io'2 HISTORICAL, INTRODUCTION TO CHEMISTRY CHAP. 



Priestley. In a paragraph headed " Fixed Air has an Acid 

 Taste," Bergman writes : 



" As this air is in form of an elastic vapour, it can hardly 

 be tasted by itself, at least distinctly ; but if it be united with 

 water, which is in itself void of flavour, being accumulated 

 and rendered less volatile by this union, it readily affects 

 the tongue with a weak but agreeable acidity. This is the 

 real spirit of the cold mineral waters, which undoubtedly 

 occasioned them to be called acidulous ; and by means of 

 which, together with a due proportion of suitable salts, we 

 may perfectly imitate the Seltzer, Spa, and Pyrmont waters. 

 Such artificial waters I have now been using for eight vears 

 with signal advantage" (JSssays, p. 12). 



(3) Its power of imparting to litmus a transient red 

 coloration : 



" Syrup of violets, and such other blue vegetable juices 

 as I have hitherto tried are not reddened by fixed air, 

 the tincture of tournsole [litmus] is of all known tinctures 

 most easily acted on by acids, therefore the slightest 

 vestiges, which cannot by any other means be discovered, 

 are by this tincture easily detected " (Essays, p. 16). 



Thus if water " be tinged with tournsole to a perfect blue, 

 when fixed air sufficient to fill about the 1/50 of the vessel has 

 passed through it, it will be manifestly red ... In like 

 manner one part of water, saturated with fixed air, makes 50 

 parts of the above tincture distinctly red. This change of 

 colour, however occasioned by the fixed air, soon disappears 

 in an open vessel, particularly if it be exposed to heat, or 

 the rays of the sun ; a circumstance which indicates the 

 volatile nature of the acid that produces the change 

 (Essays, pp. 14-15). 



(4) Its power of combining with mild alkalis to form 

 neutral crystalline compounds, as discovered by Black and 

 confirmed by Cavendish (pp. 69 and 108). 



(5) Its power of dissolving chalk and magnesia, dis- 

 covered by Cavendish (p. 103). 



(6) Its power of dissolving iron (giving rise to artificial 



