90 ESSAY V. 



proportion produces the same effect. If Mr. Westfeld had 

 not added nitre in his experiment, by which the green colour 

 of the glass was destroyed, he certainly never would have 

 effected any change of the glass, nor would he have ascribed 

 this property to the earth of alum, (d) But such a glass 

 purified by manganese should have become somewhat 

 yellowish ; for manganese was incapable, in my experiment, 

 of taking away the colour of glass tinged by calx of iron ; 

 and that a real calx of iron is present in this glass, though 

 to all appearance pure, I have already shown by the above 

 experiment (a). I performed the fusion upon a plate of 

 colourless glass. What can be the cause that it becomes 

 clear and colourless under these circumstances ? I believe 

 that the too small quantity of the calx of iron is the reason 

 why its natural yellow colour cannot be distinguished. It 

 is remarkable that the rays of light passing through such 

 an uncoloured glass when it is heated nearly red hot appear 

 yellow. Something like this is observed in red colours 

 that are not changeable by fire ; such as minium, crocus 

 martis, cinnabar, red precipitate, which, during the time 

 they are heated, appear of a black colour. Before I con- 

 clude, the following observations may deserve a place. 



SECTION XLIV. PRESENCE OF MANGANESE IN POTASHES. 



Chemists have often observed that alkaline salts, when 

 calcined, assume a bluish or greenish colour. The cause of 

 this has been said to be phlogiston present in the alkali ; 

 but I have constantly found some nitre in the fixed nitre 

 which was prepared with charcoal powder by a strong fire. 

 Its presence was immediately discovered by the aquafortis 

 smell that ensued on pouring some vitriolic acid upon the 

 mass. To the common opinion, therefore, there arises an 



