584 SCIENTIFIC RECORD FOR 1885. 



matter of observation therefore, and is independent of the effect of the 

 pressure on the supporting edges. Konig proposes the use of the anal- 

 ogous method of Kirchhoff for determining Poisson's constants ; only, 

 instead of using two horizontal mirrors and two telescopes, two parallel 

 mirrors inclined at an angle of 45° should be used at the two ends of 

 the bar, with one telescope. In this way the moduli of elasticity and 

 of torsion may be measured and the constants in question calculated. 

 (Wature, August, 1885, xxxii, 360.) 



Frederick Siemens has improved greatly the processes for tempering 

 glass, and consequently the product obtained. The scientific principle 

 underlying these consists simply in keeping the whole body of the glass 

 at a uniform temperature during the operations of heating and cooling. 

 The De la Bastie process is wrong in principle, since it leaves the glass 

 in a state of tension; while the glass treated by the new process is al- 

 most entirely free from internal stress. Three distinct processes are 

 made use of; in the first, called press-hardening, the very best quality 

 of glass is used. It is cut into the proposed shapes, softened in the 

 furnace, and cooled between metal plates, the degree of hardening de- 

 pending upon the temperature to which the glass is heated and the 

 rate at which it is cooled. It may be cooled so rapidly that a diamond 

 will not scratch it, a result obtained in the case of sheet and plate glass 

 for the most part, either plain or decorated ; increasing its strength 

 eightfold. For sheets of ordinary thickness, the heating occupies a 

 minute and the cooling half a minute, and this without injury to the 

 glass. The second process, called semi-hardening, is applied to articles 

 not adapted to the press. These are heated up to the verge of softening 

 and then placed in an iron casing having ])rojecting ribs to give the 

 necessary support, the whole being cooled in the open air. The strength 

 of the glass is increased about three times. The glass made in the third 

 process is called hard-cast glass, and is used for grindstones, sleepers, 

 car-rails, and floor plates. It is made in a continuous melting furnace 

 and run into molds made of imperfectly conducting materials, such as 

 mixtures of porcelain, glass pots, heavy spar, magnetic iron, &c., pul- 

 verized and then molded as in casting iron. After the glass is run into 

 the mold the whole is heated up and rapidly cooled. To show its homo- 

 geneity a tuning fork was made of it which gave a clear musical note. 

 {Mature, March, 1885, xxxi, 413.) 



Sherman has called attention to the relation obtained by Weber be- 

 tween the residual elasticity of glass and its chemical composition, the 

 former value being deduced from the observed depression of the column 

 after heating. Plotting the results obtained, the ordinates represent- 

 ing depression in tenths of a degree and the abscissas the ratio of pot- 

 ash to soda, Sherman notes the fact that while the greater portion of 

 the observations may be represented liy a smooth curve, yet several of 

 the points lie farther from the curve than can be accounted for by errors 

 of observation ; the composition of the glass being identically the same 



