v.] ON THE FORMATION OF COAL. 93 



products, which escape up the chimney ; and a greater 

 or less amount of residual earthy salts, which take the 

 form of ash. These products are, to a great extent, such 

 as would result from the burning of so much wood. 



These properties of coal may be made out without any 

 very refined appliances, but the microscope reveals some- 

 thing more. Black and opaque as ordinary coal is, slices 

 of it become transparent if they are cemented in Canada 

 balsam, and rubbed down very thin, in the ordinary way 

 of making thin sections of non-transparent bodies. But 

 as the thin slices, made in this way, are very apt to 

 crack and break into fragments, it is better to employ 

 marine glue as the cementing material. By the use of 

 this substance, slices of considerable size and of extreme 

 thinness and transparency may be obtained. 1 



Now let us suppose two such slices to be prepared 

 from our lump of coal one parallel with the bedding, 

 the other perpendicular to it ; and let us call the one 

 the horizontal, and the other the vertical, section. The 

 horizontal section will present more or less rounded 

 yellow patches and streaks, scattered irregularly through 

 the dark brown, or blackish, ground substance ; while 

 the vertical section will exhibit more elongated bars and 

 granules of the same yellow materials, disposed in lines 

 which correspond, roughly, with the general direction of 

 the bedding of the coal. 



This is the microscopic structure of an ordinary piece 

 of coal. But if a great series of coals, from different 

 localities and seams, or even from different parts of the 

 same seam, be examined, this structure will be found to 

 vary in two directions. In the anthracitic, or stone- 

 coals, which burn like coke, the yellow matter diminishes, 

 and the ground substance becomes more predominant, 



1 My assistant in the Museum of Practical Geology, Mr. Newton, invented 

 this excellent method of obtaining thin slices of coal 



