Still a degree of uncertainty was at tached to the subject, and to remove this, Lavoisier, associating with him in the investigation Macquer and Cadet, undertook some experiments. They first ascertained, that in close vessels the dia mond does not evaporate : having expos ed 19f grams in a hued earthen retort, connected with a glass receiver, and the jointing secured, to a very strong heat, the loss of weight amounted only to about two grains ; yet the heat applied was much higher and continued much longer, than would have been necessary to dissi pate the entire quantity of diamond in the open air ; and repeating the experiment of the jewellers, they found, that when carefully imbedded in charcoal powder, from which the air was excluded, the most violent heat produced no change in the diamonds submitted to trial. They were therefore disposed to conclude,that the dissipation of the diamond,when heat ed in the open air, was owing to its com bustion. (Memoirs de i'Acad. des Sci ences, 1772, p. 350.) Facts similar to these were established by a second series of experiments performed by Darcet and Rouelle. And Lavoisier, in another me moir, demonstrated more decisively the combustibility of the diamond, and dis covered the product of its combustion. When suddenly heated by a lens,he found it to decrepitate, and to be thrown into Small fragments; when heated more slowly. it was dissipated without this de crep,tation. When heated by a lens, in a glass vessel placed over water, it was still dissipated, and in the.first experiment no sensible product was obtained; in a se cond he observed,that when the heat was less powerful, the surface of the diamond became black, and was sensibly covered with a thin coating of charcoal. In a sequent he found that the1 air of the vessel in witch the was made was diminished in volume to the extent of about eight cubic inches i 60 ; on pouring into this residual air lime water it became milky, as it would have done from exposure to air in whic charcoal had been burnt ; and by sub jecting it to different trials, this milkiness was found to be owing to the presence o carbonic acid, which of course had been produced during the combustion of the diamond. The same results were obtain ed on heating the diamond in a glass ves sel containing common air, placed over quicksilver. Lavoisier drew the conclus sion, that the diamond is a combustible body ; and that, as objects of chemistry, there exists a great analogy between it and charcoal.
Some years afterwards Guy ton sheaved that the diamond is consumed when heat ed with the nitrate of potassa, and affords carbonic acid. This experiment was re peated with more precision by Mr. Te nant. He exposed to a strong red. heat, for an hour and a half, in a gold tube,two grains and a half of small diamonds with a quarter of as ounce of nitrate of potassa. The salt was decomposed,and,on examin ing the residuum,tlie potassawasfoundto have attracted carbonic acid, while the diamonds were entirely consumed. The quantity of carbonic acid was attempted to be ascertained, by adding to the solu tion of the residuum in water a solution of muriate of lime: a precipitate of carbo nate of lime was formed ; from this the carbonic acid was disengaged by muriatic acid, and it occupied a space equal to about 10.1 ounce measures of water. This, according to Mr. Tennanr's calcu lation, was about the quantity that ought to have been obtained from two and a half of charcoal combined with Oxy gen; and he therefore concluded that the diamond is charcoal, and differs from that substance only in its crystallized form.
Guyton at length investigated the sub ject with that precision, which was ne cessary to fix our opinion as to the na ture of the diamond. The diamond, an imperfect octahedron, was placed on a small porcelain crucible, elevated in a jar filled with oxygen gas, ascertained to be pure, placed over mercury. The • concentrated solar rays were thrown on the diamond by a large lens : it 'appear ed at first farinaceous, and was after wards sensibly blackened on its surface : the appearance of combustion was ex- tremely faint, and when it had begu% did not continue if the solar heat was withdrawn. Afterwards, when a more powerful lens was employed, the com bustion was more evident ; the diamond first became black, and of a coally ap pearance ; an instant after it became brilliant, and at some points appeared to boil ; it gradually diminished, and the application of the solar heat was repeat ed at different times until it was entirely consumed. The quantity of carbonic acid which had been formed was ascer tained by introducing a solution of ba rytes in water, and the unexpected re sult obtained, that the quantity was much greater than what would have been formed by the combustion of the same weight of charcoal as of diamond. Twen
ty-eight parts of charcoal in burning com bine with 72 of oxygen, and form 100 of carbonic acid ; while the same weight of acid, according to Guyton's experiment, is formed, from the combustion of 17.88 of diamond, which combine therefore with V.12 of oxygen. Guyton concluded, therefore, that it is not merely by its co lour, weight, hardness, transparency, and other sensible qualities, that the diamond differs from charcoal ; neither does the difference depend on the state of aggre gation, nor are the distinctive properties of charcoal owing to the two hundreth part of residue which .it leaves in the form of ashes, or to the small quantity of hydrogen which it may contain ; but to its oxydation, diamond being the simple base of which charcoal is the oxide.
