_Reprinted from the "Proceedings of the American Academy
of Arts and Sciences," Vol. VIII, May 24, 1870._
It would be difficult to find in the history of science a character more
simple, more noble, or more symmetrical in all its parts than that of
Thomas Graham, and he will always be remembered as one of the most
eminent of those great students of nature who have rendered our Saxon
race illustrious. He was born of Scotch parents in Glasgow in the year
1805, and in that city, where he received his education, all his early
life was passed. In 1837 he went to London as Professor of Chemistry in
the newly established London University, now called University College,
and he occupied this chair until the year 1855, when he succeeded Sir
John Herschel as Master of the Royal Mint, a post which he held to the
close of his life. His death, on the 16th of September last (1869), at
the age of sixty, was caused by no active disease, but was simply the
wearing out of a constitution enfeebled in youth by privations
voluntarily and courageously encountered that he might devote his life
to scientific study. As with all earnest students, that life was
uneventful, if judged by ordinary standards; and the records of his
discoveries form the only materials for his biography.
Although one of the most successful investigators of physical science,
the late Master of the Mint had not that felicity of language or that
copiousness of illustration which added so much to the popular
reputation of his distinguished contemporary, Faraday; but his influence
on the progress of science was not less marked or less important. Both
of these eminent men were for a long period of years best known to the
English public as teachers of chemistry, but their investigations were
chiefly limited to physical problems; yet, although both cultivated the
border ground between chemistry and physics, they followed wholly
different lines of research. While Faraday was so successfully
developing the principles of electrical action, Graham with equal
success was investigating the laws of molecular motion. Each followed
with wonderful constancy, as well as skill, a single line of study from
first to last, and to this concentration of power their great
discoveries are largely due.
One of the earliest and most important of Graham's investigations, and
the one which gave the direction to his subsequent course of study, was
that on the diffusion of gases. It had already been recognized that
impenetrability in its ordinary sense is not, as was formerly supposed,
a universal quality of matter. Dalton had not only recognized that
aeriform bodies exhibit a positive tendency to mix, or to penetrate
through each other, even in opposition to the force of gravity, but had
made this quality of gases the subject of experimental investigation. He
inferred, as the result of his inquiry, "that different gases afford no
resistance to each other; but that one gas spreads or expands into the
space occupied by another gas, as it would rush into a vacuum; at least,
that the resistance which the particles of one gas offer to those of
another is of a very imperfect kind, to be compared to the resistance
which stones in the channel of a stream oppose to the flow of running
water." But, although this theory of Dalton was essentially correct and
involved the whole truth, yet it was supported by no sufficient
evidence, and he failed to perceive the simple law which underlies this
whole class of phenomena.
Graham, "on entering on this inquiry, found that gases diffuse into the
atmosphere with different degrees of ease and rapidity." This was first
observed by allowing each gas to diffuse from a bottle into the air
through a narrow tube in opposition to the solicitation of gravity.
Afterward an observation of Doebereiner on the escape of hydrogen gas by
a fissure or crack in a glass receiver caused him to vary the conditions
of his experiments, and led to the invention of the well-known
"diffusion tube." In this simple apparatus a thin septum of plaster of
Paris is used to separate the diffusing gases, which, while it arrests
in a great measure all direct currents between the two media, does not
interfere with the molecular motion. Much later, Graham found in
prepared graphite a material far better adapted to this purpose than the
plaster, and he used septa of this mineral to confirm his early results,
in answer to certain ill-considered criticisms in Bunsen's work on
gasometry. These septa he was in the habit of calling his "atomic
filters."
By means of the diffusion tube, Graham was able to measure accurately
the relative times of diffusion of different gases, and he found that
_equal volumes of any two gases interpenetrate each other in times which
are inversely proportional to the square roots of their respective
densities_; and this fundamental law was the greatest discovery of our
late foreign associate. It is now universally recognized as one of the
few great cardinal principles which form the basis of physical science.
It can be shown, on the principles of pneumatics, that gases should
rush into a vacuum with velocities corresponding to the numbers which
have been found to express their diffusion times; and, in a series of
experiments on what he calls the "_effusion_" of gases, Graham confirmed
by trial this deduction of theory. In these experiments a measured
volume of the gas was allowed to find its way into the vacuous jar
through a minute aperture in a thin metallic plate, and he carefully
distinguished between this class of phenomena and the flowing of gases
through capillary tubes into a vacuum, in which case, however short the
tube, the effects of friction materially modify the result. This last
class of phenomena Graham likewise investigated, and designated by the
term "transpiration."
