GASES ABSORBED BV CHARCOAL. 257 



V. I alfo think it right to propofe a fmall alteration: it is Propofed im- 

 that the extremity of the tube which is plunged into the bowl ^"^^ 



oF mercury, fliould be a litlle more bent, that the refidual gas 

 may be transferred with greater eafe; this flight variation^ 

 however, is not at all injurious to the inftrument, becaufe the 

 heights are raeafured by that of the mercury, the level of which 

 is indicated by the fcale prepared for that purpofe; its greateft 

 inconvenience will be to make a larger and more cylindrical 

 bowl neceflary, 



VI. This machine was made by J. B. Piana, a very fKilful ^Accuracy of th« 



^ o > . . , r , I - inftrument. 



mechanic; he conitructed it with fuch accuracy, that it pre- 



ferved the mod perfe6t vacuum for eight days, which is what 



I am unacquainted with in pneumatic machines. , 



Manner of vfmg the Inftrument * 



VII. Suppofe I am defirous of examining the abforption Method of 

 effeded in atmofpheric air by charcoal. The tube A B is filled ^ ^"^ *^* 

 with the atmofperic air of the place in which the machine is: 



the Hop-cock D is open, the upper one is clofed : I place the 

 cup of mercury under the tube, into which I infert a fmall 

 glafs fyphon, to caufe the air to take the level of the mercury* 

 If I make ufe of a gas, I pump out the internal gas of the tube 

 with a glafs fyringe, to bring the level within the tube to that 

 of the exterior mercury in the bowl. Afterwards, by means 

 of the fcrevv P, I move the fcale till its zero is alfo at the 

 level of the mercury. 



I make a piece of charcoal, weighing, half a dram or thirty- 

 fix grains, red-hot, and with tongs place it in the cavity G of 

 the large ftop-cock; I then turn Ihe handle F F, and in a few 

 minutes the mercury will be feen to afcend more or lefs in the 

 tube, according as the air is more or leCs pure. 



If it is wiihed to examine other gafes, or atmofpheric air 

 brought from another place, then clofe the key D, and fill the 

 tube A B with water; afterwards difplace the water with the 

 gas, in the common way by means of a transferrer; bring it 

 to the mercury, and, by means of bibulous paper, abforb the 

 water perfedly ; then wipe the outfide of the tube carefully 

 with warm flannel, and leave the inftrument at reft for two 

 hours : or otherwife make ufe of a mercurial apparatus, which — 



is preferable. 



Vol. IX. — December, 1804. S Incon- 



