Double refraction accidental

Nitrates and Their Properties. Sodium nitrate, NaNOj, forms colorless crystals closely resembling crystals of calcite, CaCOg (Fig. 9-2). This resemblance is not accidental. The crystals have the same structure, with Na+ replacing Ca++ and NO3- replacing 003 The crystals of sodium nitrate have the same property of birefringence (double refraction) as calcite. Sodium nitrate is used as a fertilizer, and for conversion into nitric acid and other nitrates. 

Besides the double refraction which is characteristic of a given system, double refraction is also known which occurs as a consequence of particular constraints and which have up to the present usually been indicated under the tide accidental double refraction. 

A simple form of double refraction, which has long been known, is furnished by putting glm under strain, which (apart from a few kinds) becomes positively double refracting in the direction of the pressure In this case one can speak of strain double refraction The double refraction produced in bodies under the influence of electric or magnetic fields was also reckoned as accidental On the removal of the external cause the double refraction usually also dii ppears For so far as this is not the case, strains remain in the material, such strains are to be found in, for example, glass objects which have been cooled too rapidly ... 

Now as regards the nomenclature one has so far called the double refraction appearing after a particular action on a system accidental double refraction thereby thus making a distinction with the double refraction already present in a given system, which is called proper double refraction, except in particular cases (only occurring in colloid systems) to which we shall return presently and which were included in the concept shape double refraction. 

Some liquid colloid systems show a double refraction when they are set in laminar flow. The classical example is the V2O5 sol which (at least after ageing) consists of needle-shaped particles with a crystalline character the sol itself with its particles randomly oriented by Brownian motion is isotropic if now one makes it flow through a tube or between two coaxial cylinders, one of which is rotating, it becomes double refracting The same is the case with many macromolecular sols This phenomenon, streaming double refraction of sols, is also reckoned as accidental double refraction, because it only occurs through a constraint exerted on the system ... 

Tile nomenclature depicted, which is based on phenomenology, leads to various difiScuIries, if one takes account of the nature of the phenomena The double refraction of a cellulose membrane, obtained from nature, for example, would be called proper double refraction, that of an artificial cellulose membrane, which one has obtained by stretching an isotropic cellulose gel, would on the contrary have tP be reckoned as accidental double refraction, while in both cases one is dealing with the same phenomenon Furthermore the nomenclature is not entirely consequential even from a phenomenological point of view, since the shape double refraction in many cases arises through an external intervention and then would therefore have actually to be reckoned as accidental ... 

I. Accidental double refraction double refraction by the action of an external force... 

The old nomenclature accidental double refraction is only retained here for those cases in which there is no clear evidence of an orientation of molecules or larger particles and the classification is no longer based on the phenomenology but on theoretical points of view, against which however there seems no longer to be any great objection, since one now knows the nature of the phenomena well. 

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