PROPERTIES AND USES OF CELL WALLS 103 



filled with pyrites, gypsum, calcite or clay. Vitrain has been further 

 classified according to its content of jellied plant matter (ulmain 

 and collain), cork (suberain), or wood (xylairi). 



The organic constituents of coals are many and varied. Indeed, 

 one of the great problems in coal chemistry is concerned with 

 differentiations of the fundamental structural units of coal hence 

 with the development of fractionation procedures which are mild 

 enough to separate the molecular species actually present without 

 obscuring chemical alteration. The substances which can be reco- 

 gnized in the various coals include— 



Paraffins and aromatic hydrocarbons (liquid and solid): 



Cio, Cll' Cl3> Cn, C 24 , C26, C32; 

 naphthalene and polynuclear hydrocarbons. 

 Gases (at N.T.P.): 



H 2 , CH 4 , C 2 H 6 , CO, C0 2 , N 2 . 

 Salts of organic acids : 



oxalates of Ca(II) and Fe(II), aluminum mellitate. 

 Complex organic acids: 



humins, ulmins (C 50-60%, H 4.5-6.0%, N 0.5-2.0%). 

 Lipoidal and resinous substances: 



resins, resinoles, resinolic acids (C 76.8-83.6%, H 9.7-12.9%); 

 waxes (C 80.3-81.6%, H 13.1-14.1%); 

 fats, oils (C 74-78%, H 10.3-13.4%). 

 Mineral constituents: 



sulfur (as sulfides, sulfates, some organic S); 

 phosphorus (as phosphate, some organic phosphorus); 

 alkali metals and halogens (as Na + , K + ,C1-); 

 alkaline earths (Ca and Mg carbonates); 

 silicon (as silica, silicates); 

 heavy metals (FeS 2 , CuFeS 2 , FeAsS, SbS 3 ). 

 The various forms of coal may be arranged according to variety 

 and rank, a classification of importance in the consideration of 

 the genesis of coal. Peat is the incipient stage in coal formation 

 and consists of hydrated light brown material of fibrous, woody 

 or jelly-like texture. On the average dry peats contain C 53 per cent, 

 H 6 per cent, N 1.3 per cent, ash 10 per cent, but show consider- 

 able variability. The next rank (and stage) is lignite and brown 



