106 Wen HAM, on the Fracture of Polished Glass Surfaces. 
annealed glass tubes used for the water-gauges of steam- 
boilers are sometimes destroyed in this way, after the act of 
forcing a piece of cotton waste through them with a wire for 
the purpose of cleaning the bore. This will not happen if a 
piece of soft wood is employed. 
The late Andrew Ross informed me that on one occasion, 
late in the evening, he lightly pushed a piece of cotton wool 
through a number of barometer-tubes with a piece of cane, 
for the purpose of clearing out any particles of dust. The 
next morning he found most of the tubes broken up into 
small fragments, the hard siliceous coating of the cane 
proving as destructive as he had previously known a wire to 
be. 
After having drawn the point of a steel burnisher over the 
surface of a slip of polished glass, the following appearances 
will be observed under the microscope, using the polarizing 
apparatus and selenite plate, with a two-thirds object-glass. 
A coloured stripe is visible in the passage of the burnisher, 
showing that the surface of the glass has been placed in a 
state of tension in the direction of the line. The glass, too, 
seems not altogether devoid of plasticity, for the waves of 
colour show that it has been carried forward in ripples, re- 
sembling the mark left on a leather-bound book after the 
passage of a blunt point. It may be inferred from this that 
the mere burnishing of the surface of the glass with a sub- 
stance inferior in hardness will, without any scratching, 
cause an irregular strain in the bore of tubes sufficient to 
split them, and the concussion attendant upon the fracture 
often reduces the tube to small fragments. 
If the burnished lines upon the glass slip be examined a 
few days afterwards the colours will have become much less 
visible, showing that the strained portion of the glass partly 
recovers its equilibrium. 
On attempting to polish out a minute scratch on the sur- 
face of a piece of glass it sometimes appears to widen during 
the process, and at length resolves itself into two irregular 
parallel rows. Also, a clean cut made with a diamond on a 
piece of plate-glass, if left for a time, the surface in the 
vicinity of the cut will break up, forming a coarse irregular 
line. If the diamond be raised and struck lightly on the 
surface of the glass, the form of the edges of the short stroke 
thus made may be plainly seen, using the binocular polari- 
scope. A conical ridge of glass appears to be left with its 
apex under the line of the cut, and the glass is frequently 
wedged up on both sides of the ridge, explaining the cause of 
the double line of fracture which sometimes makes its appear- 
