-j^Q EDWARD DIVKKS. 



Expérimenta}. — If one of the cups is removed ii few minutes after 

 the operation has been started, much of the mercury is found m it as 

 a sublimate of tine i^loljules. mixed with only a Httle almost amor- 

 phous calomel, from which it would appear that the formation of the 

 calomel resuUs from reaction in the cup between th.' mercm-y in 

 va|)our and the active gases. 



On dipping- into the pot, uncovered during the ])rocess, a glass 

 rod with a drop of water hanging to it and then withdrawing it and 

 testing the remainder of the water with potassium iodide and starch 

 no chlorine can thus be detected. The same is true when a drop of 

 solution of potassium hydroxide is used. Xor can the slightest 

 odour of chlorine be detected in the N^apours issuing from the un- 

 covered pot. 



On passing air containing a little hydrochloric acid gas through 

 a tul)e in which mercury is freely boiling, sparkling <-aJomel is formed 

 close to and mixed up with the mercury. 



Red earth which has been used in tlie process turns moist red 

 litmus-paper blue, while fresh red earth is neuti-al. 



TheoreticaL — The nature of the materinls used and tlie observa- 

 tions gained bv the preceding experiments arc sufdcient to cstablisli 

 that the calomel is formed by a reaction Ijetween mercury vapour, 

 oxygen, and hydrochloric acid gas, in which along with mercurous 

 chloride, water is formed : — 



4 Kg + 4 HCl + 0, - 4 HgCl + 2 \{,0 



and that the formation takes place at a temperature near — above or 

 below — the boiling ])oint of mercury and nmch below that at which 

 «•alomel freely volatilises. The source of the hydrochloric acid is 

 cei-tainlv the magnesium chloride of the bittei'n and bay salt, which 

 heated in a moist atmosphere, even i]j the presence of sodium chloride, 



