273 



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 con- 

 tents 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 open- 

 ing 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, satu- 

 rated with nitrous gas ; and lest any increase might have been oc- 

 casioned 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 Vitse 17'25 



Box 20-25 



Beech 15 



Oak 17-40 



Mahogany 15'75 



Having next examined the absorbent power of charcoal, and found 

 that the weight which it gains by exposure to air is principally at- 

 tributable 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 in- 

 fluenced 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, -r^th 

 part of every degree below or above 60. 



The exact weights of certain measures of oxygen gas, and of car- 

 bonic 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 



