HISTORY OF CHEMISTRY: Joseph-Louis Gay-Lussac (1778-1850). Amedio Avogadro (1776-1856). Johan Jakob Berzelius (1779-1848). Jean Baptiste André Dumas (1800-84). Charles Fréderic Gerhardt (1816-56).

In 1808, the year of the publication of Dalton’s New System of Chemical Philosophy, Gay-Lussac made known to the world the laws of the combinations of gases by volume – to which his attention has been directed by the discovery which he and A. Von Humboldt had made, that a definite volume of oxygen combined with exactly twice its bulk of hydrogen. He pointed out that there is a simple relation between the volumes of two gases which unite together, and also between their collective volume in the uncombined and in the combined condition. Thus, three volumes of hydrogen combine with one of nitrogen to form two volumes of ammonia; one volume of chlorine with one of hydrogen produces two volumes of hydrochloric acid gas; and two volumes of nitrogen and one of oxygen give two volumes of protoxide of nitrogen. The law of definite proportions was shown to hold good with respect to the volumes as well as to the weights of combining bodies.

In 1811 Avogadro, remarking that equal variations of temperature and pressure produce in all gases and vapours the same changes of volume, enunciated the hypothesis that equal volumes of any gas or vapour contain the same number of atoms, and the same doctrine was brought forward in 1814 by Ampére. On this hypothsis the density of equal volumes of gases was shown to represent the relative weights of their atoms; and thus, as it had been discovered that gases frequently do not unite volume for volume, a distinction came to be drawn between atomic weights and equivalents. If we say atom for volume, wrote Berzelius, we find in Gay-Lussac’s discovery one of the most direct arguments in favour of Dalton’s hypothesis.

Berzelius (1779-1848), from considerations based on the law of combination by volume, accounted the atoms of elements distinct from their equivalents. Thus two volumes of hydrogen were recognized as the equivalents of one volume of oxygen, the relative weights of equal volumes of the two gases being those of their atoms. Berzelius adopted 100 parts of oxygen as his standard of atomic weights, the atomic weight of hydrogen was therefore 6·24, its equivalent, 12·48. He considered that the atoms of aluminium, arsenic, antimony, bromine, chlorine, fluorine, hydrogen, iodine, nitrogen, and some other elements had a weight equal to only half that of their equivalents, which latter were double and inseparable atoms. In place of the symbols used by Dalton to denote the proportions in which the elements combine by weight, he employed a notation in which letters were used to indicate the named of the elements. He introduced also an abridged notation, in which the equivalents or double atoms were represented by drawing a bar through the symbol of the element. A dot being used to signify an atom of oxygen, the formula of water was written [FORMULA]; and [FORMULA] denoted hydrochloric acid, which was viewed as consisting of a double atom of hydrogen united to a double atom of chlorine. – an hypothesis which left unexplained the fact that the combination of the so-called double atoms of hydrogen and chlorine resulted always in the formation of two molecules, instead of one, of hydrochloric acid gas. Berzelius constructed a table of atomic weights and equivalents, which the discovery by Dulong and Petit in 1819 of the connection between the specific heats and the atomic weights of the elements, and that of the law of isomorphism by Mitscherlich in 1820, enabled him to modify and improve. The equivalent notation of Berzelius was adopted by Gay-Lussac, and displaced in time that founded upon the atomic weights; but it was not generally applied with strictness to all compounds, molecular and not equivalent formulae being employed in some cases. It had this objection, that it masked the relative atom-fixing powers of the various acids; thus the formula [FORMULA] did not express the tibasic character of a molecule of phosphoric acid, and its relations to the molecules of acids of less basicity. Observing the ratio between the oxygen of bases and acids, Berzelius was led to the conception of the dualistic theory, according to which all compounds consist of paired constituents or groups of constituents. This theory was applied to both organic and inorganic substances, and received considerable support from the development of the doctrine of compound radicles, of which Berzelius was the chief supporter.

De Morveau, in a memoir On the Development of the Principles of Methodical Nomenclature, had in 1787 given the name of radicles to the "acidfiable bases" of acids, and Lavoisier in his Traité élémentaire had spoken of the "hydrocarbon radicles" in oils, starch, sugar, and hum. In 1817 Berzelius, following Lavoisier, held the opinion that all inorganic oxides contained simple radicles, and organic oxides compound radicles. In 1832 Liebig and Wöhler discovered the composition of bitter-almond oil, which they showed, on the assumption of the existence of a radicle [FORMULA], might be compared with the compounds of potassium and other metals.Berzelius at first accepted of potassium and other metals. Berzelius at first accepted their explanation of constitution of benzoyl-compounds, but afterwards rejected it, as his electro-chemical theory did not support the view that oxygen could be a constituent of a radicle. Benzoic acid was represented by hum as a compound containing the radicle [FORMULA], thus: [FORMULA], and alcohol as an oxide of the radicle [FORMULA], or [FORMULA].