While, however, it thus appears that the results of Graham's
investigation were in strict accordance with Dalton's theory, it must
also be evident that Graham was the first to observe the exact numerical
relation which obtains in this class of phenomena, and that
all-important circumstance entitles him to be regarded as the discoverer
of the law of diffusion. The law, however, at first enunciated, was
purely empirical, and Graham himself says that something more must be
assumed than that gases are vacua to each other, in order to explain all
the phenomena observed; and according to his original view this
representation of the process was only a convenient mode of expressing
the final result. Such has proved to be the case.
Like other great men, Graham built better than he knew. In the progress
of physical science during the last twenty-five years, two principles
have become more and more conspicuous, until at last they have
completely revolutionized the philosophy of chemistry. In the first
place, it has appeared that a host of chemical as well as of physical
facts are cooerdinated by the assumption that all substances in the state
of gas have the same molecular volume, or, in other words, contain the
same number of molecules in a given space; and in the second place, it
has become evident that the phenomena of heat are simply the
manifestations of molecular motion. According to this view, the
temperature of a body is the _vis viva_ of its molecules; and, since all
molecules at a given temperature have the same _vis viva_, it follows
that the molecules must move with velocities which are inversely
proportional to the square roots of the molecular weights. Moreover,
since the molecular volumes are equal, and the molecular weights
therefore proportional to the densities of the aeriform bodies in which
the molecules are the active units, it also follows that the velocities
of the molecules in any two gases are inversely proportional to the
square roots of their respective densities. Thus the simple numerical
relations first observed in the phenomena of diffusion are the direct
result of molecular motion; and it is now seen that Graham's empirical
law is included under the fundamental laws of motion. Thus Graham's
investigation has become the basis of the new science of molecular
mechanics, and his measurements of the rates of diffusion prove to be
the measures of molecular velocities.
From the study of diffusion Graham passed by a natural transition to the
investigation of a class of phenomena which, although closely allied to
the first as to the effects produced, differ wholly in their essential
nature. Here also he followed in the footsteps of Dalton. This
distinguished chemist had noticed that a bubble of air separated by a
film of water from an atmosphere of carbonic anhydride gradually
expanded until it burst. In like manner a moist bladder, half filled
with air and tied, if suspended in an atmosphere of the same material,
becomes in time greatly distended by the insinuation of this gas through
its substance. This effect can not be the result of simple diffusion,
for it is to be remembered that the thinnest film of water, or of any
liquid, is absolutely impermeable to a gas as such, and, moreover, only
the carbonic anhydride passes through the film, very little or none of
the air escaping outward. The result depends, first, upon the solution
of the carbonic anhydride by the water on one surface of the film;
secondly, on the evaporation into the air, from the other surface, of
the gas thus absorbed. Similar experiments were made by Drs. Mitchell
and Faust, and others, in which gases passed through a film of
India-rubber, entering into a partial combination with the material on
one surface, and escaping from it on the other.
Graham not only considerably extended our knowledge of this class of
phenomena, but also gave us a satisfactory explanation of the mode in
which these remarkable results are produced. He recognized in these
cases the action of a feeble chemical force, insufficient to produce a
definite compound, but still capable of determining a more or less
perfect union, as in the case of simple solution. He also distinguished
the influence of mass in causing the formation or decomposition of such
weak chemical compounds. The conditions of the phenomena under
consideration are simply these:
First. A material for the septum capable of forming a feeble chemical
union with the gas to be transferred.
Secondly. An excess of the gas on one side of the film and a deficiency
on the other.
Thirdly. Such a temperature that the unstable compound may form at the
surface, where the aeriform constituent is present in large mass, while
it decomposes at the opposite surface, where the quantity is less
abundant.
One of the most remarkable results of Graham's study of this peculiar
mode of transfer of aeriform matter through the very substance of solid
bodies was an ingenious method of separating the oxygen from the
atmosphere. The apparatus consisted simply of a bag of India-rubber kept
distended by an interior framework, while it was exhausted by a Sprengel
pump. Under these circumstances the selective affinity of the caoutchouc
determines such a difference in the rate of transfer of the two
constituents of the atmosphere that the amount of oxygen in the
transpired air rises to forty per cent., and by repeating the process
nearly pure oxygen may be obtained. It was at first hoped that this
method might find a valuable application in the arts, but in this Graham
was disappointed; for the same result has since been effected by purely
chemical methods, which are both cheaper and more rapid.
