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From this time Dalton made observations on the peculiarities of his own vision and that of others, and in his first paper read before the Literary and Philosophical Society in 1794, he described these peculiarities. He says, "Since the year 1790 the occasional study of botany obliged me to attend more to colour than before. With respect to colours that were white, yellow, or green, I readily a.s.sented to the appropriate term; blue, purple, pink and crimson appeared rather less distinguishable, being, according to my idea, all referable to blue. I have often seriously asked a person whether a flower was blue or pink, but was generally considered to be in jest." Dalton's colour-blindness was amusingly ill.u.s.trated at a later time, when having been created D.C.L. by the University of Oxford he continued to wear the red robes of his degree for some days; and when his attention was drawn to the somewhat strange phenomenon, even in a university town, of an elderly gentleman in the dress of a Quaker perambulating the town day after day in a scarlet robe, he remarked that to him the gown appeared to be of the same colour as the green trees.
Dalton's work during the next six or eight years dealt chiefly with problems suggested by his meteorological observations; he published a volume on "Meteorological Observations and Essays," chiefly occupied with descriptions of the instruments employed, more especially of the thermometer and barometer, and an instrument for determining the dew-point of air. By this time he had established the existence of a connection of some kind between magnetism and the aurora, and had thus laid the foundations of a most important branch of meteorology.
In 1799, in a note to a paper on rain and dew, he begins his work on aqueous vapour in the atmosphere by proving that water vapour exists as such in the air. This paper is quickly followed by another on the conducting power of water for heat.
A very important paper was published in 1801, on the "Const.i.tution of Mixed Gases, etc.," wherein Dalton a.s.serted that the total pressure of a mixture of two gases on the walls of the containing vessel is equal to the sum of the pressures of each gas; in other words, that if one gas is removed the pressure now exerted by the remaining gas is exactly the same as was exerted by that gas in the original mixture. In a paper published much later (1826), when his views and experiments on this subject were matured, he writes: "It appears to me as completely demonstrated as any physical principle, that whenever two or more ... gases or vapours ... are put together, either into a limited or unlimited s.p.a.ce, they will finally be arranged each as if it occupied the whole s.p.a.ce, and the others were not present; the nature of the fluids and gravitation being the only efficacious agents."
This conclusion was followed out and extended in a paper published in 1803, on the absorption of gases by water and other liquids, wherein he states that the amount of each gas _mechanically dissolved_ by a liquid from a mixture of gases depends only on the quant.i.ty of _that_ gas in the mixture, the other gases exerting no influence in this respect.
Dalton now considered the variation in the pressures of various gases caused by increasing or decreasing temperature, and then proceeded to discuss the relations which exist between the volumes of gases and the temperature at which these volumes are measured. He concluded that "all elastic fluids" under the same pressure expand equally by heat: and he adds the very important remark, "It seems, therefore, that general laws respecting the absolute quant.i.ty and the nature of heat are more likely to be derived from the study of elastic fluids than of other substances"--a remark the profound truth of which has been emphasized by each step in the advances made in our conception of the nature of heat since the time of Dalton.
In these papers on the "Const.i.tution of Mixed Gases" Dalton also describes and ill.u.s.trates a method whereby the actual amount of water vapour in a given bulk of atmospheric air may be found from a knowledge of the dew-point of that air, that is, the temperature at which the deposition of water in the liquid form begins. The introduction of this method for finding the humidity of air marks an important advance in the history of meteorology.
In this series of papers published within the first three years of the present century Dalton evidently had before his mind's eye a picture of a gas as a quant.i.ty of matter built up of small but independent particles; he constantly speaks of pressures between the small particles of elastic fluids, of these particles as repelling each other, etc. In his "New System" he says, "A vessel full of any pure elastic fluid presents to the imagination a picture like one full of small shot."
It is very important to notice that Dalton makes use of this conception of small particles to explain purely physical experiments and operations.
