258 On the Chemical Compositioji of Clay-slate. 



ed with hot water; and thus carbonate of potash was dissolved, 

 and was then filtered from the remaining undissolved carbonates 

 of baryta and magnesia. The dissolved carbonate of potash 

 was evaporated to dryness, and the dry mass converted into sul- 

 phate of potash and weighed. The carbonates of baryta and 

 magnesia left undissolved by the water were dissolved in mu- 

 riatic acid, decomposed by sulphuric acid, and the resulting 

 sulphates of magnesia and baryta filtered, evaporated to dryness, 

 exposed to a red heat, and weighed. The united weight of the 

 sulphate of potash and the sulphate of magnesia agreed witli 

 that which I had obtained before the separation of the two. I 

 could not discover the presence of soda in the sulphate of potash. 



The component portion of the clay-slate which was not decom- 

 posable in muriatic acid, was strongly heated with carbonate of 

 baryta after being separated from the silica of the soluble por- 

 tion. The carbonic acid was separated in the usual manner, 

 the baryta precipitated by sulphuric acid, and the analysis per- 

 formed in the same way as in the previous one. 



The different varieties of clay- slate which I have analyzed are 

 the following : — 



1. From Goslar in the Harz. 



2. From Benndorf near Coblenz. 



3. From Lehsten in Thuringia. 



They are all from the transition series, coloured greyish-black 

 by carbon. They split inl o thin slates, and belong to the kind 

 termed roofing-slate. The appearance of clay-slate before the 

 blowpipe is similar in all varieties. If held with platinum pin- 

 cers, and exposed to a very strong heat, clay-slate is melted at 

 the edges into a dark green glass. In a retort it yields water. 

 With soda a black glass is the product. It is acted on with 

 difficulty by phosphate of soda ; but, by the separation of the 

 silica, a colourless glass is formed, which, when cooled, acquires 

 a yellow tint. With borax the result is the same, though the 

 colour of the cooled glass is more intense. 



A larger mass of clay-slate from Benndorf, melted in a plati- 

 num crucible, forming a dark green glass resembling obsidian, 

 full of small cavities, and with a brown covering on the surface. 



