28 Dr. Trail 07i Mexica7i Coal. 



bituminous matter, or coal tar, so as to cake or become semi-liquid 

 in the fire. It does not decrepitate when heated, like Cannel* 

 When heated before the blowpipe, in a glass tube, its volatile 

 parts are separated ; and it leaves behind about 50 per cent, of a 

 coke which is capable of exciting a pretty strong heat. The vo- 

 latile portion affords a very pure coal gas. Six grains of it, burnt 

 in a platina crucible, left behind 0.2 grains of greyish white 

 ash, which is equivalent to 2^ per cent, of incombustible matter 

 in it. 



*' The smallness of the specimen rendered it impossible to as- 

 certain the relative quantities of carbon, hydrogen, and nitrogen, 

 which similar substances contain, but this sort of analysis is 

 rather an object of curiosity than utility." 



V. — On the Origin, Materials, Composition, and Analogies 

 of Rocks, by John Mac Culloch, M.D., F.R.S.E. 



[Communicated by the author.] 



If it is the first error of the observer to see, like the miner, but 

 a very limited number of rocks in the system of nature, it is not 

 long before he falls into one, the very reverse ; creating for him- 

 self permanent distinctions from eveiy incidental variety that 

 comes under his notice. Time, however, speedily corrects this 

 error, and teaches him, that however the aspects of rocks may 

 be multiplied, Nature has limited these productions by a very 

 confined set of general and constant characters. 



Of tlie Constituents of Rocks. 

 A small number only of the earths which chemistry has dis- 

 covered, forms the materials of all the rocks ; united, in some 

 cases, with alkalies, and with certain metallic oxides. In some, 

 a single earth is found ; in others, two or more exist ; and these 

 are either mechanically mixed, or united by the laws of chemical 

 affinity. Thus are formed those rocks which are considered 

 simple ; simplicity, as applied to rocks, meaning simplicity of 



