Aug. 27,1874] 



NATURE 



347 



give rise to an immense multitude of distinct compounds which 

 are true chemical species. But what is the arrangement of these 

 atoms? What is the structure of these organic molecules, so 

 much alike in the nature of their elements, so wonderful in the 

 infinite diversity of their properties ? Berzelius solved this ques- 

 tion without hesitation. Comparing organic compounds to the 

 bodies of inorganic chemistry, he divided both classes of atoms 

 into two lots, grouping on one side carbon and hydrogen, electro- 

 positives, and on the other, oxygen, electro-negative. And when, 

 at a later time, chlorine was artificially introduced into organic 

 compounds, the atoms of this powerful element were ranged on 

 the side of oxygen, both being invariably found in binary com- 

 binations of which they formed the electro-negi.tive element, the 

 atoms of carbon and hydrogen constitutmg the electro-positive 

 radical. 



Thus the gieat promoter of inorganic chemistry attempted to 

 fashion organic molecules according to the image of those mole- 

 cules of dead matter which he had studied so thoroughly. Tlie 

 paths which Lavoisier traced in this domain he wished to 

 extend to the world of products formed under the influence of 

 Ufe ; they resulted in a dead-lock. In proportion as the 

 riches of science increased it was necessary, in order to uphold 

 the system, to accumulate hypotheses, to invent radicals, to con- 

 struct, with insufficient or nnaginary data, formula; more and 

 more complicated — a thankless task, in which the feeling of expe- 

 rimental realities and sober appreciation of facts often gave place 

 to outrageous reasonings and vague subtleties. These barren 

 efforts of a great mind inaugurated the decline or marked the 

 termination of the dualistic ideas which were at the foundation of 

 what has been called, improperly perhaps, the old chemistry. 

 The new began at that point. Great discoveries, cleverly and 

 boldly interpreted, gave it an impulse which still endures. 



There were then — I speak of forty years ago — a number of 

 young men, with Dumas and Liebig at their head, in the oppo- 

 site camp, who cultivated with ardour the investigation of organic 

 compounds. Convinced that the constitution of these com- 

 pounds could only be deduced from the attentive investigation of 

 their properties and metamorphoses, they undertook to investi- 

 gate these bodies themselves, to transform them, to torment 

 them in some sort by the action of the most diverse reagents, in 

 the hope of discovering their intimate structure. And this is, 

 gentlea^en, the true method in chemistry ; to determine the 

 composition of bodies, and by careful analysis of their properties 

 to fix, as far as possible, the grouping of their ultimate particles. 

 This, then, is the glory of our science, and the single but precious 

 contribution which it is able to furnish for the solution of that 

 eternal problem, the constitution of matter. 



I-"rom the researches which were made at this epoch and in 

 this spirit, an all-important fact issued ; it relates to the action of 

 chlorine on organic compounds. This simple body deprives 

 them of hydrogen and may be substituted for that element, atom 

 for atom, without aflecting the molecular equilibrium and with- 

 out, adds Dumas, modifying the fundamental properties. This 

 jiroposition encountered at first the most violent contradiction, 

 llow could chlorine take the place of hydrogen and play its part 

 in combinations ? These two elements, said Berzelius, are en- 

 dowed with opposite properties, and if the one is lacking the 

 other cannot supply its place ; for, in short, they are two inimical 

 brothers, little disposed and by no means fit to be kept in the same 

 house. These critics and many others have not prevailtd against 

 facts. The theory of substitutions has come triumphantly out of 

 this great discussion, which marks a date in the history of our 

 science. Its natural development has gradually introduced into 

 it new ideas on the constitution of chemical compounds, on the 

 mode of combination of the elements which they contain. 



These ideas have come to light by various ingenious compari- 

 sons. Laurent considered organic compounds as formed of 

 nuclei with appendages, botli the one and the other admitting 

 into their structuies atoms grouped with a certain symmetry. 

 Dumas compared them to edifices of which the atoms constitute, 

 in a manner, the materials. Hence the graphic but frequently 

 correct expression, of molecular edifices capable of being modi- 

 fied, in certain cases, by the substitution of one part for another, 

 and which, in other cases, the shock of powerful reagents may 

 shatter to pieces. In both conceptions the chemical molecules 

 were regarded as forming a whole. ^\. little later Dumas compared 

 them to planetary systems ; and here he veritably shot ahead 

 of his time in giving us a glimpse" of groups of atoms main- 

 tained in eijuilibrium by affinity, but carried along by movements, 

 as the planets of a solar system are acted upon by gravitation 

 and carried into space. It is in these movements of atoms and 



molecules that at a later period the source of the physical and 

 chemical forces must be sought for ; but I must not anticipate. 

