The bottle H is depressed to D, by the rod adhering to its base, and guided by supports P'. The air escapes by B, and the liquid takes the place of the air. The rod is detached, and B D is canted over, so as to allow the bottle to slip off and into the net. If necessary, suction may be applied at the end of B. Fig. 798 shows an arrangement for forcing the liquid into a vessel :—H is an air-compressing pump ; A, a bent tube, with an enlargement at B, and a valve C opening inwards. The short arm is perforated. The handle of the pump is depressed, the air presses on the liquid in B, C is closed, and the liquid is forced through perforations against the inner surface of the vessel.
Figs. 799, 800, and 801 show different views of the apparatus used for the treatment of flat sheets of glass. The cold sheets are introduced into A (Fig. 799), where they are gradually warmed ; are then transferred to a movable slab B, where they are heated more strongly ; and pass thence to a canting shelf C, the motion of which is made apparent in Fig. 801. At this point, the sheet passes from the furnace into the bath, suitable arrangements being pro vided for preventing the flame of the furnace igniting the inflammable liquid in the bath X. When the shelf C is sufficiently tilted, the sheet of glass slips on to the support D, by the motion of which, after its immersion, it is raised in such a manner that it can be easily moved on to one of the shelves F. The support D is such an arrangement of wire netting and open bars as will allow free access of the liquid to the under surface of the sheet.
Pieper's Process.—Pieper's process for hardening or toughening glass differs from that of De la Bastie, although the results are similar.
Glass vessels are heated almost to the point of plasticity, and are then subjected to the action of injected superheated steam.
The insignificant demand for toughened glass at the present time proves that the quality of the material has not fulfilled the sanguine expectations aroused in the first instance. The reason is to be found in the physical nature of the glass. The characteristics of unannealed glass, and especially of glass which has passed very suddenly from the liquid to the solid condition, have already been discussed. Toughened glass presents all these characteristics in a modified form. The outside case is exceedingly hard, and capable of resisting the diamond, and shocks of very considerable violence. Directly, however, the case is pierced, either by external action—whether that of a blow, of hydrofluoric acid, of a file, or of the cutter's wheel—or by the internal disturbance of molecules in a state of extreme tension, the mass is entirely disintegrated. The similarity of toughened glass to Prince Rupert's drops is displayed in its fracture, in its resistance to the diamond, in its power of returning to the normal i condition of annealed glass by the action of heat, and in the porous condition of its internal sub , stance. The names by which this glass is generally known—" toughened " and "unbreakable ' —
are unfortunate ; it is certainly not unbreakable, and is not toughened, but case-hardened.
Optical Glass.—The relations of light to glass aro modified by the form, and the physical and chemical conditions of the glass. Transient effects of colour may be due to the form, to the physical condition, and, to a certain extent, to the chemical nature of a glass. Permanent effects are entirely due to the presence in the glass, whether in suspension, solution, or chemical combination, of certain metals and metallic oxides. A glass prism produces effects of colour principally by its form. Glass under certain circumstances will produce them by acquiring the physical property of double refrac tion. If the elasticity of glass becomes more modified in one direction than in another, whether by curvature, pressure, or sudden cooling, and if the glass is then traversed by a beam of polarized light, effects of colour are obtained. These effects vary, according as the glass has a circular, square, rectangular, or triangular shape, and according to the degree of tension of its particles. A transient effect of colour is also produced by the physical action of uranic sesquioxide con tained in the substance of a glass. This effect of colour is due to the power possessed by glass holding uranic sesquioxide in solution, of diminishing the refrangibility of the invisible ultra-violet rays of light, and of rendering them visible, and is known as fluorescence.
The chief points to be considered in the selection of glass for optical purposes are transparency, density, and homogeneity. In order to obtain perfect transparency, the raw material of the glass must be chosen and purified with immense care. The presence of iron in the sand must be especially guarded against, as a very small quantity will tinge the glass, and diminish its transparency. The chief troubles, however, arise from the presence of bubbles and strim in the substance of the glass ; the latter may be removed by agitation, produced either by stirring, or by the application of heat beneath the crucible ; the former, by rest. Faraday suggests the removal of bubbles by mixing spongy platinum with the raw materials. Strim are probably caused by the tendency of molten glass to become stratified according to the density of its constituents. Solid specks are due to thQ corrosion of the crucible.