earth. We live in
an atmosphere which does not vary beyond certain narrow limits in
temperature or pressure. Many of the powers of nature, such as gravity,
which constantly act upon us, are of almost fixed amount. Now it
will afterwards be shown that we cannot apply a quantitative law to
circumstances much differing from those in which it was observed. In
the other planets, the sun, the stars, or remote parts of the Universe,
the conditions of existence must often be widely different from what
we commonly experience here. Hence our knowledge of nature must remain
restricted and hypothetical, unless we can subject substances to
unusual conditions by suitable experiments.
The electric arc is an invaluable means of exposing metals or other
conducting substances to the highest known temperature. By its aid we
learn not only that all the metals can be vaporised, but that they
all give off distinctive rays of light. At the other extremity of the
scale, the intensely powerful freezing mixture devised by Faraday,
consisting of solid carbonic acid and ether mixed *in vacuo*, enables
us to observe the nature of substances at temperatures immensely below
any we meet with naturally on the earth’s surface.
We can hardly realise now the importance of the invention of the
air-pump, previous to which invention it was exceedingly difficult
to experiment except under the ordinary pressure of the atmosphere.
The Torricellian vacuum had been employed by the philosophers of the
Accademia del Cimento to show the behaviour of water, smoke, sound,
magnets, electric substances, &c., *in vacuo*, but their experiments
were often unsuccessful from the difficulty of excluding air.[331]
[331] *Essayes of Natural Experiments made in the Accademia del
Cimento.* Englished by Richard Waller, 1684, p. 40, &c.
Among the most constant circumstances under which we live is the force
of gravity, which does not vary, except by a slight fraction of its
amount, in any part of the earth’s crust or atmosphere to which we
can attain. This force is sufficient to overbear and disguise various
actions, for instance, the mutual gravitation of small bodies. It
was an interesting experiment of Plateau to neutralise the action of
gravity by placing substances in liquids of exactly the same specific
gravity. Thus a quantity of oil poured into the middle of a suitable
mixture of alcohol and water assumes a spherical shape; on being made
to rotate it becomes spheroidal, and then successively separates into a
ring and a group of spherules. Thus we have an illustration of the mode
in which the planetary system may have been produced,[332] though the
extreme difference of scale prevents our arguing with confidence from
the experiment to the conditions of the nebular theory.
[332] Plateau, *Taylor’s Scientific Memoirs*, vol. iv. pp. 16–43.
It is possible that the so-called elements are elementary only to us,
because we are restricted to temperatures at which they are fixed.
Lavoisier carefully defined an element as a substance which cannot be
decomposed *by any known means*; but it seems almost certain that some
series of elements, for instance Iodine, Bromine, and Chlorine, are
really compounds of a simpler substance. We must look to the production
of intensely high temperatures, yet quite beyond our means, for the
decomposition of these so-called elements. Possibly in this age and
part of the universe the dissipation of energy has so far proceeded
that there are no sources of heat sufficiently intense to effect the
decomposition.
*Interference of Unsuspected Conditions.*
It may happen that we are not aware of all the conditions under which
our researches are made. Some substance may be present or some power
may be in action, which escapes the most vigilant examination. Not
being aware of its existence, we are unable to take proper measures
to exclude it, and thus determine the share which it has in the
results of our experiments. There can be no doubt that the alchemists
were misled and encouraged in their vain attempts by the unsuspected
presence of traces of gold and silver in the substances they proposed
to transmute. Lead, as drawn from the smelting furnace, almost always
contains some silver, and gold is associated with many other metals.
Thus small quantities of noble metal would often appear as the result
of experiment and raise delusive hopes.
