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Recent studies have established that many compounds composed of disc-shaped molecules exhibit stable thermotropic liquid crystalline phases. They are now referred to as discotic liquid crystals. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase, in its simplest form, has long-range translational periodicity in two dimensions and liquid-like disorder in the third, whereas the nematic phase is an orientationally ordered arrangement of discs without any long-range translational order. This review article describes the detailed structures of the various discotic phases and some of their important physical properties.

Rayleigh-Jeans Law, blackbody spectrum, ultraviolet catastrophe, zero point energy, photon. T o w a rd s th e e n d o f th e n in e te e n th ce n tu ry , it b e c a m e c le a r th a t w ith in th e fra m e w o rk o f c la ssic a l p h y sic s, it w a s n o t p o ssib le to u n d e rsta n d th e p h e n o m e n o n o f B la ck -B o d y R a d ia tio n . In 1 9 0 0 M a x P la n ck c a m e u p w ith a th e o ry th a t a t o n e stro k e a c c o u n te d su c c e ssfu lly fo r a ll its o bse rv e d fe a tu re s. T h is th e o ry h a s b e e n p re se n te d a g a in st a b a ck d ro p o f th e m a jo r d isc o v e rie s th a t le d to it. In tro d u c tio n O n 2 7 th A p ril 1 9 0 0 L o rd K elv in g av e a lectu re a t th e R oy a l In stitu tio n o f G rea t B rita in o n`N in eteen th-C en tury C louds over the D yn am icalT heory of H eat an d L ight'. T h e tw o clo u d s th a t h e referred to w ere th e B la ck -B o d y S p ectru m an d th e resu lt o f th e M ich elso n {M orley ex p erim en t. H e sa id th a t th e`b ea u ty a n d clea rn ess o f th eo ry ' w as ov ersh ad ow ed b y th ese`tw o clou d s'. It is n ow a p a rt o f h isto ry h ow th e stu d y o f th e¯rst led to th e b irth of Q u a n tu m M ech an ics an d th a t of th e seco n d resu lted in th e d ev elo p m en t o f th e T h eo ry o f R ela tiv ity. T h u s B la ck -B o d y R a d ia tio n o ccu p ies a cen tra l p o sitio n in th e h isto ry o f m o d ern p h y sics. W e sh a ll lo o k a t th e ea rly attem p ts to u n d ersta n d it a n d a lso M a x P la n ck 's n o ncla ssica l th eory of th e b la ck -b o d y rad iation . A n a ttem p t h as b een m ad e to p resen t th e su b ject in its h istorica l p ersp ectiv e. E a rly S tu d ie s o n R a d ia tio n N a tu re o f T h e r m a l R a d ia tio n

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