ABSORPTION OF GASES BY SOLIDS. 317 



ind a small piece of charcoal be introduced, the 

 mercury rises upwards into the vessel, in consequence 

 rf the external pressure, and finally fills it completely. 



Carbonic acid is made by placing a quantity of small pieces of 

 narble of the size of a hazel-nut or pieces of unburned limestone or 

 iven chalk into the apparatus for making gas, shown in fig. 154 ; 

 he bottle is half filled with water, closed by the cork, and 

 ivdrochloric acid is poured in through the funnel. The gas is 

 generated with effervescence, similarly as in the case of hydrogen. 

 Co prevent the liquid from frothing a small quantity of acid is poured 

 n at first, 10 or 20 ec , and more of it is added when the evolution of 

 he gas becomes slower. 



O 



The vessel shown in fig. 162 is filled nearly to the edge with 



aercury and the liquid then poured through a wide funnel into 



n empty bottle, from which a small test-tube (about 8 cm long 



nd ~L cm wide) is quite filled and the remainder poured back into the 



*on vessel ; the reason of this manipulation is to bring into the 



on vessel a quantity of mercury which will just allow the vessel to 



aceive afterwards in addition the contents of the test-tube. The 



3st-tube is now closed by the finger, inverted, and its mouth placed 



elow the surface of the mercury. It should now be carefully 



amped in the retort stand, which has been set ready for the purpose, 



> that its mouth may be a few millimetres from the bottom of 



ie vessel. Beneath the lower aperture of the test-tube is placed the 



id of a short piece of glass tubing which has been drawn out into a 



)int, the other end of which is connected by a piece of india-rubber 



biug with the tube issuing from the gas -generating apparatus. 



ie carbonic acid rises in small bubbles to the top of the test-tube 



:.d fills it gradually, while the mercury flows out. 



The gas which first issues from the apparatus should not be con- 



cted into the test-tube, for it is mixed with air. Fill a bottle, about 



e same size as the gas apparatus, with water, and dip its mouth 



ilow the surface of water contained in a basin. Attach a small 



;'iss tube to the tubing of the apparatus and place it underneath the 



} >uth of the inverted bottle. Fill the bottle with gas, and you may 



t?n safely assume that all the air has been swept out of the appar- 



f is and that the gas which now issues is pure carbonic acid. For 



< ivering the gas into the test-tube a different piece of tubing from 



tit which served for the delivery of the gas into the bottle should 



I used, for the latter is wet, and water would thus be introduced 



i'o the test-tube, and would fill the pores of the charcoal and so 



uerfere with the success of the experiment. 



