522 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Crystals of sal-ammoniac were heated in a glass tube (Fig. 

 56), 13 mm. wide, over a plug of asbestos 20 mm. in length ; 

 a current of hydrogen was maintained on each side of the 

 asbestos. After the sal-ammoniac had vaporised, the hydro- 

 gen A from that side of the plug was found to contain 

 free hydrochloric acid and turned blue litmus-paper red, 

 whilst the hydrogen B from the other side of the plug con- 

 tained free ammonia and turned red litmus-paper blue. 

 This separation of ammonia and hydrogen chloride depends 

 on the fact that the ammonia, being a lighter gas than 

 hydrogen chloride, diffuses more quickly through the 

 asbestos, rendering the gas on the other side alkaline and 

 leaving behind a gas containing an excess of acid. Such 

 a separation could not take place, however, if the vapour 

 consisted entirely of undecomposed sal-ammoniac, since 

 this would then diffuse as a whole and would not be resolved 

 into its constituents. 



It should be noted that, as ammonia and hydrogen 

 chloride recombine immediately in the cold, the products 

 of dissociation cannot be fixed by the method of sudden 

 chilling, as used by Grove and by Deville. 



Variation of vapour-density with temperature dis- 

 covered by Cahours (1844). Another anomaly which 

 found a simple explanation in Deville's theory of dissociation 

 is the variation of vapour-density with temperature, dis- 

 covered by Cahours in the cases of acetic acid (Comptes 

 rendus, 1844, 19, 771-773) and phosphorus pentachloride 

 (Ann. Chim. Phys., 1847, \.Z\ 20, 369-378). 



For acetic acid, Cahours found the following values : 



Temperature... 145 152 219 231 C. 



Density 275 272 2-17 2*12 (relative to air) 



By diluting the vapour with hydrogen Playfair and Wanklyn 

 (Journ. Chem. Soc., 1862, 15, 153) were able to measure 

 its density below the normal boiling-point (119) and found 



