2^Q GASES ABSORBED BY CHARCOAL. 



author *. Let us pafs over thefe unpleafant events, as well as 

 the alterations and corredions which I made to bring the in- 

 ftrument to its prefent ftate: the defcription which follows, and 

 the figure will be fufficient to fliow what it now is. 



Defcription of the Injirument, (Plate XIV.) 

 Defcription of ^^* A B is a tube of clear glafs, three lines and a half in 

 the iallrumcnt. diameter and eighteen inches high : to this tube a ftop-cock C 

 is fixed, to ftiut the communication, by means of the key D, 

 fo that the tube may be filled with water or mercury, and the 

 gas to be examined may be afterwards pafled in. To this (lop- 

 cock C a large one E is foldered, an inch and a half in dia- 

 meter, and almoft three inches long. The handle F F ferves 

 to turn the key of the flop-cock E ; this key has a hole, one 

 inch and three lines in depth, and feven lines in diameter, 

 (Fii^. 2.) in which the charcoal is placed. This key isperfe^ly 

 tight, and, when turned, communicates with the cock C, Fig, 

 3, not permitting the palTage of any air whatever. The large 

 llop-cock is enclofed in a wooden frame, and well fecured by 

 means of the nut H, which confines the lower end of the flop- 

 cock E behind the board; the lower extremity of the tube dips 

 into a velfcl of mercury II; the board M M keeps the inflru- 

 ment perpendicular; N N are pads which ferve to raife the cup 

 filled with mercury; O O is a fcale mounted on a flip of wood, 

 which is graduated to inches and lines. The endlefs fcrcw P 

 ferves to raife or lower this fcale, to bring it to the level of the 

 mercury in the cup. 



III. After having ufed the inftrument for fome time, it is 

 neceflary to unfcrew the keys to clean them, for, by ufe, dufl, 

 charcoal, or alhes will get in, which deranges the inflrument, 

 and fuflers air to pafs. 



IV. From feveral experinvents, I found that the flop-cock 

 D C, which is attached to the cryflal tube fliould be of fleel, 

 for on examining the refidual air, or filling the tube with mer- 

 cury to examine any gas, the mercury will at length attack the 

 copper. 



* The author does not name the perfon here alhided to ; and in 

 a note he gives a defcription of this firft imperfcft inftrument, re- 

 ferring to the figure fo publiflied. This ts omitted, becaufe of little 

 ■utility and not intelligible without that figure, N. 



V. I 



