164 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



acid solution and combines with alkalis to form salts. 

 Stahl (1702) describes the process of collecting the acid 

 fumes with the help of linen or tow drenched in alkali ; he 

 states that the acid vapour is set free again by the action of 

 oil of vitriol, and that it is the weakest of the mineral acids, 

 though stronger than vinegar. In many respects the " sulphur- 

 ous acid " from burning sulphur resembles the " carbonic 

 acid " from burning charcoal ; but it differs in its obvious 

 acidity, in the greater solubility of the gas in water, and in 

 its power of bleaching coloured substances, such as red 

 rose leaves. 



Stahl showed that the salts of the ' volatile " sulphurous 

 acid, prepared from burning sulphur, could be converted into 

 salts of the "fixed" oil of vitriol by exposing them to the 

 air. A similar experiment is described by Cavendish (A.C.R- 

 III. 9-11), who collected the fumes of burning sulphur 

 in milk of lime, obtaining a soluble salt which was converted 

 by exposure to the air into selenite or gypsum, the ordinary 

 sulphate of lime. In order to distinguish it from sulphuric 

 acid, the French chemists described Stahl's volatile acid as 

 SULPHUROUS ACID, and its salts as SULPHITES. The name 

 sulphurous acid is, however, now restricted to aqueous 

 solutions of the gas, the pungent gas itself being dis- 

 tinguished as SULPHUROUS ANHYDRIDE. 



Priestley (1774) prepares a soluble gas from oil of 

 vitriol. On account of its great solubility, the gas produced 

 by burning sulphur in air cannot be collected over water. 

 It was therefore not isolated until Priestley had developed 

 Cavendish's method of using mercury to confine those gases 

 which were too soluble to be stored over water. 



Priestley prepared the gas in 1774 by heating oil of vitriol 

 with olive oil, and called it VITRIOLIC ACID AIR. He also 

 attempted, but without success, to separate it from oil of 

 vitriol by heat alone. In the course of this experiment 

 some mercury was drawn back into the hot acid, 



