xi MURIATIC ACID AND CHLORINE an 



oxygen are now generally described as PEROXIDES ; thus 

 pyrolusite is sometimes called PEROXIDE OF MANGANESE, but 

 is more often distinguished by its colour as BLACK OXIDE OF 

 MANGANESE. The composition of pyrolusite as a compound 

 of a calx or earth with oxygen is very similar to that of chalk 

 as a compound of lime with fixed air. But whilst the fixed 

 air contained in chalk is set free even by the weakest acids, 

 the oxygen of pyrolusite is only driven off by heating 

 strongly with oil of vitriol. Another method of separating 

 oxygen from pyrolusite was described in 1785 by Berthollet 

 (A.C.R. XIII. 1 6), who heated the mineral (compare the 

 burning of chalk to lime) and thus extracted from it " a large 

 quantity of vital air " ; in this process the pyrolusite " lost an 

 eighth of its weight." 



Scheele's discovery of chlorine (1774). The behaviour of 

 " manganese " towards spirit of salt differed from its behav- 

 iour with other acids, in that the mineral dissolved in the 

 cold acid without the addition of any reducing agent to 

 take away the oxygen. In the cold solution, as Scheele 

 pointed out, "the manganese has here attached itself so 

 loosely to the acid that water can precipitate it, and this 

 precipitate behaves like ordinary manganese." But when 

 the mixture was warmed, a pungent gas was liberated, and a 

 colourless solution was obtained, similar to those produced 

 by other acids with the help of a reducing agent. Scheele 

 describes his discovery of the new gas as follows : 



" One ounce of pure spirit of salt was poured on half an 

 ounce of finely-ground manganese. After this mixture had 

 stood one hour in the cold, the acid had assumed a dark 

 brown colour. Part of this solution was poured into a 

 bottle, which was left open in a warm place. The solution 

 gave off a smell like warm aqua regis, and after a quarter of 

 an hour it was clear and colourless as water, and the smell 

 was gone." 



p 2 



