THE WORK OF GRAHAM.
_Thomas Graham_, 1805-1869.
The work of Graham, concerned as it mostly was with the development of the
conception of atoms, connects the time of Dalton with that in which we are
now living. I have therefore judged it advisable to devote a short chapter
to a consideration of the life-work of this chemist, before proceeding to
the third period of chemical advance, that, namely, which witnessed the
development of organic chemistry through the labours of men who were
Graham's contemporaries.
The printed materials which exist for framing the story of Graham's life
are very meagre, but as he appears, from the accounts of his friends, to
have devoted himself entirely to scientific researches, we cannot go far
wrong in regarding the history of his various discoveries as also the
history of his life.
* * * * *
THOMAS GRAHAM was born in Glasgow, on December 21, 1805. His father, James
Graham, a successful manufacturer, was in a position to give his son a good
education. After some years spent in the ordinary school training, Graham
entered Glasgow University at the early age of fourteen, and graduated as
M. A. five years later. It was the intention of Graham's father that his
son should enter the Scottish Church; but under the teaching of Dr. Thomas
Thomson and others the lad imbibed so strong a love of natural science,
that rather than relinquish the pursuit of his favourite study, he
determined to be independent of his father and make a living for himself.
His father was much annoyed at the determination of his son to pursue
science, and vainly attempted to force him into the clerical profession.
The quarrel between father and son increased in bitterness, and
notwithstanding the intervention of friends the father refused to make his
son any allowance for his maintenance; and although many years after a
reconcilement was effected, yet at the time when Graham most needed his
father's help he was left to struggle alone. Graham went to Edinburgh,
where he pursued his studies under Hope and Leslie, professors of chemistry
and physics respectively--men whose names were famous wherever natural
science was studied. Graham's mother, for whom he had always the greatest
respect and warmest love, and his sister Margaret helped him as best they
could during this trying time.
The young student found some literary occupation and a little teaching in
Edinburgh, and sometimes he was asked to make investigations in subjects
connected with applied chemistry. Thus he struggled on for four or five
years, during which time he began to publish papers on chemico-physical
subjects. In the year 1829 he was appointed Lecturer on Chemistry at the
Mechanics' Institution in Glasgow, and next year he was removed to the more
important position of lecturer on the same science at the Andersonian
Institution in that city. This position he occupied for seven years, when
he was elected Professor of Chemistry in the University of London (now
University College): he had been elected to the Fellowship of the Royal
Society in the preceding year. During his stay at the Andersonian
Institution Graham had established his fame as a physical chemist; he had
begun his work on acids and salts, and had established the fundamental
facts concerning gaseous diffusion. These researches he continued in
London, and from 1837 to 1854 he enriched chemical science with a series of
papers concerned for the most part with attempts to trace the movements of
the atoms of matter.
In 1854 Graham succeeded Sir John Herschel in the important and honourable
position of Master of the Mint. For some years after his appointment he was
much engaged with the duties of his office, but about 1860 he again
returned to his atomic studies, and in his papers on "Transpiration of
Liquids" and on "Dialysis" he did much in the application of physical
methods to solve chemical problems, and opened up new paths, by travelling
on which his successors greatly advanced the limits of the science of
chemistry. Graham was almost always at work; his holidays were "few and far
between." By the year 1868 or so his general health began to grow feeble;
in the autumn of 1869, during a visit to Malvern where he sought repose and
invigorating air, he caught cold, which developed into inflammation of the
lungs. On his return to London the disease was overcome by medical
remedies, but he continued very weak, and gradually sank, till the end came
on the 16th of September 1869.