Although we know that during these years he was thinking much of "chemical combinations," yet we find that it was his observations on the weather which led him to the conception--a purely physical conception--of each chemically distinct gas as being built up of a vast number of small, equally heavy particles. A consideration of these papers by Dalton on the const.i.tution of mixed gases shows us the method which he pursued in his investigations. "The progress of philosophical knowledge," he says, "is advanced by the discovery of new and important facts; but much more when these facts lead to the establishment of general laws." Dalton always strove to attain to general laws. The facts which he describes are frequently inaccurate; he was singularly deficient in manipulation, and he cannot claim a high place as a careful experimenter. He was however able to draw general conclusions of wide applicability. He seems sometimes to have stated a generalization in definite form before he had obtained any experimental verification of it.
In the year 1802 Dalton conducted an examination of air from various localities, and concluded that one hundred volumes of air are composed of twenty-one volumes of oxygen and seventy-nine volumes of nitrogen. This appears to have been his first piece of purely chemical work. But in the next year he again returns to physical phenomena. In the paper already referred to, on the absorption of gases by water and other liquids, published in this year, he had stated that "All gases that enter into water and other liquids by means of pressure, and are wholly disengaged again by the removal of that pressure, are _mechanically_ mixed with the liquid, and not _chemically_ combined with it." But if this be so, why, he asked, does not water mechanically dissolve the same bulk of every kind of gas? The answer which he gives to this question is found at the close of the paper; to the student of chemistry it is very important:--
"This question I have duly considered, and though I am not yet able to satisfy myself completely, I am nearly persuaded that the circ.u.mstance depends upon the weight and number of the ultimate particles of the several gases, those whose particles are lightest and single being least absorbable, and the others more, accordingly as they increase in weight and complexity. An inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new. I have lately been prosecuting this inquiry with remarkable success. The principle cannot be entered upon in this paper; but I shall just subjoin the results, as far as they appear to be ascertained by my experiments." Then follows a "_Table of the relative weights of the ultimate particles of gaseous and other bodies._" The following numbers, among others, are given:--
Hydrogen 1 Sulphur 144 Oxygen 55 Alcohol 151 Azote 42 Nitrous oxide 137 Phosphorus 72 Ether 96
Here is the beginning of the atomic theory; and yet Dalton's strictly chemical experimental work lies in the future. The scope of the theory is defined in that sentence--"_An inquiry into the relative weights of the ultimate particles of bodies._" His paper on mixed gases is ill.u.s.trated by a plate, which shows how vividly Dalton at this time pictured to himself a quant.i.ty of gas as composed of many little particles, and how clearly he recognized the necessity of regarding all the particles of each elementary gas as alike, but as differing from those of every other elementary gas.
In 1804 Dalton was invited to deliver a course of lectures in the Royal Inst.i.tution of London, on heat, mixed gases and similar subjects. In these lectures he expounded his views on the const.i.tution of gases, on absorption of gases by liquids, etc. These views drew much attention in this and other countries. "They are busy with them," he writes in 1804, "at London, Edinburgh, Paris and in various parts of Germany, some maintaining one side and some another. The truth will surely out at last."
[Ill.u.s.tration: Fig. 2]
Dalton's love of numerical calculations is noticeable in a trivial circ.u.mstance which he mentions in a letter from London to his brother. He tried to count the number of coaches which he met in going to the Friends'
morning meeting: this he a.s.sures his brother he "effected with tolerable precision. The number was one hundred and four."
During vacation time Dalton usually made a walking excursion in the Lake district. He was extremely fond of mountain scenery, but generally combined the pursuit of science with that of pleasure; he carried his meteorological instruments with him, determined the dew-point at various alt.i.tudes, and measured mountain heights by the aid of his barometer.
