56 



crucible tlieii to cool, the colour of the fragment changed successively 

 from light yellow to verdigris-green, then to bright orange-yellow, 

 which became paler as the cooling proceeded, until the fragments became 

 nearly white. On being heated, the chromatic scale was reversed, so 

 that the changes could be observed both during the heating and cool- 

 ing. The changes took place very rapidly, with a kind of phosphorescent 

 glow, which was very beautiful, and could be repeated apparently any 

 number of times with the same fragments. The latter circumstance 

 shows that the phenomenon can take place after the loss of the carbonic 

 acid. 



This remarkable molecular change is, perhaps, connected with the 

 hemi-morphism to which the pyroelectric properties of the silicate of zinc 

 are due; and as it is as well, if not better, seen in the specimens containing a 

 very large excess of carbonate of zinc, it would appear that dicarbonate of 

 zinc is likewise hemi-morphic. The circumstance that the change takes 

 place as well after the decomposition of the carbonate, may be urged 

 against this conclusion, it is true. I think, however, that the objection is 

 only apparent. "When the mineral is in fragments, the phenomenon is 

 best seen; when reduced to very fine powder, it almost wholly disappears. 

 'Now, when fragments of a mineral containing carbonic acid are heated, 

 the latter goes away, but the residue retains the original form ; and as 

 the pyroelectric properties are due to the relative ]30sition of the mole- 

 cules, as long as the mineral retains its form these changes occur. This 

 view is further corroborated by the circumstance that the silicate, which 

 contains scarcely any carbonate, and which it is very difficult to rednce 

 to a very fine powder, exhibits it better when powdered than the sili- 

 cates containing very little silica, although the latter act better in frag- 

 ments. The hydrocarbonate 3(ZnO,C02) + 5(ZnO,HO) which is described 

 in the paper above quoted, and which is there considered to have a dif- 

 ferent composition from that in combination with the silicate of zinc, 

 does not exhibit this chromatic phenomenon at all ; and in the reniform 

 masses consisting of alternate shells of silicate and the hydrocarbonate in 

 question, so extremely thin that they can scarcely be distinguished by 

 the eye, the separate layers may at once be recognised on heating some 

 fragments, by the alternate lines of green and whitish-yellow, the former 

 being the silicate, and the latter the hydrocarbonate. 



Professor William K. Sullivan also read the following paper : — 



Oi^ A NEW Hydeated Silicate of Potash, and on some of the Condi- 

 tions UNDEE WHICH THE EeNIFOEM StEIFCTUEE IN MiNEEiLS MAY BE 

 DEVELOPED. 



About two years ago I wanted a solution of silicate of potash for some 

 experiments with which I was then engaged, and accordingly prepared 

 it, by fusing a mixture of finely powdered vein quartz with about four 

 times its weight of purified pearl-ash, in a Cornish crucible. The melted 

 glass was poured out on a cold plate of iron, and when cold was broken 

 into lumps, and put into a large glass jar about half full of water. On 



