212 



NA TURE 



Sjfune 20, 1878 



had arrived at precisely the same conceptions, starting from new 

 and perfectly independent points of view. 



The chemists, with Laurent and Gerhardt as leaders, were led 

 by purely chemical considerations and essentially by reasons con- 

 nected with systematics, to distinguish clearly between the ideas 

 of atom and molecule, and to find methods which, in the perfec- 

 tion at which they have now arrived, render possible the deter- 

 mination of the relative weights of the atoms and molecules, 

 and even of the absolute number of atoms in the molecules, for 

 all more perfectly examined substances, by the discussion of 

 purely chemical facts. Amongst others they arrived at the 

 result that the molecule even of elements consists of two atoms 

 as a rule. 



In physics, however, the mechanical heat theory caused a 

 probability bordeiing upon certainty to be ascribed to the funda- 

 mental thought of Avogadro's hypothesis ; and when our cele- 

 brated colleague, Clausius, in the course of his classical investi- 

 gations, had arrived at the conception that even in elements the 

 molecules consisted of several atoms, then he could express his 

 satisfaction regarding the fact that chemists before him, on totally 

 different ways, had already arrived at the same results. 



After, in this manner, Avogaf^ro's hypothesis on the nature of 

 gases had obtained general recognition, and the relative weights 

 of the gas particles could thus be deduced from the specific 

 gravities of gases ; after, on the other hand, we had learned to 

 determine the relative weights of the chemical molecults by 

 chemical c jnsiderations, then it appeared that both values were 

 identical, and thus we arrived at the conception, which anyhow 

 was probable on account of its simplicity, but which was not a 

 necessary one previously, that the gas particles and the chemical 

 molecules are identical, that heat therefore, is able to subdivide 

 matter down to the chemical molecules. 



The chemical "^zx^. of the atomic theory was essentially extended 

 some twenty years ago by that hypothesis, made by chemists, 

 which has been designated as the theory of the chemical quanti- 

 valence of atoms. In its fundamental thought this hypotheses 

 only says that besides the characteristic atomic weight which is 

 the cause that the elements combine in certain proportions of 

 weight, the atoms still possess a further fundamental property, 

 which causes them to combine exactly in those numbers in which 

 they do. As we could not, at first, arrive at a clear conception 

 of this fundamental property, we simply ascribed a certain num- 

 ber of chemical attraction units to the materially different atom*:, 

 and accordingly called them uni-, bi-, tri-, or quadrivalent. 



Now this hypothesis of the chemical quantivalence of ele- 

 mentary atoms of course still offers many dark points, but yet it 

 has led to the recognition of a law which, not only for chemistry, 

 but for the entire atomic theory is of fundamental importance, 

 and which chemists call the law of the connection of atoms ( Ver- 

 kettung der Atome). The separate atoms of a molecule are not 

 connected all with all, or all with one, but, on the contrary, tach 

 one is connected only with one or with a few niighlouring atoms, 

 just as in a chain link is connected with link. 



At the same time it is evident that the atoms within the mole- 

 cules must be in constant motion, and if, indeed, nothing certain 

 is known respecting the nature of this motion, yet it results from 

 this very law of the connection that the intramolecular atomic 

 motion must be of such a nature that the separate atoms move 

 about certain positions of equilibrium without ever leaving them, 

 as long as chemically the molecules continue to exist. The 

 motion of atoms, therefore, is certainly similar to that of the 

 molecules ia the solid state, and thus it may be said that the 

 molecule: of existing substances are solid aggregations of atoms. A 

 state of motion similar to that which the molecules of liquid 

 bodies possess, occurs only with chemical changes, by which 

 molecules of different atomic structure are formed, and then evi- 

 dently only in a transitory manner and only for single atoms. A 

 state of this kind doubtless plays an important part not only in 

 fermentation phenomena, but also in the chemical processes 

 occurring in living organisms. The nature of the motion of 

 atoms is, as I said before, unknown at present. Perhaps it may 

 be imagined as an oscillatory one in such a way that the nutnber 

 of oscillations executed in the unit of time exactly represents the 

 chemical value, and that atoms engaged in functional oscillation, 

 and perhaps striking against each other, appear in chemical com- 

 bination. Then the chemical quantivalence of atoms would have 

 to be considered as a constant one with even greater probability 

 than hitherto. Anyhow one might imagine that polyvalent 

 atoms, at temperatures which, for the substances in question, 

 might be called ultra-hot, do not meet with another atom during 

 one or even more oscillation phases, while adding a part of their 



motion-energy to the molecular motion ; a conception which 

 would correspond to the present conception of unsaturated affini- 

 ties. We would have to think it probable, further, that upon 

 raising the temperature still more, this intermediary state of 

 partial dissociation would be followed by one of total dissocia- 

 tion, during which isolated atoms move in space, just as has 

 already been proved for mercury vapour at temperatures of easy 

 access. 



