Vitreous appearance

The red form is chiefly amorphous, and of vitreous appearance and fracture. On prolonged heating it gradually turns violet and exhibits double refraction.2... 

On the surfaces of the silica brick utilized in the side walls, Insley observed an incrustation with a white, coral-like appearance. This frost was easily broken away from the surface consequently, very little evidence of corrosion of the silica brick was noticed. A petrographic examination of the frost revealed very small tridymite crystals in the part of the brick next to the frost. The contact material between the brick and the incrustation had a vitreous appearance composed of glass and large tridymite crystals. Further within the incrustation, the crystals were larger, but the quantity of glass was smaller. 

Brucite, which was named after Archibald Bruce (1777-1818) in 1824, who discovered it in Hoboken, New Jersey, occurs typically as tabular crystals. Less commonly it can occur in acicular, fibrous, and scaly form. Its color can range from white, pale green, gray, gray-blue, and blue. It can also have a transparent, pearly, waxy, or vitreous appearance see Table 2.5, which displays a variety of physical properties of brucite. 

The liquid vitreous appears first in childhood and by the seventh decade it occupies half of the vitreous [18, 19]. 

Thanks to the purity of the raw materials used, porcelain shards are white and translucent beneath the low thickness. They do not have open porosity (< 0.5%), but are likely to exhibit some large closed pores (air holes). Their fracture are brilliant and have a vitreous appearance. After enamehng, the surface of the pieces is remarkably smooth and brilliant. 

The oxidation of vitreous siUca appears to proceed by one of two mechanisms, depending on the material s hydroxyl content (109,111). In hydroxyl-containing material, the rapid oxidation probably occurs by the diffusion and removal of hydrogen, according to the following reaction ... 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

Glassy, or vitreous, carbon is a black, shiny, dense, brittle material with a vitreous or glasslike appearance (10,11). It is produced by the controUed pyrolysis of thermosetting resins phenol—formaldehyde and polyurethanes are among the most common precursors. Unlike conventional artificial graphites, glassy carbon has no filler material. The Hquid resin itself becomes the binder. 

Elimination from the vitreous occurs by one of two pathways. This can be visualized by injecting fluorescent compounds and examining the concentration distribution in frozen sections obtained after a steady state has been established [230]. If the major route of elimination is by means of the re-tina/choroid, at steady state the lowest concentration would be in the vicinity of the retina. The contours observed in frozen sections of the rabbit eye obtained after intravitreal injection of fluorescein exhibit this pattern, with the highest concentration immediately behind the lens (Fig. 16A). Compounds not chiefly eliminated through the retina exit the vitreous by passive diffusion and enter the posterior aqueous, where they are eliminated by the natural production and outflow of aqueous humor. In such a situation, the contours would be perpendicular to the retina, with the highest concentration towards the rear of the vitreous cavity. This appears to be the case for fluorescently labeled dextran polymer, whose contours decrease in concentration toward the hyaloid membrane (Fig. 16B). 

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

The behavior of D2 in the Raman experiments is strongly correlated with the Q4 chemical shift, 6, in the NMR spectra. 6 equals about -110 to -111 ppm when D2 is absent or when it exhibits low relative intensities comparable to those in conventional vitreous silica, for example the 50 and 1050°C sample spectra and the rehydrated 600°C sample spectrum. From the regression equation cited above -110 to -111 ppm corresponds to - 147 to 149°, values quite close to the average in conventional v-Si02, 151° (4 ). The average 64 is shifted downfield to about -107 ppm in the 600°C sample in which D2 is observed to be quite intense. Deconvolution of this peak reveals two Q4 resonances at -110 and -105 ppm. -105 ppm corresponds to - 138°, which is very near the equilibrium 4> calculated for the isolated cyclic trisiloxane molecule, HgSi303, ( = 136.7°) (46). The positions of the Q2 and Q3 resonances, however, appear to be totally unaffected by the presence or absence of D2 (as shown in the 600°C CP MASS sample spectrum). 

Lastly, we observe that the glass-melt transition involves a marked discontinuity at T of transition, reflecting the increase of vibrational freedom (rotational components appear). This discontinuity involves some complexity in the extrapolation of calorimetric data (usually obtained on glasses) from vitreous to molten states, discussed in detail by Richet and Bottinga (1983, 1986). 

In the first animal study, the clinical tolerance to RMN3 appeared to be good in all eyes regardless of the duration of internal tamponade, with a clear vitreous cavity and a normally looking retina. Moderate whitish deposits occurred on the... 

In the second animal experiment, involving albino rabbits, the clinical tolerance of Oxane Hd appeared to be good with no inflammatory reaction and no retinal vascular lesion. No emulsification was noted. Optical microscopy of semi-thin layers elicited only a few retinal lesions atrophy of the retinal external layers was observed in one eye treated with the mixture and one treated with standard silicone oil. We found a few rare cells in the vitreous cavity, which contained intra-cytoplasmic vacuoles. Electron microscopy confirmed the absence of damage to the pigmentary epithelium and to the retinal internal layers and atrophy of the retinal external layers in two cases treated with the mixture and in one case treated with standard silicone oil (Fig. 2). 

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