M. TECHNOLOGY. 557 



It may even replace iron and lead in the construction of gas 

 and water pipes. Bulletin Hebdomadaire, XVI., 22. 



EXPLANATION OF SO-CALLED HARDENED GLASS. 



No adequate explanation of the hardening of glass by the 

 new method of annealing seems as yet to have been given. 

 Professor Bauer, who has successfully imitated the glass pro- 

 duced by the French method, suggests the following. The 

 glass at once calls to mind the phenomenon of Rupert's drop; 

 but the latter can not be accounted for satisfactorily, since 

 it has been shown that if the point is dissolved off by fluo- 

 hydric acid, instead of pinched off, the whole drop does not 

 burst. On the other hand, the hardened glass also recalls 

 the fact that, although glass in a state of fusion may be con- 

 sidered as homogeneous, yet upon cooling it does not form a 

 perfectly homogeneous and amorphous mass, as was formerly 

 supposed ; but that all varieties, even those that are apparent- 

 ly without a trace of crystallization, are mixtures of crystal- 

 line and amorphous particles, as manifested under the action 

 of fluohydric acid. This want of homogeneity in the struct- 

 ure of glass, which is produced to a certain depth by slow 

 cooling, is evidently prevented by rapid cooling. Therefore 

 by cooling fused, or even softened glass very slowly, it may 

 easily happen that this separation into crystalline and amor- 

 phous particles may take place. The preparation of so-call- 

 ed Reaumur's porcelain depends on this fact ; and quite re- 

 cent experiments show that the crystalline portions may 

 even become visible, and the glass is said to be devitrified. 

 An explanation of the hardness and peculiar fracture of the 

 hardened glass may be connected with these facts. 14 (7, 

 CCXV., 1875, 382. ' 



PHOTOSTEREOTYPY. 



A sheet of ordinary plate glass larger than the picture to 

 be reproduced is coated in the dark room with a solution 

 made by dissolving 1 ounce of potassium bichromate in 15 

 ounces of water, warming gradually, then adding 2 ounces 

 of fine gelatin and filtering through linen at the boiling 

 heat. A diapositive is taken from an ordinary negative, and 

 laid with the collodion side to the gelatin face of the pre- 

 pared plate in diffused light for 10 to 30 minutes. The 



