348 Professors W. B. and R. E. Rogers 



cork, flask, pipette and drying tube are coated with a varnish of 

 shell-lac, to protect the cork from infiltration, and to diminish the 

 hygrometric action of the surface generally. 



In using the instrument, a weighed quantity, say 40 grains, of 

 the carbonate is placed in the flask, and if it be in powder, enough 

 water is added to moisten it throughout. The pipette is then 

 charged with hydrochloric or sulphuric acid, by placing its open 

 end in a capsule containing this liquid, compressing the gum-elas- 

 tic bag, and then allowing its elastic expansion to pump the acid 

 into the bulb. Sometimes the pipette thus charged, when held 

 upright, permits the liquid slowly to accumulate in a drop, at its 

 beak, thus endangering a premature descent of the acid upon the 

 carbonate, which would vitiate the experiment. This is effectu- 

 ally prevented by lightly pressing the bag and allowing it to re- 

 coil so as to draw a short column of air into the tube, near the 

 end. The cork bearing the pipette and drying tube, being then 

 secured in the flask, the acid will remain supported without any 

 tendency to ooze out, and the instrument is in a condition to be 

 placed in the balance to be counterpoised. This done it must be 

 removed and placed upon a clean dry surface, near the balance, 

 where, gently pressing the bag, the acid is to be projected in a 

 fine stream on the carbonate, the action being regulated so as to 

 maintain a steady but not too violent effervescence. When all 

 the carbonate has been decomposed, which in the case of a marl 

 or limestone occupies but a few minutes, the acid still in the 

 bulb must be expelled into the flask. 



To remove the carbonic acid remaining in the instrument after 

 the completion of the reaction, a large drying tube must be an- 

 nexed to the end of that belonging to the apparatus, and the gum- 

 elastic bag must then be made to operate as a pump, by alternate 

 compression and dilation. Continuing this action for some time, 

 the gas is in great part if not wholly expelled, while the air en- 

 tering from without at each alternation of the movement, deposits 

 its moisture in the large drying tube, instead of adding it to the 

 weight of the apparatus, as it would were this appendage omit- 

 ted.* The second weighing is now performed, and the loss of 



* The error here adverted to, must also arise in the use of Rose's apparatus, 

 whenever, as Parnell directs, heat is applied to espel the carbonic acid remaining 

 at the close of the action. For the air entering as the flask grows cool, must in- 

 crease the normal weight by the amount of moisture it contains. 