A striking fact with regard to the oxy genizement of the diamond, is the high temperature which is requisite to its taking place. It appears, from Guyton's statement, to be charred at about the temperature of 18 or 20 of Wedgwood's scale, (3417 or 3677 of Fahrenheit's) and at about 30 (4977°) it burns with a feeble flame ; nor does it even in oxy gen gas produce as much heat as to support its own combustion. This is no doubt owing to the very strong cohesion exerted between its particles. (Memoir by Guyton, Annales de Chimie, tom. 31.; or Abstract of it in Nicholson's Journal, 4to. viii. p. 298.) 'Flte appearances attending the com bustion of the diamond have been observ ed with perhaps more accuracy by Sir George Mackenzie, and the temperature requisite has been stated by him as less high. A diamond cut and polished, when introduced into a muffle previously heat ed red hot, soon acquired the same red ness as the muffle, but in a few minntes more became distinguished by a bright glow, and began to consume. A piece of plumbago placed beside it exhibited a si milar luminous appearance, but it began at a lower temperature. When the air was excluded from the muffle, both lost their brightness; but it returned on the admission of the air, and was much in creased by blowing on them with a bel lows. To ascertain at what temperature the combustion of the diamond took place, one of the pyrometrical pieces of Wedgwood was placed with it in the muffle. When both were perfectly red throughout, the pyrometer was with drawn, and indicated 13° of Wedgwood's scale. They were replaced, and the heat Increased until the glow appeared; it was kept at this as equal as possible until the diamond was consumed ; the pyrometrical piece then indicated 14°, and in another diamond the heat requi site to produce the glow and consume it, was 15°. These experiments are evi dently the most accurate that have been made to ascertain this point ; and in deed the temperature assigned by Guy ton was rather from conjecture than ex periment. Although they shew that a less elevation of temperature is requisite for the combustion of the diamond than was supposed, they still prove it to be much higher than that which is requisite for the combustion of charcoal.
Sir George Mackenzie, likewise, re peated and confirmed an experiment of the French chemists, in which a piece of soft iron was converted into steel by be ing heated with diamonds, in the same manner as it would have been by being heated in the usual manner with charcoal powder ; and his experiments are more satisfactory, as having been made with diamond in its purest state. (Nicholson's Journal, 4to. 'Vol. iv. p. 103.) • The diamond is scarcely acted on by any other agent than by oxygen, at an elevated temperature. Bergman states an experiment, from which it would ap pear to be capable of being partially oxyded by sulphuric acid ; this acid, when poured on the diamond powder, previous ly freed from impurities by disgestion with nitro-muriatic acid, and evaporated to a small quantity, becoming black, and depositing small pellicles, which take fire on the approach of flame, and are con sumed. The other acids, according to his observation, exert no sensible action on it ; nor does it appear from the expe riments made with the soda (tile mixture of soda and of diamond powder being ex posed toll very strong heat) that. it had suffered any chemical change from the action of the alkali ; for though a minute portion of earthy matter appeared to be produced, this might probably be deriv ed from the various agents which were employed in the experiments. Essays, vol. ii. p. 118.)