122 a: Chronicles of Scrence. [Jan., 
if a metal wire be drawn through a glass tube, a few hours after- 
wards the tube will burst into fragments. The 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. 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 micro- 
scope, using the polarizing apparatus and selenite, 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, resembling 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 substance inferior in hardness will, without any scratch- 
ing, 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 portions of the glass partly recovers its 
equilibrium. 
On attempting to polish out a minute scratch on the surface 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 clear 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 polariscope. 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 Ime of fracture which 
sometimes makes its appearance in polishing out a scratch. This 
effect may also be exemplified by observing the marks left on a 
polished glass surface from the light blows of a steel centre-punch. 
The point of the punch drives in an atom of glass, and the fracture 
extends some distance into the interior, expanding downwards in the 
form of a truncated cone. The polariscope shows that the conical 
centre is in a state of compression, and that the surrounding exterior 
portion of glass is also under strain. 
The smooth, round edge of a glazier’s diamond, when drawn of 
a polished glass surface, burnishes down and compresses the glass 
beneath the cut, and in the case of thin sheets the wedge-like force 
