3(M THE MICROSCOPE AND ITS REVELATIONS. 



lowed. Of those kinds which have sufficient tenacity, thin sections may 

 be made; but the opacity of the substance requires that such sections 

 should be ground extremely thin before they become transparent; and its 

 friability renders this process one of great difficulty. It may, however, 

 be facilitated by using Marine Glue, instead of Canada balsam, as the 

 cement for attaching the smoothed surface of the coal to the slip of glass 

 on which it is rubbed-down. Another method is recommended by the 

 authors of the " Micrographic Dictionary" (2d edit., p. 178): "The 

 coal is macerated for about a week in a solution of carbonate of potass; 

 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-color, very transparent, and exhibit the struc- 

 ture, 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 de- 

 fect." 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 parti- 

 cles may then, after being mounted in Canada balsam, be subjected 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 determined, by the comparison of the appear- 

 ances presented by such fragments with those which are more distinctly 

 exhibited elsewhere. 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 pre- 

 viously boiled in nitric acid, and then carefully mounting the ash in 

 Canada balsam; for mineral ( casts' of vegetable cells and fibres may of- 

 ten be distinctly recognized in such ash; and such casts are not unfre- 

 quently best aiforded 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. 



698. Passing on now to the Animal kingdon, 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 determinined by the assist- 

 ance of the Microscope; and shall then select a few examples 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 Geolo- 

 logical 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 still 

 going on. Thus, when we meet with an extensive stratum of fossilized 

 Diatomacem ( 299) in what is now dry land, we can entertain no doubt 

 that this siliceous deposit originally accumulated either at the bottom of 

 a fresh-water lake or beneath the waters of the ocean; just as such de- 

 posits are formed at the present time by the production and death of suc- 

 cessive generations of these bodies, whose indestructible casings accu- 

 mulate in the lapse of ages, so as to form layers whose thickness is only 

 limited by the time during which this process has been in action ( 298). 

 In like manner, when we meet with a Limestone-rock entirely composed 

 of the calcareous shells of Foramimfera, some of them entire, others 

 broken-up into minute particles (as in the case of the Fusiilina-limestonQ 



