He tells us that he had long felt a desire to make some new experiments on the combustion of the diamond and other carbonaceous substances; and that this desire was increased by the new fact ascertained with respect to iodine, which by uniting to hydrogen, affords an acid so analogous to muriatic acid, that it was for some time confounded with that body. His object in these new experiments, was to examine minutely whether any peculiar matter was separated from the diamond during its combustion, and to determine whether the gas, formed in this process, was precisely the same in its minute chemical nature, as that formed in the combustion of common charcoal. By his experiments at Florence, he satisfactorily accomplished his wishes, and established beyond a question the important fact, that "the diamond affords no other substance by its combustion than pure carbonic acid gas; and that the process is merely a solution of diamond in oxygen, without any change in the volume of the gas."
As one of the principal objects in these researches was to ascertain whether water was formed during the combustion of the diamond, with a view to decide the question of the presence of hydrogen, every possible source of fallacy was excluded. In one experiment there was an evident deposition of moisture, but it was immediately discovered to have been owing to the production of vapour from a cork connected with a part of the apparatus, during the combustion.
In the progress of this research, he ascertained a fact, the knowledge of which must not only be considered as important to the present enquiry, but as highly valuable in excluding error from our reasonings upon the delicate results of analysis[13]—I allude to the extremely minute quantity of water which becomes perceptible by deposition on a polished glass surface. He introduced a piece of paper weighing a grain into a tube of about the capacity of four cubical inches, the exterior of which was gently heated by a candle; immediately a slight but perceptible dew appeared in the interior of the upper part of the tube; the paper taken out and directly weighed in a balance, sensible to 1-100th of a grain, had not suffered any appreciable diminution. If, then, on burning 1·84 grains of diamond in oxygen gas, not even a barely perceptible dew was produced, we may consider it as fully proved that this gem cannot contain hydrogen in its composition: but to render the demonstration, if possible, still more complete, he kept a small diamond, weighing ·45 of a grain, in a state of intense ignition by the great lens of the Florentine Museum, for more than half an hour, in chlorine; but the gas suffered no change, and the diamond underwent no alteration either in weight or appearance: now had the smallest portion of hydrogen been developed, white fumes of muriatic acid would have been visible, and a certain condensation of the gas must have taken place.
The general tenor of his results was equally opposed to the idea of the diamond containing oxygen; for, in such a case, the quantity of carbonic acid generated by the combustion, would, on comparison, have indicated that fact. By combining the carbonic acid with lime, and then recovering the gas from the precipitate by muriatic acid, he found its proportion to be exactly that which was furnished by an equal weight of Carrara marble similarly treated.
The enquiry next proceeds to the examination of other forms of carbonaceous matter, such as plumbago, charcoal formed by the action of sulphuric acid on oil of turpentine, and that produced during the formation of sulphuric ether; and lastly, the common charcoal of oak.
In all these bodies, he detected the presence of hydrogen, both by the water generated during their combustion, and by the production of muriatic acid, when ignited in chlorine. The chemical difference then between the diamond and the purest charcoal, would appear to consist in the latter containing hydrogen; but Davy very justly asks whether a quantity of an element, less in some cases than 1-5000th part of the weight of the substance, can occasion so great a difference in physical and chemical characters? "It is certainly possible," says he, "yet it is contrary to analogy, and I am more inclined to adopt the opinion of Mr. Tennant, that the difference depends upon crystallization." In support of such an opinion, he farther adduces the fact, that charcoal after being intensely ignited in chlorine, is not altered in its conducting power or colour: in which case the carbon is freed from the hydrogen, and yet undergoes no alteration in its physical properties.
One distinction supposed to exist between the diamond and common carbonaceous substance, the researches of Davy have certainly removed, viz. its relative inflammability; for he has shown that the former will burn in oxygen with as much facility as plumbago.
The experiments, then, which Davy conducted at Florence and Rome, have removed several important errors with regard to the nature of carbonaceous substances; and though they may not encourage the labours of those speculative chemists who still hope to illustrate the old proverb,[14] by manufacturing diamonds out of charcoal, they certainty show that they are less chimerical than those of the wild visionaries who sought to convert the baser metals into gold.
While at Rome, Davy was engaged for several successive days in the house of Morrichini, for the purpose of repeating with that philosopher his curious experiments on magnetisation. Mr. Faraday was charged with the performance of the experiments, but never could obtain any results.
On the 8th of May he entered Naples, and remained there for three weeks, during which period he visited Mount Vesuvius, and the volcanic country surrounding it. He describes the crater, at this time, as presenting the appearance of an immense funnel, closed at the bottom, with many small apertures emitting steam; while on the side towards Torre del Greco, there was a large aperture from which flame issued to a height of at least sixty yards, producing a most violent hissing noise. He was unable to approach sufficiently near the flame to ascertain the results of the combustion; but a considerable quantity of steam ascended from it; and he says, that when the wind blew the vapours upon him, there was a distinct smell both of sulphurous and muriatic acids, but there was no indication of carbonaceous matter from the colour of the smoke; nor was any deposited upon the yellow and white saline matter which surrounded the crater, and which he found to be principally sulphate and muriate of soda, and in some specimens there was also a considerable quantity of muriate of iron. At this period, when the volcano was comparatively tranquil, he observed the solfaterra to be in a very active state, throwing up large quantities of steam, and some sulphuretted hydrogen.