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Then the tall decayed Rampinelli and, followed by a calle, the Ramo Barzizza, and next the Mengaldo, with a very choice doorway and arches, now a statuary store; then the yellow Avogadro, now an antiquity dealer's and tenements, with a fondamenta; then a new building, and we reach the fine red palace adjoining the Casa Petrarca, with its ramping garden.
Dalton made the atom a beacon-light which revealed to chemists paths that led them to wider and more accurate knowledge. Avogadro illuminated chemical, and also physical, ways by his conception of the molecule as a stable, although separable, group of atoms with particular properties different from those of the atoms which constituted it.
But this new chemistry has grown up by the help of hypotheses, such as those of Dalton and of Avogadro, and that singular conception of 'bonds' invented to colligate the facts of 'valency' or 'atomicity, the first of which took some time to make its way; while the second fell into oblivion, for many years after it was propounded, for lack of empirical justification.
Avogadro is among those who make Giovanni Bishop, or rather Archbishop, of Ravenna, and gives the following account of the circumstances which led to his resigning his diocese and going to live at the top of the inhospitable Monte Caprasio. It seems there had been a confirmation at Ravenna, during which he had accidentally forgotten to confirm the child of a certain widow.
Dalton often said: "Thou knowest thou canst not cut an atom"; but the fact that he applied the term atom to the small particles of compounds proves that he had escaped the danger of logically defining the atom, the danger of thinking of it as a particle which never can be cut. The molecule of Avogadro has always been a decomposable particle.
The compound atom, composed of two or more elementary atoms, Avogadro proposed to distinguish, for purposes of convenience, by the name molecule. It is to the molecule, considered as the unit of physical structure, that Avogadro's law applies. This vastly important distinction between atoms and molecules, implied in the law just expressed, was published in 1811.
I have been so unjust to our own century that I have made no allusion to some of its greatest scientific triumphs: its grand conceptions in natural history; its discoveries in magnetism and electricity; its invention of the beautiful art of photography; its applications of spectrum analysis; its attempts to bring chemistry under the three laws of Avogadro, of Boyle and Mariotte, and of Charles; its artificial production of organic substances from inorganic material, of which the philosophical consequences are of the utmost importance; its reconstruction of physiology by laying the foundation of that science on chemistry; its improvements and advances in topographical surveying and in the correct representation of the surface of the globe.
Cross the canal, turn to the left, and the Campo di S. Barnaba, with its archway under the houses, is before you. The direct way from S. Sebastiano to this same point and the iron bridge is by the long Calle Avogadro and Calle Lunga running straight from the bridge before the church. There is no turning.
The true explanation of Gay-Lussac's law of combination by volumes was thought out almost immediately by an Italian savant, Amadeo, Avogadro, and expressed in terms of the atomic theory. The fact must be, said Avogadro, that under similar physical conditions every form of gas contains exactly the same number of ultimate particles in a given volume.
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.
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