1875.] Physics. 411 
duced by gentle pressure into mere atoms, so thorough and so complete does 
the disorganisation of the entire mass appear to be. All these points will be 
practically demonstrated at the conclusion of my paper. A similar result is 
produced by placing a piece of toughened glass flat on the table, with a corner 
projecting over, and endeavouring to chip the corner off with a hammer. The 
corner will, after a series of smart blows, be broken off, but the whole mass 
will be at the same moment disintegrated and reduced to atoms. Another 
peculiarity about toughened glass is that the fragments are by no means so 
sharp, and therefore so capable of piercing the flesh, or of causing incised 
wounds, as are those of ordinary glass. One important point of difference be- 
tween M. de la Bastie’s toughened glass and Prince Rupert’s drops is 
that, aithough the skin of the former may be scored through with the 
diamond, the body cannot even then be broken through by ordinary force, 
much less does the mass fly to pieces and disintegrate, as in the case of the 
Rupert drops. Still wider will this difference appear when I state that 
toughened glass is readily susceptible of a high degree of polish, and it can be. 
cut by the wheel for lustre-work and such like. The glass can likewise be 
engraved, either by hydrofluoric acid, or by Mr. Tilghman’s elegant sand-blast 
process. It will thus be seen that toughened glass presents features which 
appear to some extent paradoxical. It would appear that toughened glass 
possesses enormous cohesive power, but that if the equilibrium of the mass is 
disturbed at any one point, the disturbance, or disintegration, is instantaneously 
communicated throughout the whole piece, the atoms no longer retaining the 
power of cohesion. It is as though the glass was endued with a nervous sys- 
tem, a shock to which at any one point instantly and utterly demoralised the 
whole. It is important to note that neither transparency nor colour in glass 
is in any way affected by the process of toughening, and the ring, or sound 
emitted upon the glass being struck, is nearly as clear in toughened as in plain 
glass. In order to determine the relative values of ordinary glass and the 
toughened material, as regards their strength, I suggested to Messrs. Rey the 
propriety of instituting experiments, with the view of ascertaining their respec- 
tive resistances to ordinarily applied stress. In these experiments I have been 
ably assisted by Mr. Kirkaldy, whose perfect testing machinery has, for some 
years past, supplied a want long previously felt by the enyineering profession. 
Twenty pieces of glass in all were submitted to bendiny stress, ten being 
toughened and ten untoughened. The glass was of French manufaéture, and 
was that known as “ Rive de Giers.’”’ Each piece of glass measured, as nearly 
as possible, 6 inches in length by 5 inches in breadth, and the samples had a 
mean thickness of 0°2259 of an inch. Each piece was placed with a bearing 
of half-an-inch at each end, and the weight was brought gradually upon the 
centre; in some instances by the testing-machine, and in others by dire& 
weights. Taking two pieces of glass, having about the same sectional area— 
the one tempered and the other untempered—Mr. Kirkaldy’s certificate shows 
that the untempered glass yielded under a strain of 279 Ibs., whilst the 
toughened glass did not give way until a stress of 1348 lbs. had been reached. 
The same proportion, however, did not occur throughout the series, the 
toughened glass giving in some instances lower results. This arose from two 
causes—the diminished area of some of the samples of glass, and from the fa@ 
that, in some instances, the process of toughening had not been perfectly 
carried out; for the samples were prepared by M. de la Bastie under purely 
experimental conditions. ‘The imperfect tempering was made manifest, after 
the destruction of the glass, in tnree ways chiefly; first, by the glass showing 
needle fractures, suck as are seen in untoughened glass; secondly, by a faint 
milky line presenting itself in looking at the glass in section; and, thirdly, by 
portions of the glass, a square inch in area, remaining unfractured, whilst the 
whole surrounding mass was reduced to atoms. But above and beyond alk 
this, it was evident that the strains applied were such as could not possibly 
come upon glass articles in ordinary use. They were long-sustained pressures, 
tending at every increment of weight to alter the relative position of the 
particles of the glass, but affording them no opportunity of returning to their 
normal position, or, in other words, of utilising the elasticity of the mass. 
