CH. XVTTI SALT 263 



incomplete displacement of the air the gas will not completely 

 fill the jar, but it does so if unmixed with air.) Test the 

 solution with litmus papers. It is found to be acid. 



\v,- therefore find that the "salt gas " is soluble in water, 

 and forms an acid solution. 



EXPT. 279. Pass the gas through a solution of caustic soda 

 until the liquid has become slightly acid. Evaporate the solu- 

 tion to dryness, and observe that a white solid is left. Taste 

 this solid, and satisfy yourself that it is salt. 



EXPT. 280. Do a similar experiment, using caustic potash 

 solution in place of the solution of caustic soda. Observe 

 that, as before, you obtain a white solid, which has a taste 

 somewhat resembling that of salt. Heat this solid with a 

 little sulphuric acid, and observe that the " salt gas " is again 

 produced. 



Composition of Salt. Salt, therefore, results from the 

 interaction of the "salt gas" with caustic soda. The solution of 

 the "salt gas" is, in fact, the acid which is so frequently used 

 under the name of hydrochloric acid, and the last experiment 

 should be again performed with hydrochloric acid in place of 

 the "salt gas" in order to verify this statement. If the con- 

 tents of the flask or test-tube used in the preparation of the gas 

 be examined and allowed to crystallise, crystals are obtained 

 of a solid known as sodium sulphate, and we may state that 

 Common salt and sulphuric acid yield hydrochloric acid gas 

 and sodium sulphate. 



Composition of "Salt" or Hydrochloric Acid Gas. 



EXPT. 281. In the flask A (Fig. 132) place some salt and strong 

 sulphuric acid. Pass the "salt gas" so obtained over heated 

 copper oxide in the hard glass tube BC. Observe that water 

 collects in the test-tube D, and that the copper oxide is con- 

 verted into a green substance. Examine this green substance 

 and see that it also gives off the "salt gas" when acted upon 

 by sulphuric acid. 



As water is formed the ' * salt gas " must evidently contain hydro- 

 gen, and the simplest explanation of the experiment is that the 

 hydrogen of "salt gas" combined with the oxygen of the copper 