In more than one case the unsuspected presence of common salt in the
air has caused great trouble. In the early experiments on electrolysis
it was found that when water was decomposed, an acid and an alkali
were produced at the poles, together with oxygen and hydrogen. In
the absence of any other explanation, some chemists rushed to the
conclusion that electricity must have the power of *generating*
acids and alkalies, and one chemist thought he had discovered a new
substance called *electric acid*. But Davy proceeded to a systematic
investigation of the circumstances, by varying the conditions. Changing
the glass vessel for one of agate or gold, he found that far less
alkali was produced; excluding impurities by the use of carefully
distilled water, he found that the quantities of acid and alkali were
still further diminished; and having thus obtained a clue to the cause,
he completed the exclusion of impurities by avoiding contact with his
fingers, and by placing the apparatus under an exhausted receiver,
no acid or alkali being then detected. It would be difficult to meet
with a more elegant case of the detection of a condition previously
unsuspected.[333]
[333] *Philosophical Transactions* [1826], vol. cxvi. pp. 388, 389.
Works of Sir Humphry Davy, vol. v. pp. 1–12.
It is remarkable that the presence of common salt in the air, proved
to exist by Davy, nevertheless continued a stumbling-block in the
science of spectrum analysis, and probably prevented men, such as
Brewster, Herschel, and Talbot, from anticipating by thirty years
the discoveries of Bunsen and Kirchhoff. As I pointed out,[334] the
utility of the spectrum was known in the middle of the last century
to Thomas Melvill, a talented Scotch physicist, who died at the early
age of 27 years.[335] But Melvill was struck in his examination of
coloured flames by the extraordinary predominance of homogeneous yellow
light, which was due to some circumstance escaping his attention.
Wollaston and Fraunhofer were equally struck by the prominence of the
yellow line in the spectrum of nearly every kind of light. Talbot
expressly recommended the use of the prism for detecting the presence
of substances by what we now call spectrum analysis, but he found that
all substances, however different the light they yielded in other
respects, were identical as regards the production of yellow light.
Talbot knew that the salts of soda gave this coloured light, but in
spite of Davy’s previous difficulties with salt in electrolysis, it
did not occur to him to assert that where the light is, there sodium
must be. He suggested water as the most likely source of the yellow
light, because of its frequent presence; but even substances which were
apparently devoid of water gave the same yellow light.[336] Brewster
and Herschel both experimented upon flames almost at the same time as
Talbot, and Herschel unequivocally enounced the principle of spectrum
analysis.[337] Nevertheless Brewster, after numerous experiments
attended with great trouble and disappointment, found that yellow light
might be obtained from the combustion of almost any substance. It
was not until 1856 that Swan discovered that an almost infinitesimal
quantity of sodium chloride, say a millionth part of a grain, was
sufficient to tinge a flame of a bright yellow colour. The universal
diffusion of the salts of sodium, joined to this unique light-producing
power, was thus shown to be the unsuspected condition which had
destroyed the confidence of all previous experimenters in the use of
the prism. Some references concerning the history of this curious point
are given below.[338]
[334] *National Review*, July, 1861, p. 13.
[335] His published works are contained in *The Edinburgh Physical
and Literary Essays*, vol. ii. p. 34; *Philosophical Transactions*
[1753], vol. xlviii. p. 261; see also Morgan’s Papers in
*Philosophical Transactions* [1785], vol. lxxv. p. 190.
[336] *Edinburgh Journal of Science*, vol. v. p. 79.
[337] *Encyclopædia Metropolitana*, art. *Light*, § 524; Herschel’s
*Familiar Lectures*, p. 266.
[338] Talbot, *Philosophical Magazine*, 3rd Series, vol. ix. p. 1
(1836); Brewster, *Transactions of the Royal Society of Edinburgh*
[1823], vol. ix. pp. 433, 455; Swan, ibid. [1856] vol. xxi. p. 411;
*Philosophical Magazine*, 4th Series, vol. xx. p. 173 [Sept. 1860];
Roscoe, *Spectrum Analysis*, Lecture III.
