5io HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



acids, since in the manufacture of glass it expels the acid 

 from sodium sulphate and converts it into a glassy silicate. 



Balanced actions in gases. Whilst a balanced action in 

 a liquid is often disturbed by the vaporisation of one of the 

 products, this action may be checked by increasing the 

 pressure (and thereby the "active mass") of the gas 

 (Chemical Statics, I. 188). 



An interesting example of a balanced action in which 

 the gas-concentrations are of dominant importance is shown 

 by the 'equation 



Fe 3 4 + 4 H 2 3 Fe + 4 H 2 0, 



The solid iron and its solid oxide have a constant activity, 

 but the action can be influenced strongly by varying the 

 proportions of hydrogen and steam. The strong tendency 

 for the hydrogen to reduce the oxide is shown by the upper 

 arrow; but when, as in Lavoisier's experiment (Ch. VII, 

 p. 1 1 8), large quantities of steam are passed over the solid the 

 slight back-action shown by the lower broken arrow becomes 

 important, and condensation of the steam immediately 

 reveals the small quantities of hydrogen that have been 

 produced. 



SUMMARY AND SUPPLEMENT. 



Mayow, in 1674, showed that nitric acid could be expelled 

 from nitre by oil of vitriol and suggested that this was due to 

 the fact that oil of vitriol was " more concordant " with the 

 alkali than nitric acid. 



Baume, in 1760, showed that this action could be reversed 

 and that nitre could be precipitated by adding nitric acid to 

 vitriolated tartar (potassium sulphate). The reversal of the 

 action is shown in the equation 



Cold 



Bergman (1775), following Geoffroy (1718) and Stahl (1720),; 

 drew up tables of " Elective Attractions," showing the order of 