I have said that the seven years during which Graham held the lectureship
on chemistry in the Andersonian Institution, Glasgow, witnessed the
beginning alike of his work on salts and of that on gaseous diffusion. He
showed that there exists a series of compounds of various salts, _e.g._
chloride of calcium, chloride of zinc, etc., with alcohol. He compared the
alcohol in these salts, which he called _alcoates_, to the water in
ordinary crystallized salts, and thus drew the attention of chemists to the
important part played by water in determining the properties of many
substances. Three years later (1833) appeared one of his most important
papers, bearing on the general conception of acids: "Researches on the
Arseniates, Phosphates, and Modifications of Phosphoric Acid." Chemists at
this time knew that phosphoric acid--that is, the substance obtained by
adding water to pentoxide of phosphorus--exhibited many peculiarities, but
they were for the most part content to leave these unexplained. Graham,
following up the analogy which he had already established between water and
bases, prepared and carefully determined the composition of a series of
phosphates, and concluded that pentoxide of phosphorus is able to combine
with a base--say soda--in three different proportions, and thus to produce
three different phosphates of soda. But as Graham accepted that view which
regards a salt as a metallic derivative of an acid, he supposed that three
different phosphoric acids ought to exist; these acids he found in the
substances produced by the action of water on the oxide of phosphorus. He
showed that just as the oxide combines with a base in three proportions, so
does it combine with water in three proportions. This water he regarded as
chemically analogous to the base in the three salts, one atom (we should
now rather say molecule) of base could be replaced by one atom of water,
two atoms of base by two atoms of water, or three atoms of base by three
atoms of water. Phosphoric acid was therefore regarded by Graham as a
compound of pentoxide of phosphorus and water, the latter being as
essentially a part of the acid as the former. He distinguished between
_monobasic_, _dibasic_, and _tribasic_ phosphoric acids: by the action of a
base on the _monobasic acid_, one, and only one salt was produced; the
_dibasic acid_ could furnish two salts, containing different proportions
(or a different number of atoms) of the same base: and from the _tribasic
acid_ three salts, containing the same base but in different proportions,
could be obtained.
Davy's view of an acid as a compound of water with a negative oxide was
thus confirmed, and there was added to chemical science the conception of
_acids of different basicity_.
In 1836 Graham's paper on "Water as a Constituent of Salts" was published
in the "Transactions of the Royal Society of Edinburgh." In this paper he
inquires whether the water in crystalline salts can or cannot be removed
without destroying the chemical individuality of the salts. He finds that
in some crystalline salts part of the water can be easily removed by the
application of heat, but the remainder only at very high temperatures. He
distinguishes between those atoms of water which essentially belong to the
compound atom of the salt, and those atoms which can be readily removed
therefrom, which are as it were added on to, or built up around the
exterior of the atom of salt. In this paper Graham began to distinguish
what is now called _water of crystallization_ from _water of constitution_,
a distinction pointed to by some of Davy's researches, but a distinction
which has remained too much a mere matter of nomenclature since the days of
Graham.
In these researches Graham emphasized the necessity of the presence of
hydrogen in all true acids; as he had drawn an analogy between water and
bases, so now he saw in the hydrogen of acids the analogue of the metal of
salts. He regarded the structure of the compound atom of an acid as similar
to that of the compound atom of a salt; the hydrogen atom, or atoms, in the
acid was replaced by a metallic atom, or atoms, and so a compound atom of
the salt was produced.
Davy and Berzelius had proved that hydrogen is markedly electro-positive;
hydrogen appeared to Graham to belong to the class of metals. In making
this bold hypothesis Graham necessarily paid little heed to those
properties of metals which appeal to the senses of the observer. Metals, as
a class, are lustrous, heavy, malleable substances; hydrogen is a
colourless, inodourless, invisible, very light gas: how then can hydrogen
be said to be metallic?
I have again and again insisted on the need of imagination for the
successful study of natural science. Although in science we deal with
phenomena which we wish to measure and weigh and record in definite and
precise language, yet he only is the successful student of science who can
penetrate beneath the surface of things, who can form mental pictures
different from those which appear before his bodily eye, and so can discern
the intricate and apparently irregular analogies which explain the
phenomena he is set to study.
Graham was not as far as we can learn endowed, like Davy, with the
sensitive nature of a poet, yet his work on hydrogen proves him to have
possessed a large share of the gift of imagination. Picturing to himself
the hydrogen atom as essentially similar in its chemical functions to the
atom of a metal, he tracked this light invisible gas through many tortuous
courses: he showed how it is absorbed and retained (_occluded_ as he said)
by many metals; he found it in meteors which had come from far-away regions
of space; and at last, the year before he died he prepared an alloy of
palladium and the metal hydrogen, from which a few medals were struck,
bearing the legend "Palladium-Hydrogenium 1869."
Within the last few years hydrogen has been liquified and, it is said,
solidified. Solid hydrogen is described as a steel-grey substance which
fell upon the table with a sound like the ring of a metal.
But Graham's most important work was concerned with the motion of the
ultimate particles of bodies.
He uses the word "atom" pretty much as Dalton did. He does not make a
distinction between the atom of an element and the atom of a compound, but
apparently uses the term as a convenient one to express the smallest
undivided particle of any chemical substance which exhibits the properties
of that substance. As Graham was chiefly concerned with the physical
properties of chemical substances, or with those properties which are
studied alike by chemistry and physics, the distinction between atom and
molecule, so all-important in pure chemistry, might be, and to a great
extent was, overlooked by him. In considering his work we shall however do
well to use the terms "atom" and "molecule" in the sense in which they are
now always used in chemistry, a sense which has been already discussed (see
pp. 139-143).