 I have attempted to show how the ideas on chemical combina- 

 tions have been gradually modified under the double influence of 

 the atomic hypotheses and of facts brought to light by the 

 French school concerning their reciprocal replacement in 

 combinations. Forming a whole, more or less complex, 

 the molecules of organic substances may be modified by sub- 

 stitution and give rise to a multitude of derivatives which 

 naturally attach themselves to the mother substance. The lat- 

 ter serves them as a model or type. The typical idea thus intro- 

 duced into science very soon occupied a large place. It first 

 brought to it important elements of classification. All the 

 compounds derived by suljstitution from the same body were 

 ranged in the same family, of which the latter was, so to speak, 

 the chief. Hence arose groups of bodies perfectly distinct from 

 each other, and the number of which were being constantly in- 

 cJeased by daily discoveries. It was necessary not only to intro- 

 duce order into all these tribes, but to connect them with each other 

 by a common bond. The honour of having discovered the supe- 

 rior principle of classification belongs to Laurent and Gerhardt, 

 valiant champions of French science, from whom premature 

 death has snatched, if not victory, at least the gratification of 

 victory. Laurent was the first to say that a certain number of 

 mineral and organic compounds possessed the constitution of 

 water, and this idea, brilliantly develo[)ed by Williamson, 

 was generalised by Geihardt. According lo the last named, all 

 inoiganic and organic compounds may be connected with a 

 small number of types, of which hydrochloric acid, water, 

 and ammonia, are the chief. In these compounds, relatively 

 simple, one element may be replaced by another element, or by 

 a group of atoms performing the function of a radical, so that 

 this substitution gives rise to a multitude of various compounds 

 bound together by the analogy of their structure, if not by the 

 harmony of their properties. 



This last point was novel and important. Bodies belonging 

 to one type and similar in their molecular structure may differ 

 much in their properties : these depend not only on the arrange- 

 ment of the atoms, but also on their nature. Thus the inorganic 

 and organic bodies ranged under the type water, are, according 

 to the nature of their elements or their radicals, powerful bases, 

 energetic acids, or indifferent substances — a great and bold idea, 

 which has established a connection between the most diverse 

 bodies, and which has definitely overturned the barriers which 

 use had raised, and which the weakness of theory had maintained, 

 between inorganic and organic chemistry. And yet this was 

 only a stage in the march of ideas, liy what right and by what 

 privilege, it was said, may the relatively simple compounds we 

 have named serve as types for all others, and why should nature 

 be restricted to make all bodies on the model of hydrochloric 

 acid, water, and ammonia? This was a serious difficulty, but it 

 has been removed ; it became the occasionof a profound discus- 

 sion and the germ of a real progress. 



These typical compounds represent at bottom various forms 

 of combination, the diversity of which it is necessary to refer to 

 the nature of the elements themselves. The latter impress 

 on each of these compound types a particidar character and a 

 special form. The atoms of chlorine are so formed that to one 

 of them only a single atom of hydrogen needs to be added to 

 form hydrocliloric acid ; then that an atom of oxygen takes two 

 atoms of hydrogen to form water ; that an atom of nitrogen re- 

 quires three to constitute ammonium ; and that an atom of carbon 

 demands four to become marsh-gas. What a difference in the 

 power of combination of these elements, and, so to speak, in 

 their appetites for hydrogen ! And will this difference not be 

 connected with some peculiarities in their mode of existence, to 

 some property inherent in matter itself, and which will impress 

 on each of these hydrogenic compounds a special form ? Such 

 is the case. 



It is now admitted that atoms are not motionless, even in bodies 

 apparently the most fixed and in completely formed combina- 

 tions. At the moment when these arc being formed the atoms 

 come into violent collision with each other. In this conflict a 

 disengagement of heat is ordinarily observed, resulting from the 

 expenditure of active energy which the atoms have lost in the 

 mcU'e, and the intensity of this heat-phenomenon gives the 

 measure of the entrgy of the affinities which have presided at 

 the combination. But there is another thing in chemical pheno- 

 mena besides the intensify of the forcts at work, and which are 

 more or less exhausted by a disengagement of heat ; I refer to 

 their mode ; it was of this elective attraction that Bergman spoke 



