PHYSICS. 215 



thirty meters in diameter was placed, which, being fixed, of 

 course followed the motion of the earth. The pipe, on the 

 contrary, supported by a pivot at the extremity of the 

 axis, carried large indexes, which appeared to be displaced 

 with it. 



Siemens, of Dresden, the inventor of the new compressed 

 hard glass, has recently exhibited it before the Berlin Poly- 

 technic Society. Owing to the perfection to which the proc- 

 ess has now been brought, the new hardened glass is not 

 only more easily, but more cheaply made than ordinary glass. 

 The resisting power of this glass varies from eight to ten 

 times that of ordinary glass. The serious objection made to 

 hard glass at the time of its discovery, that it often fell to 

 pieces when entirely unexposed to pressure, lias been suc- 

 cessfully avoided. This property was found to result from 

 overhardening, and it is now possible to detect all articles 

 which have acquired it, by the use of the polariscope, under 

 which overhardened glass shows a prevalence of violet tints. 

 This condition is also detected by exposure to water heated 

 to a certain and definite temperature. 



2. Of Liquids. 



Muirhead has communicated to Nature the results of some 

 experiments, undertaken at his request by Whitely, to af- 

 ford additional data in answering the question whether mat- 

 ter in the solid state will float upon the same matter in 

 the liquid condition, with reference to the solidification of 

 the earth. In the first experiments metals were used, various 

 copper and zinc alloys being employed in the earlier, and 

 cast-iron in the later, experiments. When the solid frag- 

 ment was placed on the liquid surface, a portion of the liquid 

 metal was at first chilled by it, and coated the mass; but this 

 soon re-fused, and the floating solid mass gradually melted, 

 beginning at the lower surface. The result was more mark- 

 ed in the case of the iron than the brass: a small piece of 

 cold, dry iron, when dropped endwise on the liquid metal, 

 bounded back to the surface, and melted in that position. 

 Subsequently, experiments were tried with melted rock, in 

 the first case whin -stone being placed on melted furnace 

 cinder, and then solid cinder being put into the furnace con- 

 taining the liquid mass. Pieces of five or six pounds weight 