The law of the connection of atoms based upon the hypothesis 

 of chemical quantivalence, at present accounts cnly for the 

 chemical serial connection (Aneinanderreihung) of atoms, without 

 explaining their position in space and iheform of molecules caused 

 by it. But even now, from investigations on molecular volumes 

 it results that the nature of the connection of atoms influences 

 the mean distances of atoms. 



The circumstance that with isomeric substances the boiling- 

 point of that modification is the highest for which the law of 

 the connection of atoms supposes a chain running in a straight 

 line, while volatility increases the more ramifications the chain 

 shows, the more compressed, therefore, the molecule appears- 

 from a chemical point of view ; together with the maxim, pro- 

 bable in itself, that the position of the point of gravity and the 

 moment of inertia of the rotating molecule must be of influence 

 upon volatility, seem to indicate that the views on the chemi- 

 cal connection of atoms at the same time give us some notion on 

 their mean position in space. The calculations made by Emit 

 Meyer, of the molecular diameters, molecular transverse section?, 

 and molecular volumes, also seem to support this view. Thus 

 the probability of the hypothesis pronounced by Le Bel, and 

 worked out further by Van't Hoff, of the unsymmetrical carbcnt, 

 according to which the four affinity bonds of the carbon 

 atoms, which had already been represented fetrahedrically, are 

 imagined to exist in space in a tetrahedrical position, is in- 

 creased. An hypothesis which may perhaps not merit the 

 unconditional praise which Wislicenus has bestowed upon it, 

 but which certainly much less deserves the bitter derision which 

 Kolbe was inclined to throw upon it. 



The hypothesis of chemical quantivalence further leads to the 

 supposition that also a considerably large number of single mole- 

 cules may, through polyvalent atoms, combine to net-like, and if 

 we like to say so, sponge-like masses, in order thus to produce those 

 molecular masses which resist diffusion, and which, according to 

 Graham's proposition, are called colloidal ones. The same 

 hypothesis in the most natural manner leads to the view, already 

 pronounced by our genial colleague, Pfluger, that such an accu- 

 mulation of molecules may go further yet, and may thus form 

 the elements of the form of living organisms. Of these mass- 

 molecules we may perhaps suppose further that they, through the 

 constant change of position of polyvalent atoms, show a constant 

 change in the connected single molecules, so that the whole --and 

 of course under generation of electricity— is in a sort of living 

 state, particularly since, through the same change of position, 

 adjacent molecules are drawn into the circle of combination 

 and newly-formed ones are expelled. To follow such specu- 

 lations any further at present would, however, be equivalent to- 

 leaving the basis of facts rather too far behind us. 



Really fertile hypotheses on the nature of that force, which 

 brings about the combination of atoms, have not been made up to- 

 the present. The electro chemical theory, so ingeniously worked 

 out by the great Berzeliu', of which, during whole decades, it 

 was believed that it would lead to a satisfactory explanation of 

 chemical facts and to their combination with physical phenomena, 

 has proved completely insufiicient. In all probability in a future 

 period of the development of science it will again be taken up, 

 and will then, in a modernised form, bear its fruits. 



In any case besides the chemical quantivalence, which decides 

 the number of combining atoms, the specific intensity with which 

 this combinauon takes place, must aUo be considered. Here we 

 must suppose that the atoms combined in a molecule, and there- 

 fore saturated with regard to their quantivalence, do not only 

 exercise an attraction upon each other but also upon the atoms 

 of neighbouring molecules, and that thus a molecular attraction 

 results, which is caused by the attraction of the separate atoms- 

 and therefore depends on their quality. Only in this way we 

 can explain the process of chemical decomposition and the exist- 

 ence of that infinite number of more comphcated things which 

 are supposed to be molecular additions or molecules of a higher 

 order. Unquestionably the same cause plays a part in so-called 

 mass-efficts and in catalytic decompositions. The formation of 

 solutions must also be ascribed to it, which hitherto were consi- 

 dered as chemical combinations ia varying proportions, but 



