86 ESSAY V. 



SECTION XLII. WITH GLASS FLUXES. 



All the phenomena hitherto observed with manganese 

 I have explained from the four general properties laid down 

 in Sec. xiv. ; and from the same principles I think the 

 phenomena it shows with glass fluxes may be deduced. A 

 colourless glass flux becomes constantly more or less red on 

 addition of manganese, according to the quantity (Sec. xxxvui. 

 (d)). If the flux be a little alkaline, the colour will approach 

 to violet (Sec. xxxvui. (a)). It is well known that arsenic, 

 gypsum, and calx of tin destroy the red colour in those 

 glasses, and thus render them clear and colourless. As to 

 the arsenic, the reason appears from its constituent parts 

 (Sec. xxxvui. (i)) ; for in this case the phlogiston of the 

 arsenic unites with the manganese that is dissolved in the 

 red glass, and thus takes away the colour ; and the acid of 

 arsenic unites with the alkali of the glass (Sec. xiv. (3)). 

 It is to be observed here that the experiment likewise 

 succeeds in a covered crucible, but it never succeeded with 

 me when it was made in that way with gypsum and calx of 

 tin ; but on adding powdered charcoal an effervescence 

 ensues, the red colour disappears, and the glass becomes 

 colourless. Thence it may be concluded that the ex- 

 periments made with a view to change the red colour were 

 made upon charcoal with the blowpipe ; and the phlogiston 

 of the charcoal is therefore the cause of the destruction 

 of the colour. The preceding effervescence is a necessary 

 consequence of the separation of the phlogiston from the 

 charcoal (Sec. xxn.). 



(a) If glass coloured red by manganese be mixed with 

 charcoal powder in a crucible and fused, the colour 

 disappears during the effervescence, without the addition of 



