WHITE LEAD AND ZINC WHITE. 143 



phuretted hydrogen, by putting it in a chip hat-box with a small 

 hole cut in the side through which is introduced the delivery pipe of 

 a sulphuretted hydrogen generator. In a very short time the side 

 painted with lead will be turned black, whilst the other side 

 painted with zinc white will be unchanged, provided the zinc white 

 be pure (vide also Expts. 14, 129). 



Separation of Gases by Solvent Chemical Action. 



A very frequent operation, both in laboratory experiments 

 and in chemical manufactures, is to treat a mixture of gases with 

 some solution that will act chemically upon one or more of the 

 gases present, absorbing them in consequence, and leaving the 

 others undissolved and unacted upon. Thus, in the purifica- 

 tion of coal-gas for ordinary lighting purposes, methods of this 

 description are largely used. The following experiments illustrate 

 this. 



Expt. 147. To separate a Mixture of Carbon Dioxide and Air. 

 Place a jar mouth upwards on a table, and let the delivery pipe of 

 a generator of carbon dioxide (Expt. 100) pass into it, so that some 

 of the gas may pass into the jar ; or otherwise, pour into the jar 

 some of the heavy carbon dioxide gas evolved from an aerated 

 water siphon on running some of the fluid into a tumbler, &c., as 

 directed in Expt. 71; in either case you will have a mixture of 

 carbon dioxide and air in the jar. The presence of the former gas 

 may be proved by the special test described more fully in Expt. 

 152, consisting of pouring out a little of the gas from the jar (as 

 in Expt. 101, fig. 58) into another one containing some lime-water, 

 closing this second jar with a cork, and shaking up vigorously, when 

 the lime-water will be rendered milky if carbon dioxide be present. 

 Now pour into the first jar a wine-glassful of caustic soda solution, 

 quickly cork up with a greased cork, and shake thoroughly; turn 

 the jar upside down, and remove the cork with the mouth under 

 water, so that the atmospheric pressure may force water into 

 the jar (as in Expt. 77), to supply the place of the absorbed gas. 

 On re-corking the jar under water and removing it, it will be 

 found, firstly, that on applying the lime-water test as above 

 described no milkiness is produced, showing that the admixed 

 carbon dioxide has been removed by the action of the caustic soda 

 solution ; and, secondly, that a lighted taper introduced into the 

 jar at the end of a wire will burn freely, showing that the air 

 present in the mixture of gases has not been similarly absorbed 

 and removed. 



