176 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



sulphuret of iron, is to be granulated and put into a gas 

 bottle, to which dilute sulphuric acid is to be added, 

 after which the gas comes over plentifully. When the 

 sulphuret of iron is made in a crucible from iron filings and 

 sulphur, it seldom answers well ; it often gives hydrogen 

 mixed with the sulphuretted hydrogen" (New System, II. 

 450-451). 



This action is very similar to that whereby muriatic acid 

 is liberated from salt by oil of vitriol, and served to suggest 

 that sulphuretted hydrogen might be regarded as a 

 feeble acid. This view was put forward, in I796, 1 by 

 Berthollet (Ann. de Chimie, 1798, 25, 233-272), who showed 

 that the salts of silver and lead could be converted into their 

 insoluble sulphides by the addition of sulphuretted hydrogen, 

 just as silver is converted into its insoluble muriate by the 

 addition of muriatic acid, and lead into its insoluble sulphate 

 by the addition of sulphuric acid. These facts were seen to 

 be important when Lavoisier's oxygen-theory of acids became 

 the subject of criticism, for in spite of many endeavours no 

 oxygen could be detected in sulphuretted hydrogen itself, 

 nor in the sulphides derived from it. Berthollet sums up 

 his views as follows : 



" Sulphuretted hydrogen dissolved in water, reddens 

 tincture of litmus, litmus-paper and tincture of radish ; it 

 combines with the alkalis, baryta, lime and magnesia ; it 

 forms with these substances compounds which exchange 

 bases when mixed with metallic solutions ; it decomposes 

 soap, displacing the oil from the alkali . . . ." 



"Sulphuretted hydrogen possesses then all the properties 

 which characterise the acids. If several other common 

 properties did not demand a separate class of hydrogen- 

 compounds, it would undoubtedly be ranged amongst the 

 acids." 



" I will not recall here the observations which I have 

 opposed to the opinion of those who pretend that acidity is 



1 The paper was read before the Paris Academy, 21 Ventose, 

 An. IV. 