Sometimes however he refused to have anything to do with science. A companion in one of these excursions says that he was "like a schoolboy enjoying a holiday, mocking the cuckoos, putting up and chasing the hares, stopping from time to time to point out some beautiful view, or loitering to chat with pa.s.sing pedestrians."
This side of Dalton's nature was not often apparent. In him the quiet, hard-working student generally appeared prominently marked; but on the half-holiday which he allowed himself on each Thursday afternoon, in order to enjoy the society of a few friends and to engage in his favourite amus.e.m.e.nt of a game at bowls, he laid aside something of the quietness, regularity and decorum which usually characterized him. "When it came to his turn to bowl he threw his whole soul into the game,... and it was not a little amusing to spectators to see him running after the ball across the green, stooping down as if talking to it, and waving his hands from one side to the other exactly as he wished the line of the ball to be, and manifesting the most intense interest in its coming near to the point at which he aimed."
From the year 1803-4 Dalton becomes more and more a worker in chemistry.
The establishment of the atomic theory now engaged most of his time and attention. The results of his investigation of "the primary laws which seem to obtain in regard to heat and to chemical combinations" appeared in his "New System of Chemical Philosophy," Part I. of which, "On Heat, on the Const.i.tution of Bodies and on Chemical Synthesis," was published in 1808.
We have now arrived at the time when Dalton's inquiry into the "relative weights of the ultimate particles of bodies" was in his opinion sufficiently advanced for presentation to the scientific world; but I think we shall do better to postpone our consideration of this great inquiry until we have completed our review of the chief events in the life of Dalton, other than this the greatest event of all.
Dalton did not look for rewards--he desired only the just fame of one who sought for natural truths; but after the publication of the "New System"
rewards began to come to him. In 1817 he was elected a corresponding member of the French Academy of Sciences.
In 1822, when his fame as a philosophical chemist was fully established, Dalton visited Paris. This visit gave him great pleasure. He was constantly in the society of the great men who then so n.o.bly represented the dignity of natural science in France; Laplace, Cuvier, Biot, Arago, Gay-Lussac, Milne-Edwards and others were his friends. For some time after this visit he was more vivacious and communicative than usual, and we are told by one who lived in the same house as he, "We frequently bantered him with having become half a Frenchman." Dalton especially valued the friendship of Clementine Cuvier, daughter of the great naturalist, with whom he became acquainted during his visit to Paris. All through life he greatly delighted in the society of cultivated women, and his warmest friendships were with gentlewomen. At one time, shortly after going to Manchester, he was much taken by a widow lady who combined great personal charms with considerable mental culture. "During my _captivity_," he writes to a friend, "which lasted about a week, I lost my appet.i.te, and had other symptoms of _bondage_ about me, as incoherent discourse, etc., but have now happily regained my freedom." The society of men who like himself were actively engaged in the investigation of natural science was also a source of much pleasure to Dalton. Such men used to visit him in Manchester, so that in the house of the Rev. Mr. Johns, in whose family he lived, "there were found from time to time some of the greatest philosophers in Europe."
Dalton was elected a Fellow of the Royal Society in 1822, and four years later he became the first recipient of one of the Royal Medals, then founded by the King (George IV.). In 1830 he was elected one of the eight foreign a.s.sociates of the French Academy, an honour which is generally regarded as the highest that can be bestowed on any man of science.
Dalton was one of the original members of the British a.s.sociation for the Advancement of Science, and he attended most of the meetings from the first held in York in 1831 to that held in Manchester two years before his death.
At the Oxford meeting of 1832 he was created D.C.L. by the University, and two years later the University of Edinburgh honoured herself by enrolling his name on the list of her doctors of law.
About this time some of Dalton's scientific friends, who considered his work of great national importance, endeavoured to obtain a pension for him from the civil list. At the meeting of the British a.s.sociation held at Cambridge in 1833, the president, Professor Sedgwick, was able to announce that "His Majesty, willing to manifest his attachment to science, and his regard for a character like that of Dr. Dalton, had graciously conferred on him, out of the funds of the civil list, a substantial mark of his royal favour." The "substantial mark of royal favour," the announcement of which Dalton received "with his customary quietness and simplicity of manner,"
consisted of a pension of 150 _per annum_, which was increased three years later to 300.