In the science of radiant heat, early inquirers were led to the
conclusion that radiation proceeded only from the surface of a solid,
or from a very small depth below it. But they happened to experiment
upon surfaces covered by coats of varnish, which is highly athermanous
or opaque to heat. Had they properly varied the character of the
surface, using a highly diathermanous substance like rock salt, they
would have obtained very different results.[339]
One of the most extraordinary instances of an erroneous opinion due
to overlooking interfering agents is that concerning the increase of
rainfall near to the earth’s surface. More than a century ago it was
observed that rain-gauges placed upon church steeples, house tops, and
other elevated places, gave considerably less rain than if they were
on the ground, and it has been recently shown that the variation is
most rapid in the close neighbourhood of the ground.[340] All kinds
of theories have been started to explain this phenomenon; but I have
shown[341] that it is simply due to the interference of wind, which
deflects more or less rain from all the gauges which are exposed to it.
[339] Balfour Stewart, *Elementary Treatise on Heat*, p. 192.
[340] British Association, Liverpool, 1870. *Report on Rainfall*,
p. 176.
[341] *Philosophical Magazine.*, Dec. 1861. 4th Series, vol. xxii.
p. 421.
The great magnetic power of iron renders it a source of disturbance in
magnetic experiments. In building a magnetic observatory great care
must therefore be taken that no iron is employed in the construction,
and that no masses of iron are near at hand. In some cases magnetic
observations have been seriously disturbed by the existence of masses
of iron ore in the neighbourhood. In Faraday’s experiments upon feebly
magnetic or diamagnetic substances he took the greatest precautions
against the presence of disturbing substances in the copper wire, wax,
paper, and other articles used in suspending the test objects. It was
his custom to try the effect of the magnet upon the apparatus in the
absence of the object of experiment, and without this preliminary trial
no confidence could be placed in the results.[342] Tyndall has also
employed the same mode for testing the freedom of electro-magnetic
coils from iron, and was thus enabled to obtain them devoid of any
cause of disturbance.[343] It is worthy of notice that in the very
infancy of the science of magnetism, the acute experimentalist Gilbert
correctly accounted for the opinion existing in his day that magnets
would attract silver, by pointing out that the silver contained iron.
[342] *Experimental Researches in Electricity*, vol. iii. p. 84, &c.
[343] *Lectures on Heat*, p. 21.
Even when we are not aware by previous experience of the probable
presence of a special disturbing agent, we ought not to assume the
absence of unsuspected interference. If an experiment is of really
high importance, so that any considerable branch of science rests
upon it, we ought to try it again and again, in as varied conditions
as possible. We should intentionally disturb the apparatus in various
ways, so as if possible to hit by accident upon any weak point.
Especially when our results are more regular than we have fair grounds
for anticipating, ought we to suspect some peculiarity in the apparatus
which causes it to measure some other phenomenon than that in question,
just as Foucault’s pendulum almost always indicates the movement of the
axes of its own elliptic path instead of the rotation of the globe.
It was in this cautious spirit that Baily acted in his experiments on
the density of the earth. The accuracy of his results depended upon the
elimination of all disturbing influences, so that the oscillation of
his torsion balance should measure gravity alone. Hence he varied the
apparatus in many ways, changing the small balls subject to attraction,
changing the connecting rod, and the means of suspension. He observed
the effect of disturbances, such as the presence of visitors, the
occurrence of violent storms, &c., and as no real alteration was
produced in the results, he confidently attributed them to gravity.[344]
[344] Baily, *Memoirs of the Royal Astronomical Society*, vol. xiv.
pp. 29, 30.
Newton would probably have discovered the mode of constructing
achromatic lenses, but for the unsuspected effect of some sugar of
lead which he is supposed to have dissolved in the water of a prism.