These experiments on India-rubber naturally led to the study of similar
effects produced with metallic septa, which, although to some extent
previously observed in passing gases through heated metallic tubes, had
been only imperfectly understood. Thus, when a stream of hydrogen or
carbonic oxide is passed through a red-hot iron tube, a no
inconsiderable portion of the gas escapes through the walls. The same is
true to a still greater degree when hydrogen is passed through a red-hot
tube of platinum, and Graham showed that, through the walls of a tube
of palladium, hydrogen gas passes, under the same conditions, almost as
rapidly as water through a sieve. Moreover, our distinguished associate
proved that this rapid transfer of gas through these dense metallic
septa was due, as in the case of the India-rubber, to an actual chemical
combination of its material with the metal, formed at the surface, where
the gas is in excess, and as rapidly decomposed on the opposite face of
the septum. He not only recognized as belonging to this class of
phenomena the very great absorption of hydrogen by platinum plate and
sponge in the familiar experiment of the Doebereiner lamp, but also
showed that this gas is a definite constituent of meteoric iron--a fact
of great interest from its bearing on the meteoric theory.
We are thus led to Graham's last important discovery, which was the
justification of the theory we have been considering, and the crowning
of this long line of investigation. As may be anticipated from what has
been said, the most marked example of that order of chemical compounds,
to which the metallic transpiration of aeriform matter we have been
considering is due, is the compound of palladium with hydrogen. Graham
showed that, when a plate of this metal is made the negative pole in the
electrolysis of water, it absorbs nearly one thousand times its volume
of hydrogen gas--a quantity approximatively equivalent to one atom of
hydrogen to each atom of palladium. He further showed that the metal
thus becomes so profoundly altered as to indicate that the product of
this union is a definite compound. Not only is the volume of the metal
increased, but its tenacity and conducting power for electricity are
diminished, and it acquires a slight susceptibility to magnetism, which
the pure metal does not possess. The chemical qualities of this product
are also remarkable. It precipitates mercury from a solution of its
chloride, and in general acts as a strong reducing agent. Exposed to the
action of chlorine, bromine, or iodine, the hydrogen leaves the
palladium and enters into direct union with these elements. Moreover,
although the compound is readily decomposed by heat, the gas can not be
expelled from the metal by simple mechanical means.
These facts recall the similar relations frequently observed between the
qualities of an alloy and those of the constituent metals, and suggest
the inference made by Graham, that palladium charged with hydrogen is a
compound of the same class--a conclusion which harmonizes with the
theory long held by many chemists, that hydrogen gas is the vapor of a
very volatile metal. This element, however, when combined with
palladium, is in a peculiarly active state, which sustains somewhat the
same relation to the familiar gas that ozone bears to ordinary oxygen.
Hence Graham distinguished this condition of hydrogen by the term
"hydrogenium." Shortly before his death a medal was struck at the Royal
Mint from the hydrogen palladium alloy in honor of its discovery; but,
although this discovery attracted public attention chiefly on account of
the singular chemical relations of hydrogen, which it brought so
prominently to notice, it will be remembered in the history of science
rather as the beautiful termination of a life-long investigation, of
which the medal was the appropriate seal.
Simultaneously with the experiments on _gases_, whose results we have
endeavored to present in the preceding pages, Graham carried forward a
parallel line of investigation of an allied class of phenomena, which
may be regarded as the manifestations of molecular motion in _liquid_
bodies. The phenomena of diffusion reappear in liquids, and Graham
carefully observed the times in which equal weights of various salts
dissolved in water diffused from an open-mouth bottle into a large
volume of pure water, in which the bottle was immersed. He was not,
however, able to correlate the results of these experiments by such a
simple law as that which obtains with gases. It appeared, nevertheless,
that the rate of diffusion differs very greatly for the different
soluble salts, having some relation to the chemical composition of the
salt which he was unable to discover. But he found it possible to divide
the salts into groups of equi-diffusive substances, and he showed that
the rate of diffusion of the several groups bear to one another simple
numerical ratios.
More important results were obtained from the study of a class of
phenomena corresponding to the transpiration of gases through
India-rubber or metallic septa. These phenomena, as manifested in the
transfer of liquids and of salts in solution through bladder or a
similar membrane, had previously been frequently studied under the names
of exosmose and endosmose, but to Graham we owe the first satisfactory
explanation. As in the case of gases, he referred these effects to the
influence of chemical force, combination taking place on one surface of
the membrane and the compound breaking up on the other, the difference
depending, as in the previous instance, on the influence of mass. He
also swept away the arbitrary distinctions made by previous
experimenters, showed that this whole class of phenomena are essentially
similar, and called this manifestation of power simply "osmose."
While studying osmotic action, Graham was led to one of his most
important generalizations--the recognition of the crystalline and
amorphous states as fundamental distinctions in chemistry. Bodies in the
first state he called crystalloids; those in the last state, colloids
(resembling glue). That there is a difference in structure between
crystalloids, like sugar or felspar, and colloids, like barley candy or
glass, has of course always been evident to the most superficial
observer; but Graham was the first to recognize in these external
differences two fundamentally distinct conditions of matter not peculiar
to certain substances, but underlying all chemical differences, and
appearing to a greater or less degree in every substance. He showed that
the power of diffusion through liquids depends very much on these
fundamental differences of condition--sugar, one of the least diffusible
of the crystalloids, diffusing fourteen times more rapidly than caromel,
the corresponding colloid. He also showed that, in accordance with the
general chemical rule, while colloids readily combine with crystalloids,
bodies in the same condition manifest little or no tendency to chemical
union. Hence, in osmose, where the membranes employed are invariably
colloidal, the osmotic action is confined almost entirely to
crystalloids, since they alone are capable of entering into that
combination with the material of the septum on which the whole action
depends.
