24 



two constituent compounds ; but we have not done so in the former, as 

 it is probable that the water exists in two conditions — as basic water, 

 and as saline water. Until we shall have further evidence on this point, 

 however, we prefer writing the formulae of the balls as above. 



This power of combining in endless proportions appears to us not only 

 to show that hemimorphite and dicarbonate of zinc are truly isomorphic, 

 but that the isomorphism of carbon and silicon extends to carbonic and 

 silicic acids, and thus adds an additional support to the view that silicic 

 acid is a deutoxide. - 



Globular Radiated Ily drat ed Bisilicate of Zinc. — Among the minerals 

 which were procured at the mines of Florida, was a very peculiar variety 

 of silicate of zinc. It consisted of an irregular mass, sometimes distinctly 

 botryoYdal, of globular silicate, — the largest of the globules being about 

 a centimetre in diameter. Externally the globules were covered with 

 asperities, which were the ends of crystals disposed in a radiated acieular 

 form. The fracture of a globule showed the cleavage planes of these 

 crystals, arranged in a stellated form, and inclined to each other. These 

 cleavage planes were large, and appeared to be oo P oo, parallel to 

 which the cleavage is complete. Colour, yellowish-brown ; the fresh 

 surfaces being studded with a number of extremely small black points. 

 The cleavage planes had a mother-of-pearl lustre, which soon tarnished, 

 and became dull; sp. gr. 3-267. When freshly fractured, and a per- 

 fectly undecomposed fragment examined, its hardness was nearly = 5. 

 The mineral decomposed into a brownish-yellow, ochry substance with 

 remarkable facility. Its composition was found to be: — 



If we deduct the oxide of iron, and calculate the proportions in 100 

 parts of the oxide of zinc, silica, and water, alone, and compare the re- 

 sults with the theoretical composition deduced from the formula 2 ZnO, 

 Si02,II0, we shall find that the silica and water are too high in the 

 experimental results, and consequently the oxide of zinc too low. In 

 what state is the sesquioxide of iron in this mineral ? Is it in combi- 

 nation, or merely mixed mechanically with it ? The property which 

 silicate of zinc has of dissolving in a solution of caustic potash, sug- 

 gested itself at once as a means of answering this question. On treating 

 the mineral in the state of fine powder with a solution of potash in the 

 cold during several days, the whole of the silicate of zinc was dissolved, 

 and a reddish-brown powder was left ; the composition of which may 

 be represented by the formula 2Fe203,Si02,II0. This is exactly the 

 silicate of iron, which is found in Glauber's iron-tree, obtained by 



Oxide of zinc, . 

 Silicic acid, . . 

 Sesqui-oxide of iron, . 



Lime, 



Water, 



62-195 

 24-883 

 5-182 

 trace. 

 7-121 



99-381 