Many years before Graham began his work a curious fact had been recorded
but not explained. In 1823 Doebereiner filled a glass jar with hydrogen and
allowed the jar to stand over water: on returning after twelve hours he
found that the water had risen about an inch and a half into the jar. Close
examination of the jar showed the presence of a small crack in the glass.
Many jars, tubes and flasks, all with small cracks in the glass, were
filled with hydrogen and allowed to stand over water; in every case the
water rose in the vessel. No rise of the water was however noticeable if
the vessels were filled with ordinary air, nitrogen or oxygen.
In 1831 Graham began the investigation of the peculiar phenomenon observed
by Doebereiner. Repeating Doebereiner's experiments, Graham found that a
portion of the hydrogen in the cracked vessels passed outwards through the
small fissures, and a little air passed inwards: the water therefore rose
in the jar, tube or flask, because there was a greater pressure on the
surface of the water outside than upon that inside the vessel. Any gas
lighter than air behaved like hydrogen; when gases heavier than air were
employed the level of the water inside the vessel was slightly lowered
after some hours.
Graham found that the passage of gases through minute openings could be
much more accurately studied by placing the gas to be examined in a glass
tube one end of which was closed by a plug of dry plaster of Paris, than by
using vessels with small fissures in the glass.
The _diffusion-tube_ used by Graham generally consisted of a piece of glass
tubing, graduated in fractions of a cubic inch and having a bulb blown near
one end; the short end was closed by a thin plug of dry plaster of Paris
(gypsum), the tube was filled with the gas to be examined, and the open end
was immediately immersed in water. The water was allowed to rise until it
had attained a constant level, when it was found that the whole of the gas
originally in the tube had passed outwards through the porous plug, and air
had passed inwards. The volume of gas originally in the tube being known,
and the volume of air in the tube at the close of the experiment being
measured, it was only necessary to divide the former by the latter number
in order to obtain the number of volumes of gas which had passed outwards
for each one volume of air which had passed inwards; in other words to
obtain the _rate of diffusion_ compared with air of the gas under
examination.
Graham's results were gathered together in the statement, "The
diffusion-rates of any two gases are inversely as the square roots of their
densities." Thus, take oxygen and hydrogen: oxygen is sixteen times
heavier than hydrogen, therefore hydrogen diffuses four times more rapidly
than oxygen. Take hydrogen and air: the specific gravity of hydrogen is
0.0694, air being 1; the square root of 0.0694 is 0.2635, therefore
hydrogen will diffuse more rapidly than air in the ratio of 0.2635:1.
In the years 1846-1849 Graham resumed this inquiry; he now distinguished
between _diffusion_, or the passage of gases through porous plates, and
_transpiration_, or the passage of gases through capillary tubes. He showed
that if a sufficiently large capillary tube be employed the rate of
transpiration of a gas becomes constant, but that it is altogether
different from the rate of diffusion of the same gas. He established
the fact that there is a connection of some kind between the
transpiration-rates and the chemical composition of gases, and in doing
this he opened up a field of inquiry by cultivating which many important
results have been gained within the last few years, and which is surely
destined to yield more valuable fruit in the future.
Returning to the diffusion of gases, Graham, after nearly thirty years'
more or less constant labour, begins to speculate a little on the causes of
the phenomena he had so studiously and perseveringly been examining. In his
paper on "The Molecular Mobility of Gases," read to the Royal Society in
1863, after describing a new diffusion-tube wherein thin plates of
artificial graphite were used in place of plaster of Paris, Graham says,
"The pores of artificial graphite appear to be really so minute that a gas
_in mass_ cannot penetrate the plate at all. It seems that molecules only
can pass; and they may be supposed to pass wholly unimpeded by friction,
for the smallest pores that can be imagined to exist in the graphite must
be tunnels in magnitude to the ultimate atom of a gaseous body." He then
shortly describes the molecular theory of matter, and shows how this
theory--a sketch of which so far as it concerns us in this book has been
given on pp. 123-125--explains the results which he has obtained. When a
gas passed through a porous plate into a vacuum, or when one gas passed in
one direction and another in the opposite direction through the same plate,
Graham saw the molecules of each gas rushing through the "tunnels" of
graphite or stucco. The average rate at which the molecules of a gas rushed
along was the diffusion-rate of that gas. The lighter the gas the more
rapid was the motion of its molecules. If a mixture of two gases, one much
lighter than the other, were allowed to flow through a porous plate, the
lighter gas would pass so much more quickly than the heavier gas that a
partial separation of the two might probably be effected. Graham
accomplished such a separation of oxygen and hydrogen, and of oxygen and
nitrogen; and he described a simple instrument whereby this process of
_atmolysis_, as he called it, might be effected.