On the above principles Graham based a simple method of separating
crystalloids from colloids, which he calls "dialysis," and which was a
most valuable addition to the means of chemical analysis. A shallow
tray, prepared by stretching parchment paper (an insoluble colloid) over
a gutta-percha hoop, is the only apparatus required. The solution to be
"dialyzed" is poured into this tray, which is then floated on pure
water, whose volume should be eight or ten times greater than that of
the solution. Under these conditions the crystalloids will diffuse
through the porus septum into the water, leaving the colloids on the
tray, and in the course of a few days a more or less complete separation
of the two classes of bodies will have taken place. In this way
arsenious acid and similar crystalloids may be separated from the
colloidal materials with which, in the case of poisoning, they are
usually found mixed in the animal juices or tissues.
But, besides having these practical applications, the method of dialysis
in the hands of Graham yielded the most startling results, developing an
almost entirely new class of bodies, as the colloidal forms of our most
familiar substances, and justifying the conclusion that the colloidal as
well as the crystalline condition is an almost universal attribute of
matter. Thus, he was able to obtain solutions in water of the colloidal
states of aluminic, ferric, chromic, stannic, metastannic, titanic,
molybdic, tungstic, and silicic hydrates, all of which gelatinize under
definite conditions like a solution of glue. The wonderful nature of
these facts can be thoroughly appreciated only by those familiar with
the subject, but all may understand the surprise with which the chemist
saw such hard, insoluble bodies as flint dissolved abundantly in water
and converted into soft jellies. These facts are, without doubt, the
most important contributions of Dr. Graham to pure chemistry.
In this sketch of the scientific career of our late associate, we have
followed the logical, rather than the chronological, order of events,
hoping thus to render the relations of the different parts of his work
more intelligible. It must be remembered, however, that the two lines of
investigation we have distinguished were in fact inter-woven, and that
the beautiful harmony which his completed life presents was the result,
not of a preconceived plan, but of a constant devotion to truth, and a
childlike faith, which unhesitatingly pressed forward whenever nature
pointed out the way.
Although the investigations of the phenomena connected with the
molecular motion in gases and liquids were by far the most important of
Dr. Graham's labors, he also contributed to chemistry many researches
which can not be included under this head. Of these, which we may regard
as his detached efforts, the most important was his investigation of the
hydrates and other salts of phosphorus. It is true that the
interpretation he gave of the results has been materially modified by
the modern chemical philosophy, yet the facts which he established form
an important part of the basis on which that philosophy rests. Indeed,
it seems as if he almost anticipated the later doctrines of types and
polybasic acids, and in none of his work did he show more discriminating
observation or acute reasoning. A subsequent investigation on the
condition of water in several crystalline salts and in the hydrates of
sulphuric acid is equally remarkable. Lastly, Graham also made
interesting observations on the combination of alcohol with salts, on
the process of etherification, on the slow oxidation of phosphorus, and
on the spontaneous inflammability of phosphureted hydrogen. It would
not, however, be appropriate in this place to do more than enumerate the
subjects of these less important studies; and we have therefore only
aimed in this sketch to give a general view of the character of the
field which this eminent student of nature chiefly cultivated, and to
show how abundant was the harvest of truth which we owe to his faithful
toil.
Graham was not a voluminous writer. His scientific papers were all very
brief, but comprehensive, and his "Elements of Chemistry" was his only
large work. This was an admirable exposition of chemical physics, as
well as of pure chemistry, and gave a more philosophical account of the
theory of the galvanic battery than had previously appeared. Our late
associate was fortunate in receiving during life a generous recognition
of the value of his labors. His membership was sought by almost all the
chief scientific societies of the world, and he enjoyed to a high degree
the confidence and esteem of his associates. Indeed, he was singularly
elevated above the petty jealousies and belittling quarrels which so
often mar the beauty of a student's life, while the great loveliness and
kindliness of his nature closely endeared him to his friends.
In concluding, we must not forget to mention that most genial trait of
Graham's character, his sympathy with young men, which gave him great
influence as a teacher in the college with which he was long associated.
There are many now prominent in the scientific world who have found in
his encouragement the strongest incentive to perseverance, and in his
approval and friendship the best reward of success.