He tried, by means of a glass prism combined with a water prism, to
produce dispersion of light without refraction, and if he had succeeded
there would have been an obvious mode of producing refraction without
dispersion. His failure is attributed to his adding lead acetate to
the water for the purpose of increasing its refractive power, the lead
having a high dispersive power which frustrated his purpose.[345]
Judging from Newton’s remarks, in the *Philosophical Transactions*,
it would appear as if he had not, without many unsuccessful trials,
despaired of the construction of achromatic glasses.[346]
[345] Grant, *History of Physical Astronomy*, p. 531.
[346] *Philosophical Transactions*, abridged by Lowthorp, 4th
edition, vol. i. p. 202.
The Academicians of Cimento, in their early and ingenious experiments
upon the vacuum, were often misled by the mechanical imperfections
of their apparatus. They concluded that the air had nothing to do
with the production of sounds, evidently because their vacuum was not
sufficiently perfect. Otto von Guericke fell into a like mistake in the
use of his newly-constructed air-pump, doubtless from the unsuspected
presence of air sufficiently dense to convey the sound of the bell.
It is hardly requisite to point out that the doctrine of spontaneous
generation is due to the unsuspected presence of germs, even after the
most careful efforts to exclude them, and in the case of many diseases,
both of animals and plants, germs which we have no means as yet of
detecting are doubtless the active cause. It has long been a subject
of dispute, again, whether the plants which spring from newly turned
land grow from seeds long buried in that land, or from seeds brought
by the wind. Argument is unphilosophical when direct trial can readily
be applied; for by turning up some old ground, and covering a portion
of it with a glass case, the conveyance of seeds by the wind can be
entirely prevented, and if the same plants appear within and without
the case, it will become clear that the seeds are in the earth. By
gross oversight some experimenters have thought before now that crops
of rye had sprung up where oats had been sown.
*Blind or Test Experiments.*
Every conclusive experiment necessarily consists in the comparison of
results between two different combinations of circumstances. To give a
fair probability that A is the cause of X, we must maintain invariable
all surrounding objects and conditions, and we must then show that
where A is X is, and where A is not X is not. This cannot really be
accomplished in a single trial. If, for instance, a chemist places
a certain suspected substance in Marsh’s test apparatus, and finds
that it gives a small deposit of metallic arsenic, he cannot be sure
that the arsenic really proceeds from the suspected substance; the
impurity of the zinc or sulphuric acid may have been the cause of its
appearance. It is therefore the practice of chemists to make what they
call a *blind experiment*, that is to try whether arsenic appears in
the absence of the suspected substance. The same precaution ought to be
taken in all important analytical operations. Indeed, it is not merely
a precaution, it is an essential part of any experiment. If the blind
trial be not made, the chemist merely assumes that he knows what would
happen. Whenever we assert that because A and X are found together A
is the cause of X, we assume that if A were absent X would be absent.
But wherever it is possible, we ought not to take this as a mere
assumption, or even as a matter of inference. Experience is ultimately
the basis of all our inferences, but if we can bring immediate
experience to bear upon the point in question we should not trust to
anything more remote and liable to error. When Faraday examined the
magnetic properties of the bearing apparatus, in the absence of the
substance to be experimented on, he really made a blind experiment
(p. 431).
We ought, also, to test the accuracy of a method of experiment whenever
we can, by introducing known amounts of the substance or force to be
detected. A new analytical process for the quantitative estimation of
an element should be tested by performing it upon a mixture compounded
so as to contain a known quantity of that element. The accuracy of the
gold assay process greatly depends upon the precaution of assaying
alloys of gold of exactly known composition.[347] Gabriel Plattes’
works give evidence of much scientific spirit, and when discussing the
supposed merits of the divining rod for the discovery of subterranean
treasure, he sensibly suggests that the rod should be tried in places
where veins of metal are known to exist.[348]
[347] Jevons in Watts’ *Dictionary of Chemistry*, vol. ii. pp. 936,
937.
[348] *Discovery of Subterraneal Treasure.* London, 1639, p. 48.
