a history of science-4-第11章
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Though the presence of the alkalies and acids in the water was explained； however； their respective migrations to the negative and positive poles of the battery remained to be accounted for。 Davy's classical explanation assumed that different elements differ among themselves as to their electrical properties； some being positively； others negatively； electrified。 Electricity and 〃chemical affinity；〃 he said； apparently are manifestations of the same force； acting in the one case on masses； in the other on particles。 Electro…positive particles unite with electro…negative particles to form chemical compounds； in virtue of the familiar principle that opposite electricities attract one another。 When compounds are decomposed by the battery； this mutual attraction is overcome by the stronger attraction of the poles of the battery itself。
This theory of binary composition of all chemical compounds； through the union of electro…positive and electro…negative atoms or molecules； was extended by Berzelius； and made the basis of his famous system of theoretical chemistry。 This theory held that all inorganic compounds； however complex their composition； are essentially composed of such binary combinations。 For many years this view enjoyed almost undisputed sway。 It received what seemed strong confirmation when Faraday showed the definite connection between the amount of electricity employed and the amount of decomposition produced in the so…called electrolyte。 But its claims were really much too comprehensive； as subsequent discoveries proved。
ORGANIC CHEMISTRY AND THE IDEA OF THE MOLECULE
When Berzelius first promulgated his binary theory he was careful to restrict its unmodified application to the compounds of the inorganic world。 At that time； and for a long time thereafter； it was supposed that substances of organic nature had some properties that kept them aloof from the domain of inorganic chemistry。 It was little doubted that a so…called 〃vital force〃 operated here； replacing or modifying the action of ordinary 〃chemical affinity。〃 It was； indeed； admitted that organic compounds are composed of familiar elementschiefly carbon； oxygen； hydrogen； and nitrogen； but these elements were supposed to be united in ways that could not be imitated in the domain of the non…living。 It was regarded almost as an axiom of chemistry that no organic compound whatever could be put together from its elementssynthesizedin the laboratory。 To effect the synthesis of even the simplest organic compound； it was thought that the 〃vital force〃 must be in operation。
Therefore a veritable sensation was created in the chemical world when； in the year 1828； it was announced that the young German chemist； Friedrich Wohler； formerly pupil of Berzelius； and already known as a coming master； had actually synthesized the well…known organic product urea in his laboratory at Sacrow。 The 〃exception which proves the rule〃 is something never heard of in the domain of logical science。 Natural law knows no exceptions。 So the synthesis of a single organic compound sufficed at a blow to break down the chemical barrier which the imagination of the fathers of the science had erected between animate and inanimate nature。 Thenceforth the philosophical chemist would regard the plant and animal organisms as chemical laboratories in which conditions are peculiarly favorable for building up complex compounds of a few familiar elements； under the operation of universal chemical laws。 The chimera 〃vital force〃 could no longer gain recognition in the domain of chemistry。
Now a wave of interest in organic chemistry swept over the chemical world； and soon the study of carbon compounds became as much the fashion as electrochemistry had been in the； preceding generation。
Foremost among the workers who rendered this epoch of organic chemistry memorable were Justus Liebig in Germany and Jean Baptiste Andre Dumas in France； and their respective pupils； Charles Frederic Gerhardt and Augustus Laurent。 Wohler； too； must be named in the same breath； as also must Louis Pasteur； who； though somewhat younger than the others； came upon the scene in time to take chief part in the most important of the controversies that grew out of their labors。
Several years earlier than this the way had been paved for the study of organic substances by Gay…Lussac's discovery； made in 1815； that a certain compound of carbon and nitrogen； which he named cyanogen； has a peculiar degree of stability which enables it to retain its identity and enter into chemical relations after the manner of a simple body。 A year later Ampere discovered that nitrogen and hydrogen； when combined in certain proportions to form what he called ammonium； have the same property。 Berzelius had seized upon this discovery of the compound radical； as it was called； because it seemed to lend aid to his dualistic theory。 He conceived the idea that all organic compounds are binary unions of various compound radicals with an atom of oxygen； announcing this theory in 1818。 Ten years later； Liebig and Wohler undertook a joint investigation which resulted in proving that compound radicals are indeed very abundant among organic substances。 Thus the theory of Berzelius seemed to be substantiated； and organic chemistry came to be defined as the chemistry of compound radicals。
But even in the day of its seeming triumph the dualistic theory was destined to receive a rude shock。 This came about through the investigations of Dumas； who proved that in a certain organic substance an atom of hydrogen may be removed and an atom of chlorine substituted in its place without destroying the integrity of the original compoundmuch as a child might substitute one block for another in its play…house。 Such a substitution would be quite consistent with the dualistic theory； were it not for the very essential fact that hydrogen is a powerfully electro…positive element； while chlorine is as strongly electro…negative。 Hence the compound radical which united successively with these two elements must itself be at one time electro…positive； at another electro…negativea seeming inconsistency which threw the entire Berzelian theory into disfavor。
In its place there was elaborated； chiefly through the efforts of Laurent and Gerhardt； a conception of the molecule as a unitary structure； built up through the aggregation of various atoms； in accordance with 〃elective affinities〃 whose nature is not yet understood A doctrine of 〃nuclei〃 and a doctrine of 〃types〃 of molecular structure were much exploited； and； like the doctrine of compound radicals； became useful as aids to memory and guides for the analyst； indicating some of the plans of molecular construction； though by no means penetrating the mysteries of chemical affinity。 They are classifications rather than explanations of chemical unions。 But at least they served an important purpose in giving definiteness to the idea of a molecular structure built of atoms as the basis of all substances。 Now at last the word molecule came to have a distinct meaning； as distinct from 〃atom；〃 in the minds of the generality of chemists； as it had had for Avogadro a third of a century before。 Avogadro's hypothesis that there are equal numbers of these molecules in equal volumes of gases； under fixed conditions； was revived by Gerhardt； and a little later； under the championship of Cannizzaro； was exalted to the plane of a fixed law。 Thenceforth the conception of the molecule was to be as dominant a thought in chemistry as the idea of the atom had become in a previous epoch。
CHEMICAL AFFINITY
Of course the atom itself was in no sense displaced； but Avogadro's law soon made it plain that the atom had often usurped territory that did not really belong to it。 In many cases the chemists had supposed themselves dealing with atoms as units where the true unit was the molecule。 In the case of elementary gases； such as hydrogen and oxygen； for example； the law of equal numbers of molecules in equal spaces made it clear that the atoms do not exist isolated； as had been supposed。 Since two volumes of hydrogen unite with one volume of oxygen to form two volumes of water vapor； the simplest mathematics show； in the ligh