The second part of Volume I. of his "New System" was published by Dalton in 1810, and the second volume of the same work in 1827. In 1844 a paper by him was read before the British a.s.sociation, in which he announced some important discoveries with regard to the water in crystallizable salts, and thus brought a new cla.s.s of facts within the range of the atomic theory.
He was seized with paralysis in 1837, but recovered to a great extent; a second attack in 1844 however completely prostrated him. On the 16th of July in that year he made the last entry in his book of "Observations on the Weather"--"_Little rain_;" next morning he became insensible and quietly pa.s.sed away.
It is as the founder of the chemical atomic theory that Dalton must ever be remembered by all students of physical and chemical science.
To the Greek philosophers Leucippus and Democritus (flourished about 440-400 B.C.) we owe the conception that "The bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies, which we cannot see or handle, which are always in motion, and which can neither be stopped, nor broken in pieces, nor in any way destroyed or deprived of the least of their properties" (Clerk Maxwell). The heavier among these small indivisible bodies or atoms were regarded as always moving downwards. By collisions between these and the lighter ascending atoms lateral movements arose. By virtue of the natural law (as they said) that things of like weight and shape must come to the same place, the atoms of the various elements came together; thus larger ma.s.ses of matter were formed; these again coalesced, and so finally worlds came into existence.
This doctrine was extended by Epicurus (340-270 B.C.), whose teaching is preserved for us in the poem of Lucretius (95-52 B.C.), "De Rerum Natura;"
he ascribed to the atoms the power of deviating from a straight line in their descending motion. On this hypothesis Epicurus built a general theory to explain all material and spiritual phenomena.
The ceaseless change and decay in everything around them was doubtless one of the causes which led men to this conception of atoms as indivisible, indestructible substances which could never wear out and could never be changed. But even here rest could not be found; the mind was obliged to regard these atoms as always in motion. The dance of the dust-motes in the sunbeam was to Lucretius the result of the more complex motion whereby the atoms which compose that dust are agitated. In his dream as told by Tennyson--
"A void was made in Nature: all her bonds Cracked: and I saw the flaring atom-streams And torrents of her myriad universe, Ruining along the illimitable inane, Fly on to clash together again, and make Another and another frame of things For ever."
The central quest of the physicist, from the days of Democritus to the present time, has been to explain the conception of "atom"--to develop more clearly the observed properties of the things which are seen and which may be handled as dependent on the properties of those things which cannot be seen, but which yet exist. For two thousand years he has been trying to penetrate beneath the ever-changing appearances of Nature, and to find some surer resting-place whence he may survey these shifting pictures as they pa.s.s before his mental vision. The older atomists thought to find this resting-place, not in the atoms themselves, but in the wide s.p.a.ces which they supposed to exist between the worlds:--
"The lucid inters.p.a.ce of world and world Where never creeps a cloud, or moves a wind, Nor ever falls the least white star of snow, Nor ever lowest roll of thunder moans, Nor sound of human sorrow mounts to mar Their sacred everlasting calm."
To the modern student of science the idea of absolute rest appears unthinkable; but in the most recent outcome of the atomic theory--in the vortex atoms of Helmholtz and Thomson--he thinks he perceives the very "foundation stones of the material universe."
Newton conceived the atom as a "solid, ma.s.sy, hard, impenetrable, movable particle." To the mind of D. Bernoulli the pressure exerted by a gas on the walls of a vessel enclosing it was due to the constant bombardment of the walls by the atoms of which the gas consisted.
Atomic motion was the leading idea in the explanation of heat given by Rumford and Davy, and now universally accepted; and, as we have seen, Dalton was himself accustomed to regard all "elastic fluids" (_i.e._ gases) as consisting of vast numbers of atoms.
