_Elasticity and Ductility of Glass._--In a series of glasses investigated by Winkelmann and Schott, the modulus of elasticity (Young’s Modulus) varied from 3,500 to 5,100 tons per sq. in., the value being largely dependent upon the chemical composition of the glass. Measurable ductility has not been observed in glass under ordinary conditions except in the case of champagne bottles under test by internal hydraulic pressure; in these tests it was found that a permanent increase of volume of a few tenths of a cubic centimetre could be obtained by the application of an internal pressure just short of that required to burst the bottle--pressure of the order of 18 to 30 atmospheres being involved. This small permanent set has been ascribed to incipient fissuring of the glass, and this explanation is probably correct. On the other hand, it is in the writer’s opinion very probable that glass is capable of decided flow under the _prolonged_ action of relatively small forces; the behaviour of large discs of worked optical glass suggests some such action, but the view as yet lacks full experimental confirmation. _The Hardness_ of glass is a property of some importance in most of the applications of glass. The durability of glass objects which are exposed to handling or to periodical cleaning must largely depend upon the power of the glass to resist scratching; this applies to such objects as plate-glass windows and mirrors, spectacle and other lenses, and in a minor degree to table-ware. On the other hand, the exact definition and means of measuring hardness are not yet satisfactorily settled. Experimenters have found it very difficult to measure the direct resistance to scratching, since it is found, for example, that two glasses of very different hardness are yet capable of decidedly scratching each other under suitable conditions. Resort has therefore been had to other methods of measuring hardness; the method which, from the experimental point of view, is, perhaps, the most satisfactory, depends upon principles laid down by Hertz and elaborated experimentally by Auerbach. This depends upon measuring the size of the circular area of contact produced when a spherical lens is pressed against a flat plate of the same glass with a known pressure. Auerbach himself found some difficulty in deciding the exact connection between the “indentation modulus” thus determined and the actual hardness of the glass. This method is, therefore, of theoretical interest rather than of use in testing glasses for hardness. A test of a more practical kind consists in exposing specimens of the glasses to be tested to abrasion against a revolving disc of cast-iron fed with emery or other abrasive, and to measure the loss of weight which results from a given amount of abrading action under a known contact pressure. If a number of specimens of different glasses are exposed to this test at one time, a very good comparison of their power of resisting abrasion can be obtained.