a history of science-4-第8章
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swan…song of the doctrine of phlogiston。 Despite the defiance of this single warrior the battle was really lost and won； and as the century closed 〃antiphlogistic〃 chemistry had practical possession of the field。
III。 CHEMISTRY SINCE THE TIME OF DALTON
JOHN DALTON AND THE ATOMIC THEORY
Small beginnings as have great endingssometimes。 As a case in point； note what came of the small； original effort of a self…trained back…country Quaker youth named John Dalton； who along towards the close of the eighteenth century became interested in the weather； and was led to construct and use a crude water…gauge to test the amount of the rainfall。 The simple experiments thus inaugurated led to no fewer than two hundred thousand recorded observations regarding the weather； which formed the basis for some of the most epochal discoveries in meteorology； as we have seen。 But this was only a beginning。 The simple rain…gauge pointed the way to the most important generalization of the nineteenth century in a field of science with which； to the casual observer； it might seem to have no alliance whatever。 The wonderful theory of atoms； on which the whole gigantic structure of modern chemistry is founded； was the logical outgrowth； in the mind of John Dalton； of those early studies in meteorology。
The way it happened was this： From studying the rainfall； Dalton turned naturally to the complementary process of evaporation。 He was soon led to believe that vapor exists； in the atmosphere as an independent gas。 But since two bodies cannot occupy the same space at the same time； this implies that the various atmospheric gases are really composed of discrete particles。 These ultimate particles are so small that we cannot see themcannot； indeed； more than vaguely imagine themyet each particle of vapor； for example； is just as much a portion of water as if it were a drop out of the ocean； or； for that matter； the ocean itself。 But； again； water is a compound substance； for it may be separated； as Cavendish has shown； into the two elementary substances hydrogen and oxygen。 Hence the atom of water must be composed of two lesser atoms joined together。 Imagine an atom of hydrogen and one of oxygen。 Unite them； and we have an atom of water； sever them； and the water no longer exists； but whether united or separate the atoms of hydrogen and of oxygen remain hydrogen and oxygen and nothing else。 Differently mixed together or united； atoms produce different gross substances； but the elementary atoms never change their chemical naturetheir distinct personality。
It was about the year 1803 that Dalton first gained a full grasp of the conception of the chemical atom。 At once he saw that the hypothesis； if true； furnished a marvellous key to secrets of matter hitherto insolublequestions relating to the relative proportions of the atoms themselves。 It is known； for example； that a certain bulk of hydrogen gas unites with a certain bulk of oxygen gas to form water。 If it be true that this combination consists essentially of the union of atoms one with another （each single atom of hydrogen united to a single atom of oxygen）； then the relative weights of the original masses of hydrogen and of oxygen must be also the relative weights of each of their respective atoms。 If one pound of hydrogen unites with five and one…half pounds of oxygen （as； according to Dalton's experiments； it did）； then the weight of the oxygen atom must be five and one…half times that of the hydrogen atom。 Other compounds may plainly be tested in the same way。 Dalton made numerous tests before he published his theory。 He found that hydrogen enters into compounds in smaller proportions than any other element known to him； and so； for convenience； determined to take the weight of the hydrogen atom as unity。 The atomic weight of oxygen then becomes （as given in Dalton's first table of 1803） 5。5； that of water （hydrogen plus oxygen） being of course 6。5。 The atomic weights of about a score of substances are given in Dalton's first paper； which was read before the Literary and Philosophical Society of Manchester； October 21； 1803。 I wonder if Dalton himself； great and acute intellect though he had； suspected； when he read that paper； that he was inaugurating one of the most fertile movements ever entered on in the whole history of science？
Be that as it may； it is certain enough that Dalton's contemporaries were at first little impressed with the novel atomic theory。 Just at this time； as it chanced； a dispute was waging in the field of chemistry regarding a matter of empirical fact which must necessarily be settled before such a theory as that of Dalton could even hope for a bearing。 This was the question whether or not chemical elements unite with one another always in definite proportions。 Berthollet； the great co…worker with Lavoisier； and now the most authoritative of living chemists； contended that substances combine in almost indefinitely graded proportions between fixed extremes。 He held that solution is really a form of chemical combinationa position which； if accepted； left no room for argument。
But this contention of the master was most actively disputed； in particular by Louis Joseph Proust； and all chemists of repute were obliged to take sides with one or the other。 For a time the authority of Berthollet held out against the facts； but at last accumulated evidence told for Proust and his followers； and towards the close of the first decade of our century it came to be generally conceded that chemical elements combine with one another in fixed and definite proportions。
More than that。 As the analysts were led to weigh carefully the quantities of combining elements； it was observed that the proportions are not only definite； but that they bear a very curious relation to one another。 If element A combines with two different proportions of element B to form two compounds； it appears that the weight of the larger quantity of B is an exact multiple of that of the smaller quantity。 This curious relation was noticed by Dr。 Wollaston； one of the most accurate of observers； and a little later it was confirmed by Johan Jakob Berzelius； the great Swedish chemist； who was to be a dominating influence in the chemical world for a generation to come。 But this combination of elements in numerical proportions was exactly what Dalton had noticed as early as 1802； and what bad led him directly to the atomic weights。 So the confirmation of this essential point by chemists of such authority gave the strongest confirmation to the atomic theory。
During these same years the rising authority of the French chemical world； Joseph Louis Gay…Lussac； was conducting experiments with gases； which he had undertaken at first in conjunction with Humboldt； but which later on were conducted independently。 In 1809； the next year after the publication of the first volume of Dalton's New System of Chemical Philosophy； Gay…Lussac published the results of his observations； and among other things brought out the remarkable fact that gases； under the same conditions as to temperature and pressure； combine always in definite numerical proportions as to volume。 Exactly two volumes of hydrogen； for example； combine with one volume of oxygen to form water。 Moreover； the resulting compound gas always bears a simple relation to the combining volumes。 In the case just cited； the union of two volumes of hydrogen and one of oxygen results in precisely two volumes of water vapor。
Naturally enough； the champions of the atomic theory seized upon these observations of Gay…Lussac as lending strong support to their hypothesisall of them； that is； but the curiously self…reliant and self…sufficient author of the atomic theory himself； who declined to accept the observations of the French chemist as valid。 Yet the observations of Gay…Lussac were correct； as countless chemists since then have demonstrated anew； and his theory of combination by volumes became one of the foundation…stones of the atomic theory； despite the opposition of the author of that theory。
The true explanation of Gay…Lussac's law of combination by volumes was thought out almost immediately by an Italian savant； Amadeo； Avogadro； and expressed in
