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" Inst.i.tution in Glasgow, and next year he was removed to the more important position of lecturer on the same science at the Andersonian Inst.i.tution 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 Inst.i.tution 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 Inst.i.tution, 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 a.r.s.eniates, 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 a.n.a.logy 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 a.n.a.logous 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 _mon.o.basic_, _dibasic_, and _tribasic_ phosphoric acids: by the action of a base on the _mon.o.basic 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 Const.i.tuent 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 const.i.tution_, 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 a.n.a.logy between water and bases, so now he saw in the hydrogen of acids the a.n.a.logue 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 cla.s.s 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 cla.s.s, are l.u.s.trous, 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 a.n.a.logies 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 s.p.a.ce; 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 Dobereiner filled a gla.s.s 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 gla.s.s.

Many jars, tubes and flasks, all with small cracks in the gla.s.s, 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 Dobereiner. Repeating Dobereiner"s experiments, Graham found that a portion of the hydrogen in the cracked vessels pa.s.sed outwards through the small fissures, and a little air pa.s.sed 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 pa.s.sage of gases through minute openings could be much more accurately studied by placing the gas to be examined in a gla.s.s tube one end of which was closed by a plug of dry plaster of Paris, than by using vessels with small fissures in the gla.s.s.

The _diffusion-tube_ used by Graham generally consisted of a piece of gla.s.s 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 pa.s.sed outwards through the porous plug, and air had pa.s.sed 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 pa.s.sed outwards for each one volume of air which had pa.s.sed 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 00694, air being 1; the square root of 00694 is 02635, therefore hydrogen will diffuse more rapidly than air in the ratio of 02635:1.

In the years 1846-1849 Graham resumed this inquiry; he now distinguished between _diffusion_, or the pa.s.sage of gases through porous plates, and _transpiration_, or the pa.s.sage 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 ma.s.s_ cannot penetrate the plate at all. It seems that molecules only can pa.s.s; and they may be supposed to pa.s.s 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 pa.s.sed through a porous plate into a vacuum, or when one gas pa.s.sed 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 pa.s.s 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 gla.s.s; the pipe stem was fixed by pa.s.sing through two corks, one at each end of the gla.s.s tube; through one of these corks there also pa.s.sed a short piece of gla.s.s tubing.

When the instrument was employed, the piece of short gla.s.s 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 pa.s.sed outwards through the pipe stem into the wide gla.s.s 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 quant.i.ty 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 pa.s.sed 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 ent.i.tled "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 pa.s.ses 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 quant.i.ty 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 const.i.tuents 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 a.n.a.lysis," 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 gla.s.s 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 pa.s.sed 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 pa.s.sed 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 cla.s.s 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 cla.s.ses 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 ma.s.s 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 ma.s.s.

"Can any facts," asks Graham, "more strikingly ill.u.s.trate the maxim that in Nature there are no abrupt transitions, and that distinctions of cla.s.s 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 ma.s.ses 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 ma.s.s 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 ma.s.s of matter constructed of these parts would not have the same properties as a ma.s.s 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 s.p.a.ce. 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 n.o.bler 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.

CHAPTER VI.

RISE AND PROGRESS OF ORGANIC CHEMISTRY--PERIOD OF LIEBIG AND DUMAS.

_Justus Liebig, 1803-1873. Jean Baptiste Andre Dumas, born in 1800._

I have as yet said almost nothing with regard to the progress of organic chemistry, considered as a special branch of the science. It is however in this department that the greatest triumphs which mark the third period of chemical advance have been won. We must therefore now turn our attention to the work which has been done here.