August. 1841.] 73 



Written Communication. Mr. Speakman submitted 

 some views on the nature of Light and Heat ; which commu- 

 nication was referred to a committee composed of Dr. God- 

 dard, Mr. Lukens and Dr. Griscom. > 



Verbal Communications. 



Professor Johnson exhibited several specimens of crystalloid 

 anthracite and bituminous coal, and stated some of the circum- 

 stances which seem to determine the assumption by this mineral, 

 of certain figures, contrary to the assertion of many mineralogists 

 that it has no definite form. 



The forms which anthracite occasionally presents are, 1. Ovoid, 

 which causes it, in breaking, especially when the fracture takes 

 place from long exposure to the atmosphere, to fall into spheroidal 

 masses with salient portions strongly reminding us of truncated 

 angles in ordinary crystals. This figure has often been observed 

 in one of the beds of anthracite found on Beaver creek; and a large 

 specimen furnished by Mr. Jacob Thomas of Beaver Meadow, was 

 exhibited, showing the truncated borders along the lines of separa- 

 tion in a very remarkable manner, and also displaying beautiful 

 iridescent colours. It was remarked that coal affecting this form, 

 is frequently found to leave a considerable portion of oxide of 

 iron among its earthy residue; but that the whole of the latter 

 was not usually a large per centage of the coal. 



2. The next definite form mentioned as affected by coal, is a 

 radiated structure well characterized in several specimens from the 

 third bed, from the bottom of the formation at Bear-gap, Dauphin 

 county, Penna. This structure was likewise illustrated in a sample 

 of Welsh bituminous coal. The radiations generally proceed from 

 two points at no great distance from each other and forming two 

 sections of conical surface, unite at certain distances from their 

 points of departure into a single cylindrical section near the termi- 

 nation of the rays. The exterior of these radii is of a silky lustre, 

 striated, and sometimes interrupted by the interposition of frag- 

 ments of organic remains, in the state of fossil charcoal. Very 

 large stems are occasionally found with radiated, crystalloid an- 

 thracite adhering to the opposite sides, the directions of the 

 striae being on the two sides very nearly the same. 



3. The rhombic hexaedron was exhibited in a well marked 

 specimen of anthracite adhering to its accompanying slate. Few 

 mineral forms are better defined than this mass of anthracite. The 

 angles can be determined with the goniometer with tolerable pre- 

 cision, liable however to the slight uncertainty arising from the 

 presence of organic bodies, tending to oppose the crystalline ar- 

 rangement. 



