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atoms。 But we have no proof that under any circumstances it could hold more than two。 Its affinities seem satisfied when it has two bonds。 But； on the other hand； the atom of nitrogen is able to hold three atoms of hydrogen； and does so in the molecule of ammonium （NH3）； while the carbon atom can hold four atoms of hydrogen or two atoms of oxygen。
Evidently； then； one atom is not always equivalent to another atom of a different kind in combining powers。 A recognition of this fact by Frankland about 1852； and its further investigation by others （notably A。 Kekule and A。 S。 Couper）； led to the introduction of the word equivalent into chemical terminology in a new sense； and in particular to an understanding of the affinities or 〃valency〃 of different elements； which proved of the most fundamental importance。 Thus it was shown that； of the four elements that enter most prominently into organic compounds； hydrogen can link itself with only a single bond to any other elementit has； so to speak； but a single hand with which to graspwhile oxygen has capacity for two bonds； nitrogen for three （possibly for five）； and carbon for four。 The words monovalent； divalent； trivalent； tretrava…lent； etc。； were coined to express this most important fact； and the various elements came to be known as monads； diads； triads； etc。 Just why different elements should differ thus in valency no one as yet knows； it is an empirical fact that they do。 And once the nature of any element has been determined as regards its valency； a most important insight into the possible behavior of that element has been secured。 Thus a consideration of the fact that hydrogen is monovalent； while oxygen is divalent； makes it plain that we must expect to find no more than three compounds of these two elementsnamely； HO（written HO by the chemist； and called hydroxyl）； HOH （H2O； or water）； and HOOH （H2O2； or hydrogen peroxide）。 It will be observed that in the first of these compounds the atom of oxygen stands； so to speak； with one of its hands free； eagerly reaching out； therefore； for another companion； and hence； in the language of chemistry； forming an unstable compound。 Again； in the third compound； though all hands are clasped； yet one pair links oxygen with oxygen； and this also must be an unstable union； since the avidity of an atom for its own kind is relatively weak。 Thus the well…known properties of hydrogen peroxide are explained； its easy decomposition； and the eagerness with which it seizes upon the elements of other compounds。
But the molecule of water； on the other hand； has its atoms arranged in a state of stable equilibrium； all their affinities being satisfied。 Each hydrogen atom has satisfied its own affinity by clutching the oxygen atom； and the oxygen atom has both its bonds satisfied by clutching back at the two hydrogen atoms。 Therefore the trio； linked in this close bond； have no tendency to reach out for any other companion； nor； indeed； any power to hold another should it thrust itself upon them。 They form a 〃stable〃 compound； which under all ordinary circumstances will retain its identity as a molecule of water； even though the physical mass of which it is a part changes its condition from a solid to a gas from ice to vapor。
But a consideration of this condition of stable equilibrium in the molecule at once suggests a new question： How can an aggregation of atoms； having all their affinities satisfied； take any further part in chemical reactions？ Seemingly such a molecule； whatever its physical properties； must be chemically inert； incapable of any atomic readjustments。 And so in point of fact it is； so long as its component atoms cling to one another unremittingly。 But this； it appears； is precisely what the atoms are little prone to do。 It seems that they are fickle to the last degree in their individual attachments； and are as prone to break away from bondage as they are to enter into it。 Thus the oxygen atom which has just flung itself into the circuit of two hydrogen atoms； the next moment flings itself free again and seeks new companions。 It is for all the world like the incessant change of partners in a rollicking dance。 This incessant dissolution and reformation of molecules in a substance which as a whole remains apparently unchanged was first fully appreciated by Ste。…Claire Deville； and by him named dissociation。 It is a process which goes on much more actively in some compounds than in others； and very much more actively under some physical conditions （such as increase of temperature） than under others。 But apparently no substances at ordinary temperatures； and no temperature above the absolute zero； are absolutely free from its disturbing influence。 Hence it is that molecules having all the valency of their atoms fully satisfied do not lose their chemical activitysince each atom is momentarily free in the exchange of partners； and may seize upon different atoms from its former partners； if those it prefers are at hand。
While； however； an appreciation of this ceaseless activity of the atom is essential to a proper understanding of its chemical efficiency； yet from another point of view the 〃saturated〃 moleculethat is； the molecule whose atoms have their valency all satisfiedmay be thought of as a relatively fixed or stable organism。 Even though it may presently be torn down； it is for the time being a completed structure； and a consideration of the valency of its atoms gives the best clew that has hitherto been obtainable as to the character of its architecture。 How important this matter of architecture of the moleculeof space relations of the atomsmay be was demonstrated as long ago as 1823； when Liebig and Wohler proved； to the utter bewilderment of the chemical world； that two substances may have precisely the same chemical constitutionthe same number and kind of atomsand yet differ utterly in physical properties。 The word isomerism was coined by Berzelius to express this anomalous condition of things； which seemed to negative the most fundamental truths of chemistry。 Naming the condition by no means explained 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 d