ON MANGANESE 89 



through which the want of phlogiston is constantly supplied. 

 This is proved by the constant effervescence which takes 

 place as long as the glass globule remains in a liquid state 

 upon the charcoal (Sec. XXIL). 



SECTION XLIIL 



From this explanation it plainly appears how manganese 

 purifies glass. If the colour of the glass were to depend 

 on a kind of carbonaceous matter, it would be imprudent 

 to add more of the manganese than is required to saturate 

 the phlogiston of that matter, for the natural colour of the 

 manganese would certainly be produced. With regard to 

 the green colour of the common bottle-glass, I was not yet 

 fully convinced that it depended upon iron, and I therefore 

 took this opportunity to examine whether I could separate 

 iron from it. (a) I melted green glass with alkali of tartar 

 by the blowpipe upon a piece of the same substance (for 

 in using a crucible one may be deceived by the iron it 

 contains). Upon this mass I poured a large quantity of 

 pure muriatic acid, and added some drops of lixivium san- 

 guinis, when the mixture grew a little bluish, and there 

 is consequently some iron in the green glass. (b) This 

 iron must be present nearly in a metallic form; for the 

 calx of iron renders glass always yellowish. It is therefore 

 the phlogiston to which the green colour is owing. As long 

 as the iron retains part of its phlogiston, it also gives such 

 a green colour to its solution in acids. But if manganese 

 be added to such a solution, the green colour disappears 

 during digestion, and a yellowish one is seen in its place. 

 Mtrous acid likewise takes away this green colour during 

 digestion, (c) If nitre be added to green glass in fusion, 

 the green colour also disappears. Manganese added in due 



