Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/478

46G CHEMISTRY [HISTORY. powers of the various acids ; thus the formula AgO.JPO 5 did not express the tribasic 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 " acidifiable bases " of acids, and Lavoisier in his Traits elementaire had spoken of the " hydrocarbon radicles " in oils, starch, sugar, and gum. In 1817 Berzelius, following Lavoisier, held the opinion that all inorganic oxides contained simple radicles, and organic oxides compound radicles. In 1832 Liebig and Wohler discovered the composition of bitter-almond oil, which they showed, on the assumption of the existence of a radicle C 7 H 5 O, might be compared with the compounds of potassium and other metals. Berzelius at first accepted their explanation of the 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. Beuzoic acid was represented by him as a com pound containing the radicle C 14 H 3 , thus : C 14 H 5 .O 3 + HO, and alcohol as an oxide of the radicle CH 3 , or 2(CH 3 ).O. In 1834 Dumas made known his observations on the substitution of hydrogen by other elements ; and Laurent subsequently concluded from 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 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 fhe 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) C 2 C1 3 + C. 2 O 3 + HO ; acetic acid, on the other hand, was a trioxide of acetyl (C! 4 H 3 ) with the formula C 4 H 3 .O 3 + HO. The substitution-compounds discovered by Malaguti and Regnault were in like manner represented dualistically ; dichloroformic ether, for example, was written 2C 2 H.0 3 + C,H.C1 3 + 2C 4 H 3 .O 3 4- C 4 H 3 .C1 3 ; such complex formuhe, 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 com pounds. 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 bhe smallest quantities by weight of carbonic acid gas and water pro duced in reactions were expressed by the formulae C 2 O 4 and H 2 2 , he concluded that these must represent the molecules of the two bodies, which might be more con veniently written CO 2 and H 2 0, 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 enabl ing the formula? 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 constitu tion 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 alteration 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 RO. 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 similarly obtained must be of the same chemical type with the hydrogenated substances from which they are derived. The discovery by Wurtz of the com pound ammonias, and by Hofmann of diethylamine and triethylarnine, led to the creation of the ammonia type ; and Williamson, by the discovery in 1850 of the mixed ethers, was enabled to refer ether, alcohol, and acids to the ivaicr type, and to predict the existence of acetic anhydride, which, as well as benzoic anhydride, was discovered in 1852 by Gerhardt. To these types Gerhardt added two others, those of hydrogen and hydrochloric acid, and with the former associated the aldehydes, ketones, and many hydro carbons, 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 represent ing the relations of compounds by the use of mixed types ; and by Kekul6 and numerous other investigators. The foundation of electro-chemical science may be said to have been laid by Nicholson and Carlisle, who in 1800 discovered the decomposition of water by the agency of the voltaic pile ; but the earliest electro-chemical experi ments were those made by Priestley in 1775 upon am monia gas, and by Deiman and Van Troostwijk in 1789 upon water, by means of frictional electricity. Cruickshank by experiments with the chlorides of magnesium, sodium, and ammonium demonstrated that when those salts are decomposed by the electric current, alkali always appears at the negative, and acid at the positive pole. In 1803 Berzelius and Hisinger published their observa tions on the electrical decomposition of salts and some of their bases ; oxygen, acids, and oxidized bases, they said, appear at the positive pole ; combustible bodies, alkalies, and earths at the negative. Later experimenters, however, showed that it is the metal, not the oxide, that appears at the negative pole when salts are electrolyzed, and that oxides cannot be supposed to exist ready formed in salts. Davy, whose electrical experiments were commenced in 1800, undertook in 1806 a course of investigations which led him to the discovery of the metals of the alkalies and alkaline earths. In 1807 he expressed the opinion that bodies having an affinity for one another are in different

states of electricity, and " that chemical and electrical