xx DISSOCIATION 529 



for the formula NH 4 C1. A similar value, 28*8, was 

 obtained (Trans. Chem. Soc., 1898, 73, 426) by Dumas' 

 method, the dried salt being vaporised into a dried bulb 

 protected from the moisture of the atmosphere by a long 

 tube of phosphoric anhydride. 



Baker also found that ammonia and hydrogen chloride, 

 after being dried during a week by contact with purified 

 phosphoric anhydride, 1 did not contract on mixing and 

 gave no fumes of ammonium chloride. But " If a trace of 

 moisture be admitted to the mixture of dried gases, dense 

 white fumes are at once produced, and the mercury rushes 

 up into the tube" (Trans. Chem. Soc., 1894, 65, 615). 



Carefully-prepared quicklime and ammonium chloride, 

 after drying during 1 7 days, gave no ammonia when mixed 

 and heated, the ammonium chloride subliming unchanged 

 from the lime. But when the rest of the ammonium 

 chloride was mixed with lime, after allowing moist air to 

 enter the tube, ammonia was evolved freely on heating the 

 mixture. 



From such experiments it appears that moisture is 

 necessary both for the production of sal-ammoniac from 

 ammonia and hydrogen chloride and for its decomposition 

 by heat or by the action of quicklime. 



Influence of moisture on the dissociation of calomel. 

 In 1900 (Trans. Chem. Soc., 1900, 77, 646) Baker showed 

 that calomel or mercurous chloride, dried by sulphuric 

 acid and vaporised in a Victor Meyer apparatus containing 

 nitrogen and heated to 444 C, gave a vapour-density 118-4 

 (hydrogen = i) ; similar material, dried during three weeks 

 in presence of phosphoric anhydride, gave in five experi- 

 ments the average value 217-4, approaching the value 

 235 required for the formula Hg 2 Cl 2 . Calomel vapour 

 dried by phosphoric anhydride did not amalgamate gold 



1 If the phosphoric anhydride is pure it does not absorb ammonia 

 that has been dried by potash. 



M M 