But in the year 1802 or so, Dalton thought that by the study of chemical combinations it would be possible to determine the relative weights of atoms. a.s.sume that any elementary gas is composed of small, indivisible, equally heavy parts; a.s.sume that the weight of an atom of one element is different from that of the atom of any other element; and, lastly, a.s.sume that when elements combine the atom of the compound so produced is built up of the atoms of the various elements. Make these a.s.sumptions, and it follows that the relative weights of two or more elements which combine together must represent the relative weights of the atoms of these elements.
We know that the fixity of composition of chemical compounds had been established before this time, largely by the labours of Black and Lavoisier. Fixity of composition had however been called in question by Berthollet, who held that elements combine together in very varying quant.i.ties; that, in fact, in place of there being two or three, or a few definite compounds of, say, iron and oxygen, there exists a graduated series of such bodies; and that the amount of iron which combines with oxygen depends chiefly on such physical conditions as the temperature, the pressure, etc., under which the chemical action occurs. But by the date of the publication of the first part of Dalton's "New System," the long dispute between Berthollet and Proust regarding fixity of composition of compounds had nearly closed in favour of the latter chemist, who strongly upheld the affirmative side of the argument. But if Dalton's a.s.sumptions are correct, it is evident that when two elements form more than one compound, the quant.i.ty of element A in one of these must be a simple multiple of the quant.i.ty in the other of these compounds; because there must be a greater number of atoms of element A in the atom of one compound than in that of the other compound, and an elementary atom is a.s.sumed to be indivisible. Hence it follows that if one element be taken as a standard, it must be possible to affix to any other element a certain number which shall express the smallest quant.i.ty of that element which combines with one part by weight of the standard element; and this number shall also represent how many times the atom of the given element is heavier than the atom of the standard element, the weight of which has been taken to be _one_. If this element forms two compounds with the standard element, the amount of this element in the second compound must be expressed by a simple multiple of the number a.s.signed to this element, because it is not possible, according to the fundamental a.s.sumptions of the theory, to form a compound by the combination of fractions of elementary atoms.
By pondering on the facts regarding chemical combinations which had been established by various workers previous to the year 1802, Dalton had apparently come to such conclusions as those now indicated.
In his paper on the properties of the gases const.i.tuting the atmosphere, read to the Manchester Society on November 12, 1802, he stated that one hundred measures of common air would combine with thirty-six measures of "nitrous gas" in a narrow tube to produce an oxide of nitrogen, but with seventy-two measures of the same gas in a wide vessel to produce another oxide of nitrogen. These facts, he says, "clearly point out the theory of the process: the elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quant.i.ty."
In the concluding paragraph of his paper on absorption of gases by liquids, read on October 21, 1803, we found (see p. 116) that he had got so far in his inquiry into the "relative weights of the ultimate particles of bodies"
as to give a table of twenty-one such weights. About this time Dalton made a.n.a.lyses of two gaseous compounds of carbon--olefiant gas and carburetted hydrogen or marsh-gas. He found that both are compounds of carbon and hydrogen; that in one 4.3 parts by weight of carbon are combined with one part by weight of hydrogen, and in the other the same amount (4.3) of carbon is combined with two parts by weight of hydrogen.
This was a striking confirmation of his views regarding combination in multiple proportions, which views followed as a necessary deduction from the atomic hypothesis. From this time he continued to develop and extend this hypothesis, and in the year 1808 he published his "New System of Chemical Philosophy."
The first detailed account of the atomic theory was however given to the chemical world the year before Dalton's book appeared. During a conversation with Dalton in the autumn of 1804 Dr. Thomas Thomson learned the fundamental points of the new theory, and in the third edition of his "System of Chemistry," published in 1807, he gave an account of Dalton's views regarding the composition of bodies.