138 ON THE FORMATION OF COAL v 



Occasionally one of tlie faces of a lump of coal 

 will present impressions, which are obviously 

 those of the stem, or leaves, of a plant ; but 

 though hard mineral masses of p}Tites, and even 

 fine mud, may occur here and there, neither sand 

 nor pebbles are met with. 



When the coal burns, the chief ultimate 

 products of its combustion are carbonic acid, 

 water, and ammoniacal 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 something more. Black and 

 opaque as ordinary coal is, slices of it become 

 transparent if they are cemented in Canada 

 balsam, and rubbed dovni 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.^ 



^ My assistant in the Museum of Practical Geolo^, Mr. 

 Newton, invented this excellent method of obtaining thin slices 

 of coal. 



