ROCKS FORMED OF MINUTE ORflANISMS. 629 



that such sections should be ground extremely thin, before they 

 become transparent; and its friability renders this process one of 

 great difficulty. Any section must either cross the woody tissue 

 transversely, so that the appearance it presents will resemble that 

 of Fig. 170 ; or it must traverse it vertically, in which case the 

 fibrous structure will be brought into view, either as in Fig. 171, 

 or as in Fig. 172 ; or it must pass in an intermediate direction. 

 The following method, which would seem not only to be more 

 simple, but also to give more satisfactory results, is recommended 

 by the authors of the " Micrographic Dictionary" (p. 150): — 

 " The coal is macerated for about a week in a solution of carbo- 

 nate of potass ; at the end of that time, it is possible to cut tole- 

 rably 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 color, very transparent, and exhibit the struc- 

 ture, when existing, most clearly. The specimens are best pre- 

 served in glycerine, in cells ; we find that spirit renders them 

 opaque, and even Canada balsam has the same eftect." When 

 the coal is so friable, that no sections can be made of it by either 

 of these methods, it may be ground to fine powder, and the par- 

 ticles may then, after being mounted in Canada balsam, be sub- 

 jected to microscopic examination ; the results which this method 

 affords, are by no means satisfactory in themselves ; but they 

 will often enable the organic structure to be sufficiently deter- 

 mined, by the comparison of the appearances presented by such 

 fragments, with those which are more distinctly exhibited else- 

 where. Valuable information may often be obtained, too, by 

 treating the ash of an ordinary coal- fire in the same manner, or 

 (still better) by burning to a white ash a specimen of coal that 

 has been previously boiled in nitric acid, and then carefully 

 mounting the ash in Canada balsam ; for mineral casts of vege- 

 table cells and fibres may often be distinctly recognized in such 

 ash; and such casts are not unfrequently best afforded by samples 

 of coal, in which the method of section is least successful in 

 bringing to light the traces of organic structure, as is the case, 

 for example, with the " anthracite" of Wales.' 



444. Passing on now to the Animal kingdom, we shall first 

 cite some parallel cases in which the essential nature of deposits 

 that form a very important part of the Earth's crust, has been 

 determined by the assistance of the microscope ; and shall then 

 select a few exsiraples of the most important contributions which 

 it has afforded, to our acquaintance with types of Animal life 

 long since extinct. It is an admitted rule in Geological science, 

 that the past history of the Earth is to be interpreted, so far as 

 may be found possible, by the study of the changes which are 



' See Prof Qiiekptt's Memoir on the Minute Strncture of the Torbane-hill Mineral, in 

 "Transact, of the Microsc. Societ." Ser. 2, vol. ii, p. 41, et seq. 