Graham's _tube atmolyser_ consisted of a long tobacco-pipe stem placed
inside a rather shorter and considerably wider tube of glass; the pipe stem
was fixed by passing through two corks, one at each end of the glass tube;
through one of these corks there also passed a short piece of glass tubing.
When the instrument was employed, the piece of short glass tubing was
connected with an air-pump, and one end of the pipe stem with the gaseous
mixture--say ordinary air. The air-pump being set in motion, the gaseous
mixture was allowed to flow slowly through the pipe stem; the lighter
ingredient of the mixture passed outwards through the pipe stem into the
wide glass tube more rapidly than the heavier ingredient, and was swept
away to the air-pump; the heavier ingredient could be collected, mixed with
only a small quantity of the lighter, at the other end of the pipe stem. As
Graham most graphically expressed it, "The stream of gas diminishes as it
proceeds, like a river flowing over a pervious bed."
Graham then contrived a very simple experiment whereby he was able to
measure the rate of motion of the molecules of carbonic acid. He introduced
a little carbonic acid into the lower part of a tall cylindrical jar, and
at the close of certain fixed periods of time he determined the amount of
carbonic acid which had diffused upwards through the air into the uppermost
layer of the jar. Knowing the height of the jar, he now knew the distance
through which a small portion of carbonic acid passed in a stated time,
and regarding this small portion as consisting of a great many molecules,
all moving at about equal rates, he had determined the average velocity of
the molecules of carbonic acid. A similar experiment was performed with
hydrogen. The general results were that the molecules of carbonic acid move
about in still air with a velocity equal to seventy-three millimetres per
minute, and that under the same conditions the molecules of hydrogen move
with a velocity equal to about one-third of a metre per minute.[12]
The Bakerian Lecture for 1849, read by Graham before the Royal Society, was
entitled "On the Diffusion of Liquids." In this paper he describes a very
large number of experiments made with a view to determine the rate at which
a salt in aqueous solution diffuses, or passes upwards into a layer of pure
water above it, the salt solution and the water not being separated by any
intervening medium. Graham's method of procedure consisted in completely
filling a small bottle with a salt solution of known strength, placing this
bottle in a larger graduated vessel, and carefully filling the latter with
water. Measured portions of the water in the larger vessel were withdrawn
at stated intervals, and the quantity of salt in each portion was
determined. Graham found that under these conditions salts diffused with
very varying velocities. Groups of salts showed equal rates of diffusion.
There appeared to be no definite connection between the molecular weights
of the salts and their diffusion-rates; but as Graham constantly regarded
diffusion, whether of gases or liquids, as essentially due to the movements
of minute particles, he thought that the particles which moved about as
wholes during diffusion probably consisted of groups of what might be
called chemical molecules--in other words, Graham recognized various orders
of small particles. As the atom was supposed to have a simpler structure
than the molecule (if indeed it had a structure at all), so there probably
existed groups of molecules which, under certain conditions, behaved as
individual particles with definite properties.
As Graham applied the diffusion of gases to the separation of two gases of
unequal densities, so he applied the diffusion of liquids to the separation
of various salts in solution. He showed also that some complex salts, such
as the alums, were partially separated into their constituents during the
process of diffusion.
The prosecution of these researches led to most important results, which
were gathered together in a paper on "Liquid Diffusion applied to
Analysis," read to the Royal Society in 1861.
Graham divided substances into those which diffused easily and quickly into
water, and those which diffused very slowly; he showed that the former
were all crystallizable substances, while the latter were
non-crystallizable jelly-like bodies. Graham called these jelly-like
substances _colloids;_ the easily diffusible substances he called
_crystalloids_. He proved that a colloidal substance acts towards a
crystalloid much as water does; that the crystalloid rapidly diffuses
through the colloid, but that colloids are not themselves capable of
diffusing through other colloids. On this fact was founded Graham's process
of _dialysis_. As colloid he employed a sheet of parchment paper, which he
stretched on a ring of wood or caoutchouc, and floated the apparatus so
constructed--_the dialyser_--on the surface of pure water in a glass dish;
he then poured into the dialyser the mixture of substances which it was
desired to separate. Let us suppose that this mixture contained sugar and
gum; the crystalloidal sugar soon passed through the parchment paper, and
was found in the water outside, but the colloidal gum remained in the
dialyser.
If the mixture in the dialyser contained two crystalloids, the greater part
of the more diffusible of these passed through the parchment in a short
time along with only a little of the less diffusible; a partial separation
was thus effected.
