224 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



In the case of sulphuretted hydrogen, which was known 

 to contain its own volume of hydrogen, the volume 

 changes were similar : 



Sulphuretted + chlorine > hydrogen chloride + sulphur, 

 hydrogen 

 i vol. i vol. 2 vols. solid 



Davy showed further that hydrogen chloride liberates half 

 its volume of hydrogen when the chlorine which it contains 

 is absorbed by a metal. Thus in : 



" the decomposition of muriatic acid gas, by heated tin 

 and zinc, hydrogen equal to about half its volume was 

 disengaged, and metallic muriates the same as those pro- 

 duced by the combustion of tin and zinc in [chlorine] 

 resulted" (A.C.R. IX. 28). 



" With potassium, in experiments made over very dry 

 mercury, the quantity of hydrogen is always from 9 to 11, 

 the volume of the muriatic acid gas used being 20 " 

 (A.C.R. IX. 27). 



" When mercury is made to act upon i in volume of 

 muriatic acid gas, by Voltaic electricity, all the acid 

 disappears, calomel is formed, and about 0-5 of hydrogen 

 evolved" (A.C.R. IX. 27). 



This decomposition of the gas by sparking in presence of 

 mercury was discovered by Priestley (Experiments on Air, 

 1775, II. 239) and studied in detail by Henry in 1800 

 (Phil. Trans. 1800, 191). It is of special interest as an 

 example of a reversible change. Usually the action 

 proceeds as shown by the upper arrow in the equation : 



Hydrogen + chlorine ^ hydrogen chloride. 



But chlorine acts upon mercury so strongly that a very 

 slight decomposition of the hydrogen chloride, as shown by 

 the lower arrow, is sufficient to corrode the mercury and 

 ultimately to remove the whole of the chlorine from the gas, 

 leaving a residue of pure hydrogen. The compound formed 



