432 On the Red Colour of Crystallized Felspar. [June, 



sponges, a fact which several naturalists, since the appearance 

 of my former paper, have appeared to doubt ; and lastly, which 

 is of much more consequence, it proves a considerable affinity to 

 exist between the Sponges (both the marine and Jiuviat He) and the 

 Gorgonia, which latter are known to be the habitation and pro- 

 duction of individuals belonging to the animal kingdom ; and 

 this greatly strengthens the idea of Ray, Lamarck, and others, 

 that the sponges are true corals, nearly allied to Anthipates and 

 Gorgonia, and not vegetables ; nor anomalous animals, like the 

 Infusoria. 



Article VI. 



On the Red Colour of Cr-ystallized Felspar. 



(To the Editors of the Annals of Philosophy,) 



GENTLEMEN, 



It has often occurred to me as a peculiarity in crystallized 

 felspar, that it exhibits a decided red colour, though analysis 

 points out no substance in its composition to which that colour 

 can be attributed ; and that this colour, after exposure to a strong 

 heat, entirely disappears, leaving a very pure colourless glass. 

 It cannot be attributed to iron even if a minute quantity should 

 exist in the felspar, because the colour of iron is not destructible 

 in this manner ; and if a few iron stains exist on the felspar, the 

 same heat which destroys the red colour of the crystal only 

 makes those stains stronger. Hence a question has suggested 

 itself, are chemists justified in supposing, as they uniformly do, 

 that the colour of a mineral may always be referred to some 

 specific colouring ingredient ? It appears to me that felspar is 

 an instance to the contrary, and an accidental experiment has 

 enabled me to show that substances may be produced, which, 

 though composed of perfectly colourless materials, shall, under 

 certain circumstances, exhibit a decided colour. If a chemist 

 should undertake to analyze the substance (of which I now send 

 specimens), he would find only Ume, alumina, silica, soda, and 

 boracic acid, and he would be much puzzled to account for the 

 red colour which it exhibits, particularly when he should find 

 that the colour might be made to appear and disappear at 

 pleasure, according to the degree of heat and of comminution to 

 which he might expose it. 



The method of producing the substance was as follows : — 

 The ingredients above indicated were coarsely mixed together 

 and exposed to a strong white heat, which produced a semi- 

 vitrified mass of a pure white ; a portion of this was finely 

 ground, and after exposure to a low red heat not above that of 

 melting silver, was found, much to my surprise, to have assumed 