This method of dialysis was applied by Graham to separate and obtain in the
pure state many colloidal modifications of chemical compounds, such as
aluminium and tin hydrates, etc. By his study of these peculiar substances
Graham introduced into chemistry a new class of bodies, and opened up
great fields of research.
Matter in the colloidal state appears to be endowed with properties which
are quite absent, or are hidden, when it is in the ordinary crystalloidal
condition. Colloids are readily affected by the smallest changes in
external conditions; they are eminently unstable bodies; they are, Graham
said, always on the verge of an impending change, and minute disturbances
in the surrounding conditions may precipitate this change at any moment.
Crystalloids, on the other hand, are stable; they have definite properties,
which are not changed without simultaneous large changes in surrounding
conditions. But although, to use Graham's words, these classes of bodies
"appear like different worlds of matter," there is yet no marked separating
line between them. Ice is a substance which under ordinary conditions
exhibits all the properties of crystalloids, but ice formed in contact with
water just at the freezing point is not unlike a mass of partly dried gum;
it shows no crystalline structure, but it may be rent and split like a lump
of glue, and, like glue, the broken pieces may be pressed together again
and caused to adhere into one mass.
"Can any facts," asks Graham, "more strikingly illustrate the maxim that in
Nature there are no abrupt transitions, and that distinctions of class are
never absolute?"
In the properties of colloids and crystalloids Graham saw an index of
diversity of molecular structure. The smallest individual particle of a
colloid appeared to him to be a much more complex structure than the
smallest particle of a crystalloid. The colloidal molecule appeared to be
formed by the gathering together of several crystalloidal molecules; such a
complex structure might be expected readily to undergo change, whereas the
simpler molecule of a crystalloid would probably present more definite and
less readily altered properties.
In this research Graham had again, as so often before, arrived at the
conception of various orders of small particles. In the early days of the
Daltonian theory it seemed that the recognition of atoms as ultimate
particles, by the placing together of which masses of this or that kind of
matter are produced, would suffice to explain all the facts of chemical
combinations; but Dalton's application of the term "atom" to elements and
compounds alike implied that an atom might itself have parts, and that one
atom might be more complex than another. The way was thus already prepared
for the recognition of more than one order of atoms, a recognition which
was formulated three years after the appearance of Dalton's "New System" in
the statement of Avogadro, "Equal volumes of gases contain equal numbers of
molecules;" for we have seen that the application of this statement to
actually occurring reactions between gases obliges us to admit that the
molecules of hydrogen, oxygen and many other elementary gases are composed
of two distinct parts or atoms.
Berzelius it is true did not formally accept the generalization of
Avogadro; but we have seen how the conception of atom which runs through
his work is not that of an indivisible particle, but rather that of a
little individual part of matter with definite properties, from which the
mass of matter recognizable by our senses is constructed, just as the wall
is built up of individual bricks. And as the bricks are themselves
constructed of clay, which in turn is composed of silica and alumina, so
may each of these little parts of matter be constructed of smaller parts;
only as clay is not brick, and neither silica nor alumina is clay, so the
properties of the parts of the atom--if it has parts--are not the
properties of the atom, and a mass of matter constructed of these parts
would not have the same properties as a mass of matter constructed of the
atoms themselves.
Another feature of Graham's work is found in the prominence which he gives
to that view of a chemical compound which regards it as the resultant of
the action and reaction of the parts of the compound. As the apparent
stability of chemical compounds was seen by Davy to be the result of an
equilibrium of contending forces, so did the seemingly changeless character
of any chemical substance appear to Graham as due to the orderly changes
which are continually proceeding among the molecules of which the substance
is constructed.
A piece of lime, or a drop of water, was to the mind of Graham the scene of
a continual strife, for that minute portion of matter appeared to him to be
constructed of almost innumerable myriads of little parts, each in more or
less rapid motion, one now striking against another and now moving free for
a little space. Interfere with those movements, alter the mutual action of
those minute particles, and the whole building would fall to pieces.
For more than thirty years Graham was content to trace the movements of
molecules. During that time he devoted himself, with an intense and
single-minded devotion, to the study of molecular science. Undaunted in
early youth by the withdrawal of his father's support; unseduced in his
middle age by the temptations of technical chemistry, by yielding to which
he would soon have secured a fortune; undazzled in his later days by the
honours of the position to which he had attained; Graham dedicated his life
to the nobler object of advancing the bounds of natural knowledge, and so
adding to those truths which must ever remain for the good and furtherance
of humanity.
FOOTNOTES:
[12] A metre is equal to about thirty-nine inches; a millimetre is the
one-thousandth part of a metre.