*Negative Results of Experiment.*
When we pay proper regard to the imperfection of all measuring
instruments and the possible minuteness of effects, we shall see
much reason for interpreting with caution the negative results of
experiments. We may fail to discover the existence of an expected
effect, not because that effect is really non-existent, but because it
is of a magnitude inappreciable to our senses, or confounded with other
effects of much greater amount. As there is no limit on *à priori*
grounds to the smallness of a phenomenon, we can never, by a single
experiment, prove the non-existence of a supposed effect. We are always
at liberty to assume that a certain amount of effect might have been
detected by greater delicacy of measurement. We cannot safely affirm
that the moon has no atmosphere at all. We may doubtless show that the
atmosphere, if present, is less dense than the air in the so-called
vacuum of an air-pump, as did Du Sejour. It is equally impossible to
prove that gravity occupies *no time* in transmission. Laplace indeed
ascertained that the velocity of propagation of the influence was at
least fifty million times greater than that of light;[349] but it does
not really follow that it is instantaneous; and were there any means
of detecting the action of one star upon another exceedingly distant
star, we might possibly find an appreciable interval occupied in the
transmission of the gravitating impulse. Newton could not demonstrate
the absence of all resistance to matter moving through empty space; but
he ascertained by an experiment with the pendulum (p. 443), that if
such resistance existed, it was in amount less than one five-thousandth
part of the external resistance of the air.[350]
[349] Laplace, *System of the World*, translated by Harte, vol. ii.
p. 322.
[350] *Principia*, bk. ii. sect. 6, Prop. xxxi. Motte’s translation,
vol. ii. p. 108.
A curious instance of false negative inference is furnished by
experiments on light. Euler rejected the corpuscular theory on the
ground that particles of matter moving with the immense velocity of
light would possess momentum, of which there was no evidence. Bennet
had attempted to detect the momentum of light by concentrating the rays
of the sun upon a delicately balanced body. Observing no result, it
was considered to be proved that light had no momentum. Mr. Crookes,
however, having suspended thin vanes, blacked on one side, in a nearly
vacuous globe, found that they move under the influence of light. It
is now allowed that this effect can be explained in accordance with
the undulatory theory of light, and the molecular theory of gases. It
comes to this--that Bennet failed to detect an effect which he might
have detected with a better method of experimenting; but if he had
found it, the phenomenon would have confirmed, not the corpuscular
theory of light, as was expected, but the rival undulatory theory. The
conclusion drawn from Bennet’s experiment was falsely drawn, but it was
nevertheless true in matter.
Many incidents in the history of science tend to show that phenomena,
which one generation has failed to discover, may become accurately
known to a succeeding generation. The compressibility of water which
the Academicians of Florence could not detect, because at a low
pressure the effect was too small to perceive, and at a high pressure
the water oozed through their silver vessel,[351] has now become the
subject of exact measurement and precise calculation. Independently of
Newton, Hooke entertained very remarkable notions concerning the nature
of gravitation. In this and other subjects he showed, indeed, a genius
for experimental investigation which would have placed him in the first
rank in any other age than that of Newton. He correctly conceived that
the force of gravity would decrease as we recede from the centre of
the earth, and he boldly attempted to prove it by experiment. Having
exactly counterpoised two weights in the scales of a balance, or rather
one weight against another weight and a long piece of fine cord, he
removed his balance to the top of the dome of St. Paul’s, and tried
whether the balance remained in equilibrium after one weight was
allowed to hang down to a depth of 240 feet. No difference could be
perceived when the weights were at the same and at different levels,
but Hooke rightly held that the failure arose from the insufficient
elevation. He says, “Yet I am apt to think some difference might be
discovered in greater heights.”[352] The radius of the earth being
about 20,922,000 feet, we can now readily calculate from the law of
gravity that a height of 240 would not make a greater difference than
one part in 40,000 of the weight. Such a difference would doubtless
be inappreciable in the balances of that day, though it could readily
be detected by balances now frequently constructed. Again, the
mutual gravitation of bodies at the earth’s surface is so small that
Newton appears to have made no attempt to demonstrate its existence
experimentally, merely remarking that it was too small to fall under
the observation of our senses.[353] It has since been successfully
detected and measured by Cavendish, Baily, and others.
