356 



NA TURE 



[August 8, 1889 



they fall asunder into two parts, Hg + Hj or CH4 + Hg, as 

 soon as they are even temporarily formed, and are capable of 

 separate existence, and therefore can take no part in the element- 

 ary act of substitution. With respect to the simplest molecules 

 which we shall select — that is to say, those of which the parts 

 have no separate existence, and therefore cannot appear in sub- 

 stitutions— we shall consider them according to the periodic 

 law, arranging them in direct dependence on the atomic weight 

 of the elements. 



Thus, for example, the molecules of the simplest hydrogen 

 compounds — 



HF H2O H3N H4C 



Hydrcfl icr'c acid Water Ammonia Methane 



CDrrespond to elements the atomic weights of which decrease 

 consecutively — 



F = 19, O = 16, N = 14, C = 12. 



Neither the arithmetical order (i, 2, 3, 4 atoms of hydrogen) nor 

 the total information we possess respecting the elements will 

 permit us to interpolate into this typical series one more addi- 

 tional element ; and therefore we have here, for hydrogen com- 

 pounds, a natural base upon which are built up those simple 

 chemical combinations which we take as typical. But even 

 they are competent to unite with each other, as we see, for 

 instance, in the property which hydrofluoric acid has of forming 

 a hydrate— that is, of combining with water ; and the similar 

 attribute of ammonia, resulting in the formation of a caustic 

 alkali, NH3H2O, or NH4OH. 



Having made these indispensable preliminary observations, I 

 may now attack the problem itself, and attempt to explain the 

 so-called structure, or rather construction of molecules — that is 

 to say, their constitution and transformations — without having 

 recourse to the teaching of " striictionists," but on Newton's 

 dynamical principles. 



Of Newton's three laws of motion, only the third can bs 

 applied directly to chemical molecules when regarded as systems 

 of atoms among which it must be supposed that there exist 

 common influences or forces, and resulting compounded relative 

 motions. Ciiemical reactions of every kind are undoubtedly 

 accomplished by changes in these internal movements, respecting 

 the nature of which nothing is known at present, but the exist- 

 ence of which the mass of evidence collected in modern times 

 forces us to acknowledge as forming part of the common motion 

 of the universe, and as a fact further established by the circum- 

 stance that chemical reactions are always characterized by 

 changes of volume or the relations between the atoms or the 

 molecules. Newton's third law, which is applicable to every 

 system, declares that "action is always associated with reaction, 

 and is equal to it." The brevity and conciseness of this axiom 

 was, however, qualified by Newton in a more expanded state- 

 ment : "The actions of bodies one upon another are always 

 ■equal, and in opposite directions." This simple fact constitutes 

 the point of departure for explaining dynamic equilibrium — that 

 is to say, systems of conservancy. It is capable of satisfying 

 «ven the dualists, and of explaining, without additional assump- 

 tions, the preservation of those chemical types which Dumas, 

 Laurent, and Gerhardt created unit types, and those views of 

 atomic combinations which the structionists express by atom- 

 icity or the valency of the elements, and, in connection with 

 them, the various numbers of affinities. In reality, if a system 

 of atoms or a molecule be given, then in it, according to the 

 third law of Newton, each portioa of atoms acts on the re- 

 maining portion in the same manner and with the same force as 

 the second set of atoms acts on the first. We infer directly 

 from this consideration that both sets of atoms forming a 

 molecule are not only equivalent with regard to themselves, 

 as they must be according to Dalton's law, but also that they 

 may, if united, replace each other. Let there be a molecule 

 ■containing atoms A B C, it is clear that, according to Newton's 

 law, the action of A on B C must be equal to the action of 

 B C on A, and if the first action is directed on B C, then 

 the second must be directed on A, and consequently then, 

 where A can exist in dynamic equilibrium, B C may take its 

 place and act in a like manner. In the same way the action 

 of C is equal to the action of A B. In one word every two 

 sets of atoms forming a molecule are equivalent to each other, 

 and may take each other's place in other molecules, or, having 

 the power of balancing each other, the atoms or their comple- 

 ments are endowed with the power of replacing each other. Let 



us call this consequence of an evident axiom " the principle of 

 substitution," and let us apply it to those typical forms of 

 hydrogen compounds which we have already discussed, and 

 which, on account of their simplicity and regularity, have served 

 as starting-points of chemiral argument long before the appearance 

 of the doctrine of structure. 



