the sun’s rays, by means of a powerful lens; but, considering the uncertainty of a favourable opportunity in this country, they resolved to employ an apparatus consisting of two mercurial gas-holders, with a tube of platina interposed between them in a horizontal position. and passing through a small furnace, by which the tube and its contents might be heated to any degree requisite for the combustion of the substance employed.
Into this tube the diamond or other variety of carbonaceous matter was introduced in a small tray, also of platina; after which, by opening a due communication with each gas-holder, the oxygen was made to pass freely over the surface, from one gasometer to the other, during the continuance of the heat, and subsequently examined by means of the eudiometer lately described by Mr. Pepys.
Having found that oxygen gas was liable, notwithstanding every precaution, to be deteriorated by keeping, the authors were careful to prepare it, on all occasions, within an hour or two of the time of using it, from the hyperoxygenized muriate of potash, Its purity was also ascertained before every experiment. The solution employed for this purpose was the solution of green sulphate of iron, saturated with nitrous gas; and lest any increase might have been occasioned by the extrication of this gas from the solution, the simple sulphate alone was subsequently employed, so that the residuum enabled them to determine exactly the quantity of oxygen contained in the gas.
Their charcoal was prepared from different kinds of wood, sawed into slips, and gradually heated in small crucibles covered with sand, and ultimately retained in a white heat for forty minutes. By this treatment
| Fir yielded | 18·17 per cent. |
| Lignum Vitæ | 17·25 per cent. |
| Box | 20·25 per cent. |
| Beech | 15 per cent. |
| Oak | 17·40 per cent. |
| Mahogany | 15·75 per cent. |
Having next examined the absorbent power of charcoal, and found that the weight which it gains by exposure to air is principally attributable to water, the charcoal to be employed in any experiment was always subjected to a red heat immediately before using it, and weighed as expeditiously as was consistent with accuracy.
Since the volumes of gas employed and produced would be influenced by temperature, as well as barometric pressure, the states of both barometer and thermometer were noted at the time of every experiment, and allowance was made by adding, or subtracting, 1480th part of every degree below or above 60°.
The exact weights of certain measures of oxygen gas, and of carbonic acid gas, were also carefully examined, by allowing a glass globe, previously exhausted and weighed, to receive a given measure of either of these gases from a gasometer. By the increase of weight acquired in each case, it was found that 100 inches of oxygen gas