In 1834 Dumas made known his observations on the substitution of hydrogen by other elements; and Laurent subsequently concluded form numerous experiments that, when hydrogen is substituted by an equivalent of chlorine or bromine, these elements take the place occupied by the hydrogen, performing to a certain extent the functions of the latter in the new compound which is therefore analogous to that from which it has been produced. Thus was given the first b blow to the dualistic theory. The discovery of trichloracetic acid by Dumas gave considerable aid in the establishment of the new doctrine; but Berzelius and others, who were unable to reconcile with the electro-chemical theory the substitution of an electro-positive by an electro-negative element, sought to explain the facts of substitution in accordance with dualistic notions. Berzelius considered that a compound in which oxygen was a fourth element was at once an oxide and a chloride; trichloracetic acid was, in his opinion, a copulated compound of sesquichloride of carbon with sesquioxide of carbon (oxalic acid) [FORMULA]; acetic acid, on the other hand, was a trioxide of acetyl ([FORMULA]) with the formula [FORMULA]. The substitution –compounds discovered by Malaguti and Regnault were in like manner, represented dualistically; dichloroformic ether, for example, was written [FORMULA]; such complex formulae, however, which implied in most cases a widely different; constitution for bodies obviously related in properties, were received with but little favour by chemists. At length Melsens, by converting trichloracetic acid into acetic acid, gave decisive evidence as to the truth of the principle of the substitution theory.

Gerhardt, who regarded all compounds as simple molecules, certain atoms of which were displaceable by double decomposition, discarded entirely the conception of radicles. He classified organic substances according to the number of carbon atoms contained in their molecules; hence the recognition of homologous series of organic compounds. What he termed residues – molecules deprived of certain of their constituents – were in many instances identical with the radicles employed by Berzelius, but were not held necessarily to pre-exist in compounds. Gerhardt was the means of re-introducing, in a modified form, the atomic notation of Berzelius. Observing that the smallest quantities by weight of carbonic acid gas and water produced in reactions were expressed by the formulae [FORMULA] and [FORMULA], he concluded that these must represent the molecules of the two bodies which might be more conveniently written [FORMULA] and [FORMULA], the atomic weights of hydrogen, oxygen, and carbon being taken as 1, 16, and 12 respectively. He made the atomic weights of bromine, chlorine, fluorine, hydrogen, iodine, nitrogen, and other of the elements equal to those given by Berzelius, thus enabling the formulae of water and most volatile compounds to be expressed in agreement with the law of Gay-Lussac; but he halved the old atomic weights of most of the metals, supposing that all metallic oxides were similar in constitution to water, or contained two atoms of metal to one of oxygen. Regnault afterwards proved that, according to the law of Dulong and Petit, this alternation ought to be made in the case only of the atomic weights of the metals lithium, potassium, sodium, and silver; many metals were accordingly to be regarded as having oxides of the general formula [FORMULA]. Such metals, being compared with the diatomic radicles of organic chemistry, were called diatomic by Cannizzaro in 1858. Thus originated the idea of the polyatomicity of the metals.

The theory of types dates from the time of the discovery of trichloracetic acid by Dumas, who observed that this body and others similarity obtained must be of the same chemical type with the hydrogenated substances from which they are derived. The discovery of Wurtz of the the compound ammonias, and by Hofmann of diethylamine and triethylamine, led to the creation of the ammonia type; and Williamson, by the discovery in 1850 of the mixed ethers, was enables to refer ether, alcohol, and acids to the water type, and to predict the existence of acetic anhydride, which, as well as benzoice anhydride, was discovered in 1852 by Gerhardt. To these types Gerhardt added to others, those of hydrogen and hydrochloric acid, and with the former associated the aldehydes, ketones, and many hydrocarbons, e.g., the radicles discovered by Frankland and Kolbe. The theory of types was still further extended by Williamson, to whom the conception of condensed types is due; by Odling, who first suggested the idea of representing the relations of compounds by the use of mixed types; and by Kekulé and numerous other investigators.






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