In the type of hydrofluoric acid, HF, or in systems of double 

 stars, are included a multitude of the .simplest molecules. It 

 will be sufficient for our purpose to recall a few : for example, 

 the molecules of chlorine, C]„, and of hydrogen, Hg, and 

 hydrochloric acid, HCl, which is familiar to all in aqueous 

 solution as spirit of salt, and which has many points of 

 resemblance with HF, HBg, HI. In these cases division 

 into two parts can only be made in one way, and therefore the 

 principle of substitution renders it probable that exchanges 

 between the chlorine and the hydrogen can take place, if they 

 are competent to unite with each other. There was a time when 

 no chemist would even admit the idea of any such action ; it 

 was then thought that the power of combination indicated a 

 polar difference of the molecules in combination, and this 

 thought set aside all idea of the substitution of one component 

 element by another. , 



Thanks to the observations and experiments of Dumas and 

 Laurent fifty years ago, such fallacies were dispelled, and in this 

 manner this same principle of substitution was exhibited. 

 Chlorine and bromine, acting on many hydrogen compounds, 

 occupy immediately the place of their hydrogen, and the dis- 

 placed hydrogen, with another atom of chlorine or bromine, 

 forms hydrochloric acid or bromide of hydrogen. This takes 

 place in all typical hydrogen compounds. Thus chlorine acts 

 on this principle on gaseous hydrogen— reaction, under the influ- 

 ence of light, resulting in the formation of hydrochloric acid. 

 Chlorine, acting on the alkalies, constituted similarly to water, 

 and even on water itself — only, however, under the influence of 

 light, and only partially because of the instability of HCIO — 

 forms, by this principle, bleaching salts, which are the same as 

 the alkalies, but with their hydrogen replaced by chlorine. In 

 ammonia and in methane, chlorine can also replace the 

 hydrogen. From ammonia is formed in this manner the so- 

 called chloride of nitrogen, NCI3, which decomposes very 

 readily with violent explosion on account of the evolved gases, 

 and falls asunder as chlorine and nitrogen. Out of marsh gas, 

 or methane, CH4, may be obtained consecutively, by this 

 method, every possible substitution, of which chloroform, 

 CHCI3, is the best known, and chloro-carbonic acid, CCI4, the 

 most instructive. But by virtue of the fact that chlorine and 

 bromine act in the manner shown on the simplest typical 

 hydrogen compounds, their action on the more complicated 

 ones may be assumed to be the same. This can be easily 

 demonstrated. The hydrogen of benzole, CgHg, reacts feebly 

 under the influence of light on liquid bromine, but Gustavson 

 has shown that the addition of the smallest quantity of metallic 

 aluminium causes energetic action, and the evolution of large 

 volumes of bromide of hydrogen. 



If we pass on to the second typical hydrogen compound — that 

 is to say, water — its molecule, HOH, may be split up in two 

 ways : either into an atom of hydrogen and a molecule of oxide 

 of hydrogen, HO, or into oxygen, O, and two atoms < f 

 hydrogen, H ; and therefore, according to the principle of sub- 

 stitution, it is evident that one atom of hydrogen can exchange 

 with oxide of hydrogen, HO, and two atoms of hydrogen, H, 

 with one atom of oxygen, O. 



Both these forms of substitution will constitute methods ot 

 oxidation — that is to say, of the entrance of oxygen into the 

 compound — a reaction which is so common in Nature as well as 

 in the arts, taking place at the expense of the oxygen of the air 

 or by the aid of various oxidizing substances or bodies which 

 part easily with their oxygen. There is no occasion to reckon 

 up the unlimited number of cases of such oxidizing reactions. 

 It is sufficient to state that, in the first of these, oxygen i> 

 directly transferred, and the position, the chemical function, 

 which hydrogen originally occupied is, after the substitution, 

 occupied by the hydroxyl. Thus ammonia, NH3, yields 

 hydroxylamine, NH2(0H), a substance which retains many of 

 the properties of ammonia. 



Methane and a number of other hydrocarbons yield, by sub- 

 stitution of the hydrogen by its oxide, methylic, CH3(0H), 

 and other alcohols. The substitution of one atom of oxygen for 

 two atoms of hydrogen is equally common with hydrogen com- 

 pounds. By this means aicohoHc liquids containing ethyl 



