From the results of his different experiments, which were communicated to the Royal Society on June 23rd, 1814, it appeared that the diamond affords no other substance by its combustion in oxygen than pure carbonic acid gas, and that the only chemical difference perceptible between diamond and the purest charcoal is that the latter contains a minute proportion of hydrogen. “But,” he asks, “can a quantity of an element, less in some cases than 1/50000 part of the weight of the substance, occasion so great a difference in physical and chemical characters?” This he concludes is most unlikely, for, as he points out, even when the minute quantity of hydrogen is expelled by heating the charcoal in chlorine, the specific differences remain.
The doctrine at that time current, and which seemed indeed almost axiomatic, “That bodies cannot be exactly the same in composition or chemical nature, and yet totally different in all their physical properties,” received its first great shock. Davy’s work, no doubt, paved the way for the recognition of the fact of allotropy, and thereafter of isomerism.
In May he went to Naples and made his first ascent of Vesuvius, which he revisited on several subsequent journeys. He commissioned one of the guides to inform him from time to time of the condition of the volcano, and the man’s letters, in spite of their phonetic address—“Siromfredevi-Londra”—duly found their way to Albemarle Street. He also interested himself in the excavations at Pompeii instituted by direction of Murat, then King of Naples, and he performed a number of experiments on the colours used by the ancients in painting, an account of which was communicated to the Royal Society on February 23rd, 1815.
He then passed northwards with the intention of spending the summer at Geneva. On his way he called at Milan to pay his respects to Volta. Of this visit he wrote:—
“Volta I saw at Milan, in 1814, at that time advanced in years,—I think nearly seventy and in bad health. His conversation was not brilliant; his views rather limited, but marking great ingenuity. His manners were perfectly simple. He had not the air of a courtier, or even of a man who had seen the world.”
If Dr. Paris’s story is to be credited, the lack of brilliancy in the conversation of the great Italian physicist may be attributed to the circumstances of this meeting. Davy, we are told, had written to announce his intended visit, and on the appointed day and hour Volta, in full dress, awaited his arrival.
“On the entrance of the great English philosopher into the apartment, not only in déshabille, but in a dress of which an English artisan would have been ashamed, Volta started back in astonishment, and such was the effect of his surprise, that he was for some time unable to address him.”
The party remained at Geneva until the middle of September, partaking freely of the intellectual life which that charming city afforded. Here he met Saussure Pictet, De la Rive, Madame de Staël, Benjamin Constant, Necker, and Talma, whose society he greatly enjoyed. With the approach of winter he returned to Italy viâ the Brenner and Venice, and on November 2nd arrived at Rome, where he remained until March 1st, 1815, occupying himself with his inquiry into the composition of ancient colours. In this he was greatly assisted by the kindness of his friend Canova, the celebrated sculptor, who was then charged with the care of the works connected with ancient art in Rome, and who supplied him with material from the colours found in the Baths of Titus and of Livia, and other palaces and baths of ancient Rome and Pompeii. Davy’s memoir, which appears in the Philosophical Transactions for 1815, displays considerable antiquarian and bibliographical research, and, considering his limited means, much analytical skill and ingenuity. The ancient reds he found to consist of minium, several varieties of iron ochre, and vermilion or cinnabar. The yellows were mixtures of ochres and chalks, or of ochre with minium. He was unable to discover that orpiment was used; a deep orange yellow on stucco in the ruins near the monument of Caius Cestius consisted of a mixture of massicot and minium. The blues were mainly mixtures of the Egyptian or Alexandrine blue, with more or less chalk. This Egyptian blue, he found, was a frit, made by heating soda, sand, and copper, either used as an ore or as metal. He gives a method of making it, and speaks highly of its permanence and beauty. The greens were, as a rule, compounds of copper. The exact nature of the purples he was unable to determine; they were probably organic, but whether obtained from shell-fish or madder could not be ascertained. The purplish reds in the Baths of Titus were found to be mixtures of red ochres, and the blues were copper compounds. The blacks and browns were mixtures of carbonaceous matter with oxides of iron or manganese. The whites were mainly chalk, or occasionally clay; cerusse, or white-lead, was apparently not used.
Before leaving Italy he again went to Naples, for the purpose of witnessing Vesuvius in eruption, and on several occasions he was as near the crater as he could get. He left Naples on March 21st, and came home by way of Verona, Innsbruck, Ulm, Stuttgart, Heidelberg, and the Rhine, arriving in London April 23rd, 1815. A few days after his arrival he wrote to his mother:—
“We have had a very agreeable and instructive journey and Lady Davy agrees with me in thinking that England is the only country to live in, however interesting it may be to see other countries.
“I yesterday bought a good house in Grosvenor Street, and we shall sit down in this happy land.
“I beg you to give my best and kindest love to my sisters, and to remember me with all affection to my aunts.”