COAL. 



[ 150 ] 



COAL. 



15. 



f Lower margin of cell inflated at middle { ff 



V. ll-/ 



\ Cell not inflated at middle { Je ^8 



16. 



f Cell inflated at middle 



\l-90tol-l6o". 



um, 



1 Cell slender, not inflated at middle .. { ^"f 



J Lower margin of cell inclined up- 

 wards at truncate ends ; longitudi- f didymotocum 

 i/ . , nal striae-none or indistinct ...... I 1. l-65"f. 



I Ends of cell inclined downwards ; ) , a 

 I strise distinct .................. J 8 



f Longitudinal strise 3 to 7, prominent 1 9 

 1 Longitudinal striae numerous, fine. . 20 



f Cell semilunar or crescent-shaped . . { co 



^"lni! 



\, Cell hnear 



20> C Cell narrowly linear, nearly straight {'ft 

 \ Cell tapering, curved .............. 21 



{Longitudinal striae crowded, sutures r striolntum, 

 1 to 3 .......................... U.l-80tol-68". 

 Longitudinal strise not crowded, su- i intermedium 

 tures usually more than 3 ........ U.l-77tol-54 // . 



* PI. 10. fig. 43. t PL 10- fig. 44. 



BIBL. Meneghini, Syn. Desmid. Linnaa, 

 xiv. 201 (1840); Lobarzewski, ibid. p. 2/8; 

 Ehrenb. Infus.-, Ralfs, Brit. Desmidiete-, 

 Dalrymple, Ann. Nat. Hist. 1840. v. 415 ; 

 Smith, ibid. 1850. v. 1 ; Meyen, Pflanzen- 

 physiologie, iii. 436, Sec.; Brebisson, Alg. 

 Falais.; Kiitzing, Spec. Alg. 163; Berkeley, 

 Ann. N. Hist. 2 ser. xiii. 256 ; Al. Braun, 

 Rejuvenescence, fyc., Ray Soc. Vol. 1853. 

 289, 292 ; Morren, Ann. des Sc. nat. 2 ser. 

 v. 257 ; Focke, Physiologische Studien, 1 

 Heft, 1847; Osborne, Quart. Journ. Mic. Sc. 

 iii. 54. 



COAL. This substance, although classed 

 from its mode of occurrence in nature in 

 the mineral kingdom, is in all cases of vege- 

 table origin. The degree, however, in which 

 traces of organic structure may be detected 

 in it vary extremely. Coal may be either 

 tolerably pure, containing but slight admix- 

 ture of earthy matters, or it may contain 

 large quantities of earthy substance, and 

 pass gradually into a carbonaceous impreg- 

 nation of an earthy basis, as in the various 

 modifications of bituminous shales. In the 

 next place the degree of metamorphosis of 

 the vegetable matter may be equally varied, 

 so that we have it still retaining its structure 

 very evidently, as in lignites, &c., or with 

 the structure greatly destroyed, or alto- 

 gether lost, as in much ordinary coal and 

 anthracite, which however are apparently of 

 somewhat different origin from the more 

 recent lignites. The old coal beds appear 

 to have been formed from deposits analogous 

 to our peat-bogs, and hence naturally consist 

 in great part of vegetables whose remains 

 soon became indistinguishable; but that arbo- 



rescent vegetation was also present and 

 contributed to form the coal, seems proved 

 by the detection of woody structure like 

 that of the Coniferse in certain specimens of 

 coal. Sometimes the woody structure is even 

 evident to the naked eye in a charcoal-like 

 appearance of the fractured surface of coal. 

 In many lignites the coal consists of trunks 

 of trees converted into coal without much 

 alteration of the appearance of texture of 

 the wood, and in these the structure is very 

 readily made out by means of the micro- 

 scope. It would be out of place here to 

 enter upon the geological and chemical 

 questions connected with coal; the object 

 of applying the microscope to it is to ascertain 

 the existence or absence of organic structure. 

 For this purpose various methods are em- 

 ployed. That most in use is the preparation 

 of exceedingly thin slices in the manner 

 usually adopted for fossil structures, but the 

 brittle and opaque character of coal opposes 

 great difficulties here. Traces of structure 

 may be made out in some cases by grinding 

 coal to fine powder and examining the 

 fragments, but this plan is very unsatisfac- 

 tory. A third method is to burn the coal to 

 a white ash, and examine this under the 

 microscope; it often exhibits perfect skele- 

 tons of vegetable cells, but these are very 

 fragile and require great care in their 

 management. By imbuing them very cau- 

 tiously with turpentine and Canada balsam, 

 and placing on the covering glass w r hen the 

 latter has become rather firm, permanent 

 preparations may be often obtained. Schulze 

 recommends boiling in nitric acid before 

 incinerating the coal. The method which 

 has been attended with most success in our 

 hands is as follows. The coal is macerated 

 for about a week in a solution of carbonate 

 of potash ; at the end of that time it is 

 possible to cut tolerably thin slices with a 

 razor. These slices are then placed in a 

 watch-glass with strong nitric acid, covered 

 and gently heated; they soon turn brownish, 

 then yellow, when the process must be 

 arrested by dropping the whole into a saucer 

 of cold water, or else the coal would be 

 dissolved. The slices thus treated appear of 

 a darkish amber colour, very transparent, and 

 exhibit the structure, when existing, most 

 clearly. We have obtained longitudinal and 

 transverse sections of Coniferous wood from 

 various coals in this way. The specimens 

 are best preserved in glycerine, in cells ; we 

 find that spirit renders them opaque, and 

 even Canada balsam has the same defect. 