[351] *Essayes of Natural Experiments*, &c. p. 117.
[352] Hooke’s *Posthumous Works*, p. 182.
[353] *Principia*, bk. iii. Prop. vii. Corollary 1.
The smallness of the quantities which we can sometimes observe is
astonishing. A balance will weigh to one millionth part of the load.
Whitworth can measure to the millionth part of an inch. A rise of
temperature of the 8800th part of a degree centigrade has been
detected by Dr. Joule. The spectroscope has revealed the presence of
the 10,000,000th part of a gram. It is said that the eye can observe
the colour produced in a drop of water by the 50,000,000th part of a
gram of fuschine, and about the same quantity of cyanine. By the sense
of smell we can probably feel still smaller quantities of odorous
matter.[354] We must nevertheless remember that quantitative effects of
far less amount than these must exist, and we should state our negative
results with corresponding caution. We can only disprove the existence
of a quantitative phenomenon by showing deductively from the laws of
nature, that if present it would amount to a perceptible quantity. As
in the case of other negative arguments (p. 414), we must demonstrate
that the effect would appear, where it is by experiment found not to
appear.
[354] Keill’s *Introduction to Natural Philosophy*, 3rd ed., London,
1733, pp. 48–54.
*Limits of Experiment.*
It will be obvious that there are many operations of nature which we
are quite incapable of imitating in our experiments. Our object is to
study the conditions under which a certain effect is produced; but
one of those conditions may involve a great length of time. There
are instances on record of experiments extending over five or ten
years, and even over a large part of a lifetime; but such intervals
of time are almost nothing to the time during which nature may have
been at work. The contents of a mineral vein in Cornwall may have been
undergoing gradual change for a hundred million years. All metamorphic
rocks have doubtless endured high temperature and enormous, pressure
for inconceivable periods of time, so that chemical geology is
generally beyond the scope of experiment.
Arguments have been brought against Darwin’s theory, founded upon the
absence of any clear instance of the production of a new species.
During an historical interval of perhaps four thousand years, no
animal, it is said, has been so much domesticated as to become
different in species. It might as well be argued that no geological
changes are taking place, because no new mountain has risen in Great
Britain within the memory of man. Our actual experience of geological
changes is like a point in the infinite progression of time. When we
know that rain water falling on limestone will carry away a minute
portion of the rock in solution, we do not hesitate to multiply that
quantity by millions, and infer that in course of time a mountain may
be dissolved away. We have actual experience concerning the rise of
land in some parts of the globe and its fall in others to the extent
of some feet. Do we hesitate to infer what may thus be done in course
of geological ages? As Gabriel Plattes long ago remarked, “The sea
never resting, but perpetually winning land in one place and losing in
another, doth show what may be done in length of time by a continual
operation, not subject unto ceasing or intermission.”[355] The action
of physical circumstances upon the forms and characters of animals by
natural selection is subject to exactly the same remarks. As regards
animals living in a state of nature, the change of circumstances which
can be ascertained to have occurred is so slight, that we could not
expect to observe any change in those animals whatever. Nature has made
no experiment at all for us within historical times. Man, however, by
taming and domesticating dogs, horses, oxen, pigeons, &c., has made
considerable change in their circumstances, and we find considerable
change also in their forms and characters. Supposing the state of
domestication to continue unchanged, these new forms would continue
permanent so far as we know, and in this sense they are permanent. Thus
the arguments against Darwin’s theory, founded on the non-observation
of natural changes within the historical period, are of the weakest
character, being purely negative.
[355] *Discovery of Subterraneal Treasure*, 1639, p. 52.