as illustrative of Geology. 75 



rapid transition into the solid state, no time was allowed to 

 the ingredients to combine in intimate relations : consequently, 

 the accompanying power of crystallization likewise did not 

 come into operation, but the whole followed the laws of com- 

 mon attraction. On the contrary, if the cooling is slow, the 

 chemical attraction which arranges the ingredients in fixed re- 

 lative proportions, and the power of crystallization closely asso- 

 ciated with it, come into activity at once, and that in a degree of 

 perfection proportioned to the slowness of the cooling. Accord- 

 ing to the diflPerence of texture, there is then formed either one 

 crystalline substance only, or there arise different substances 

 which are chemically and crystallographically distinct. If the 

 texture is not of such a nature, that the whole body is formed 

 into a single or compound crystalline mass, either one crystal- 

 line body, or it may be several of them, are separated in the 

 glassy mass. The stony state, in which the crystalline forma- 

 tion is confused and indistinct, occupies a middle place between 

 the glassy state and the plainly crystalline aggregation ; in conse- 

 quence of which a mass is formed as a basis, which contains more 

 perfectly crystalline bodies. 



These different modifications of the state of aggregation of sili- 

 cates are displayed in great variety in the slags that result from 

 various foundry processes. Now, it must not seem surprising 

 that perfectly fused pure slags appear usually glassy, since they 

 most frequently quickly become solid, either from communication 

 with the air, or from falling upon cold humid ground. But as 

 soon as circumstances occur that are favourable to a more gra- 

 dual transition from the fused to the solid state, a crystalline 

 formation conformable to the structure of the slags takes place ; 

 and by this means a great difference in the tendency of various 

 slags to crystallize is made apparent. Porphyritic separation 

 of single crystalline particles, which are not unfrequently sphe- 

 rical, and of perfectly formed ci'ystals, is often met with in va- 

 rious kinds of blast-furnace slags. The Kieselschmelz as de- 

 scribed by Mr Koch, is found beneath the blast-furnace slags 

 in the iron-foundries on the Harz, in its regular six-sided 

 prisms, resembling crystals of the Nepheline imbedded in a 

 glassy basis of porphyry ; and something exactly similar is 

 displayed in the iron -foundries of other parts by slags crystal- 



