Vitreous pressure

The importance of viscosity at rest in ophthalmic surgery lies in the fact that viscoelastic fluids with a high viscosity value are particularly effective in deepening the anterior chamber, especially with raised vitreous pressure. Fluids with a very low value, such as irrigation solutions, require a constant flow to deepen the anterior chamber. 

When performing phacoemulsification under topical anesthesia, the outer eye muscles retain their tone explaining the absence of a "soft" eye and, for example, possibly a raised vitreous pressure. Raised vitreous pressure may encumber cap-sulorhexis or lOL implantation because of narrowed anterior chamber conditions (Fig. 38). 

HPMC products are not suitable to maintain anterior chamber depth with concomitant increased vitreous pressure due to their low elasticity. Higher molecular weight sodium hyaluronate was proven to be superior for this clinical application (Strobel, 1997). 

Vitreous sihca does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubihty is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 sihca dissolves mainly as soluble sihcates. Solubihty also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10 300 atm), for example, the solubihty, S, of Si02 in g/kg H2O can be expressed as foUows, where d ls the density of the vapor phase and T is the absolute temperature in Kelvin. 

Va.por Pressure. Vitreous sihca is a highly refractory glass. Volatilization is significant only at elevated temperatures (Fig. 1), occurring under a... 

Cristobahte can also form on vitreous siUca at temperatures as low as 400°C when the pressure is equal to 35 MPa (<350 atm) and the glass is immersed in weak NaOH solutions (108). In stronger NaOH solutions, quart2 is formed. The formation of the crystalline phases is a result of the hydrolysis of the anions present. No crystallisation occurs with HF, H2SO4, and H PO in KHSO solutions or in pure water. 

The density of vitreous sihca is lower than those of the low pressure crystalline phases of sihcon dioxide (121), especially that of quartz. 

Heat Capacity. The mean heat capacity (0—900°C) at constant pressure, 6, in J/(kg- " C), can be estimated in vitreous silica using the following expression, where / is temperature in °C. 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

Many of the high-pressure forms of ice are also based on silica structures (Table 14.9) and in ice II, VIII and IX the protons are ordered, the last 2 being low-temperature forms of ice VII and III respectively in which the protons are disordered. Note also that the high-pressure polymorphs VI and VII can exist at temperatures as high as 80°C and that, as expected, the high-pressure forms have substantially greater densities than that for ice I. A vitreous form of ice can be obtained by condensing water vapour at temperatures of — 160°C or below. 

Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

Air or water cooled mercury discharge lamps find many uses, one of the more obvious of which is the study of photochemical reactions. These lamps are usually made of vitreous silica because of its low thermal expansion, high melting point and its transparency to ultraviolet radiation. Their operating pressure has a profound effect on the spectral distribution of the radiation produced and therefore it is important to consider the requirements in the design of such lamps. 

The densities of some Si02 modifications are a-quartz 2.65 gem, /3-quartz 2.53 gem, /3-tridymite 2.27 gem-3, /3-cristobalite 2.33 gem-3, vitreous Si02 2.20 gem-3. Should it be possible to convert /3-cristobalite to some of the other modifications by applying pressure ... 

Optical absorption, of hydrogenated and hydrogen- free films, 17 206 Optical amplifiers, 11 145-146 Optical applications U.S. patents in, 12 614t of vitreous silica, 22 440-441 Optical cavities, 14 849 Optical cells, for high pressure measurements, 13 417-419 Optical coatings, cerium application,... 

Steel-gray crystalline brittle metal hexagonal crystal system atomic volume 13.09 cc/g atom three allotropes are known namely, the a-metaUic form, a black amorphous vitreous solid known as P-arsenic, and also a yellow aUotrope. A few other allotropes may also exist but are not confirmed. Sublimes at 613°C when heated at normal atmospheric pressure melts at 817°C at 28 atm density 5.72 g/cc (P-metallic form) and 4.70 g/cm (p-amor-phous form) hardness 3.5 Mohs electrical resistivity (ohm-cm at 20°C) 33.3xlCh (B—metallic polycrystalline form) and 107 (p—amorphous form) insoluble in water. 

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