a history of science-4-第13章
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it； but the fact was made clear that something besides the mere number and kind of atoms is important in the architecture of a molecule。 It became certain that atoms are not thrown together haphazard to build a molecule； any more than bricks are thrown together at random to form a house。
How delicate may be the gradations of architectural design in building a molecule was well illustrated about 1850； when Pasteur discovered that some carbon compoundsas certain sugarscan only be distinguished from one another； when in solution； by the fact of their twisting or polarizing a ray of light to the left or to the right； respectively。 But no inkling of an explanation of these strange variations of molecular structure came until the discovery of the law of valency。 Then much of the mystery was cleared away； for it was plain that since each atom in a molecule can hold to itself only a fixed number of other atoms； complex molecules must have their atoms linked in definite chains or groups。 And it is equally plain that where the atoms are numerous； the exact plan of grouping may sometimes be susceptible of change without doing violence to the law of valency。 It is in such cases that isomerism is observed to occur。
By paying constant heed to this matter of the affinities； chemists are able to make diagrammatic pictures of the plan of architecture of any molecule whose composition is known。 In the simple molecule of water （H2O）； for example； the two hydrogen atoms must have released each other before they could join the oxygen； and the manner of linking must apparently be that represented in the graphic formula HOH。 With molecules composed of a large number of atoms； such graphic representation of the scheme of linking is of course increasingly difficult； yet； with the affinities for a guide； it is always possible。 Of course no one supposes that such a formula； written in a single plane； can possibly represent the true architecture of the molecule： it is at best suggestive or diagrammatic rather than pictorial。 Nevertheless； it affords hints as to the structure of the molecule such as the fathers of chemistry would not have thought it possible ever to attain。
PERIODICITY OF ATOMIC WEIGHTS
These utterly novel studies of molecular architecture may seem at first sight to take from the atom much of its former prestige as the all…important personage of the chemical world。 Since so much depends upon the mere position of the atoms； it may appear that comparatively little depends upon the nature of the atoms themselves。 But such a view is incorrect； for on closer consideration it will appear that at no time has the atom been seen to renounce its peculiar personality。 Within certain limits the character of a molecule may be altered by changing the positions of its atoms （just as different buildings may be constructed of the same bricks）； but these limits are sharply defined； and it would be as impossible to exceed them as it would be to build a stone building with bricks。 From first to last the brick remains a brick； whatever the style of architecture it helps to construct； it never becomes a stone。 And just as closely does each atom retain its own peculiar properties； regardless of its surroundings。
Thus； for example； the carbon atom may take part in the formation at one time of a diamond； again of a piece of coal； and yet again of a particle of sugar； of wood fibre； of animal tissue； or of a gas in the atmosphere； but from first to lastfrom glass…cutting gem to intangible gasthere is no demonstrable change whatever in any single property of the atom itself。 So far as we know； its size； its weight； its capacity for vibration or rotation； and its inherent affinities； remain absolutely unchanged throughout all these varying fortunes of position and association。 And the same thing is true of every atom of all of the seventy…odd elementary substances with which the modern chemist is acquainted。 Every one appears always to maintain its unique integrity； gaining nothing and losing nothing。
All this being true； it would seem as if the position of the Daltonian atom as a primordial bit of matter； indestructible and non…transmutable； had been put to the test by the chemistry of our century； and not found wanting。 Since those early days of the century when the electric battery performed its miracles and seemingly reached its limitations in the hands of Davy； many new elementary substances have been discovered； but no single element has been displaced from its position as an undecomposable body。 Rather have the analyses of the chemist seemed to make it more and more certain that all elementary atoms are in truth what John Herschel called them； 〃manufactured articles〃primordial； changeless； indestructible。
And yet； oddly enough； it has chanced that hand in hand with the experiments leading to such a goal have gone other experiments arid speculations of exactly the opposite tenor。 In each generation there have been chemists among the leaders of their science who have refused to admit that the so…called elements are really elements at all in any final sense； and who have sought eagerly for proof which might warrant their scepticism。 The first bit of evidence tending to support this view was furnished by an English physician； Dr。 William Prout； who in 1815 called attention to a curious relation to be observed between the atomic weight of the various elements。 Accepting the figures given by the authorities of the time （notably Thomson and Berzelius）； it appeared that a strikingly large proportion of the atomic weights were exact multiples of the weight of hydrogen； and that others differed so slightly that errors of observation might explain the discrepancy。 Prout felt that it could not be accidental； and he could think of no tenable explanation； unless it be that the atoms of the various alleged elements are made up of different fixed numbers of hydrogen atoms。 Could it be that the one true elementthe one primal matteris hydrogen； and that all other forms of matter are but compounds of this original substance？
Prout advanced this startling idea at first tentatively； in an anonymous publication； but afterwards he espoused it openly and urged its tenability。 Coming just after Davy's dissociation of some supposed elements； the idea proved alluring； and for a time gained such popularity that chemists were disposed to round out the observed atomic weights of all elements into whole numbers。 But presently renewed determinations of the atomic weights seemed to discountenance this practice； and Prout's alleged law fell into disrepute。 It was revived； however； about 1840； by Dumas； whose great authority secured it a respectful hearing； and whose careful redetermination of the weight of carbon； making it exactly twelve times that of hydrogen； aided the cause。
Subsequently Stas； the pupil of Dumas； undertook a long series of determinations of atomic weights； with the expectation of confirming the Proutian hypothesis。 But his results seemed to disprove the hypothesis； for the atomic weights of many elements differed from whole numbers by more； it was thought； than the limits of error of the experiments。 It was noteworthy； however； that the confidence of Dumas was not shaken； though he was led to modify the hypothesis； and； in accordance with previous suggestions of Clark and of Marignac； to recognize as the primordial element； not hydrogen itself； but an atom half the weight； or even one…fourth the weight； of that of hydrogen； of which primordial atom the hydrogen atom itself is compounded。 But even in this modified form the hypothesis found great opposition from experimental observers。
In 1864； however； a novel relation between the weights of the elements and their other characteristics was called to the attention of chemists by Professor John A。 R。 Newlands； of London； who had noticed that if the elements are arranged serially in the numerical order of their atomic weights； there is a curious recurrence of similar properties at intervals of eight elements This so…called 〃law of octaves〃 attracted little immediate attention； but the facts it connotes soon came under the observation of other chemists； notably of Professors Gustav Hinrichs in Am
