258 CASES ABSORfifcD BV CHAKCOAL. ' 



Inconveniences of the Injlrument. 



Inconveniences. VIII. Nothing is more certain than the propei fy pofTefled 

 by charcoal of abforbing a more or lefs confiderable quantity 

 of gas, and of atraofpheric air ; the experiments which I have 

 publiflied are a complete demonftration of it; but I then made 

 ufeof white glafs tubes, hermetically fealed at the upper ex- 

 tremity, and paiTed the charcoal under the mercury, by which 

 its interftices were filled with that fluid. At prefent, on en* 

 deavouring to make comparative experiments with my new 

 machine, which I was defirous fliould be a perfe6l eudiometer, 

 1 found that the cavity G, in which I placed the charcoal, and 

 Ihe fmall fpace between the two keys of the (lop-cock, con- 

 tained air ; that by the heat of the charcoal this air was di- 

 lated, and partly expelled, and that confequently, on opening 

 the key of the (lop-cock, a fmall abforption was occalioned 

 "by the vacuum thus made, and the mercury then rofe. To 

 Convince myfelf, and to difcover at what quantity this abforp- 

 tion might be calculated, the apparatus being arranged as for 

 Other experiments, inftead of charcoal I introduced a piece of 

 fed-hot pumice-ilone (I gave the preference to this fubdance, 

 becaufe I was certain that it had not the property of abforbing 

 the air, and alfo that it would not take a greater heat than 

 charcoal) ; on turning the key an abforption of an inch took 

 place, and from feveral experiments, I am convinced that this 

 quantity is uniform within half a line. 



Congestion of the I?)Jiru?nent. 



Correflion of IX, From the experiment made with the pumice-ftone, it is 



the inftrument. obvious that if an inch is deduded from the total abforption, 



the real abforption produced by the charcoal will be obtained. 



Befides, in comparative experiments this imperfe6lion caufes 



no error; becaufe as they are all condu6led in equal circum- 



' fiances, the variations produced, either by the different gafes, 



or by the different quality of the charcoal made ufe of, will be 



always proportional, 



X. With this iuftrumept I made feveral experiments whicb 

 I (liall now rela.te* 

 Or(J«T of the ex- XI. I began by examining the a6lIon of charcoal when cold, 

 Venmcat*. ^^^ afterwards at a low heat ; I then examined the abforption 



by 



