xlvi GENERAL SUMMARY OF SCIENTIFIC AND 



olar state to liavc altered by the division. Placing the one 

 plate directly upon the other in the original position, both 

 bands and fringes disappear ; while if reversed and super- 

 posed, the effect is increased, being that due to a plate of 

 double thickness; hence the tension in the plate is symmet- 

 rical with reference to the saw -cut. We may conclude, 

 therefore, that while hardened glass is in a state of tension, 

 it may always be cut in certain directions when the result- 

 ing pieces can take a condition of stable equilibrium. This 

 is easily determined by examination with polarized light. 

 In the case of fracture the fragments are always symmetrical- 

 ly arranged with relation to the point where the equilibrium 

 was first destroyed. The authors have also examined into 

 the cause of the bubbles so generally seen in hardened glass. 

 They find them to be produced at the moment of hardening, 

 and to disappear, or nearly so, when the glass is annealed. 

 They hence conclude that they are due to the imprisoning of 

 minute masses of gas in the glass, these masses becoming 

 enormously dilated when the glass is hardened; this dilata- 

 tion, which is actually seventeen or eighteen hundred times 

 the original volume, being caused by the contraction of the 

 surrounding glass produced in the process of hardening. 



Boudreaux has published a simple and more general meth- 

 od of demonstrating the Archimedean law of buoyancy in 

 liquids. A glass vessel with a slightly conical lateral spout 

 is placed beneath the pan of a hydrostatic balance, to which 

 is suspended the body to be experimented upon. This ves- 

 sel is filled previously, the excess of liquid being allowed to 

 flow off through the spout. Two thin capsules are then pro- 

 vided ; one of them is placed on the pan supporting the 

 body, and is balanced by shot. The body is then immersed, 

 the overflow of liquid being collected in the second capsule. 

 The inclination of the balance beam shows the upward press- 

 ure. But on replacing the first capsule by the second, which 

 contains the liquid displaced, the equilibrium is restored. 



Carl has devised a simple apparatus for showing lateral 

 pressure in liquids. It consists of a cylinder to hold the liq- 

 uid, hung at its top upon a knife edge, and having a lateral 

 opening near the bottom which can be closed at pleasure. 

 An index attached at the top moves over a graduated scale 

 as the cylinder varies from perpendicularity. The condition 



