Professor Johnston on Middletonite* 26 1 



A mathematical friend having suggested to me that the 

 mode of arriving at the equations (7.)j just mentioned, wants 

 a little more explanation, 1 have reconsidered this part of the 

 investigation, and find that I have left it obscure. The pre- 

 vious equations give for n^ two values, denoted by n^ and 

 w^/, and for p two corresponding values denoted by p^ and p^^, 

 It follows, therefore, that either of the values of n, and the 

 corresponding value of p, may be substituted for n and p in 

 the expression (2.). But since the equations (1.) are of the 

 first degree, they may be satisfied not only by the values of 

 )} and ? corresponding to each value of w, but by taking for 

 ») and ^ the sums of these particular values, in which we may 

 change the value of « as 72 changes. Hence the equations (1.) 

 may be satisfied by the equations (?.)• Compare Poisson, 

 Traitc de Mecanique, No. 546. 



I hope Mr. Archibald Smith will communicate to your 

 Journal that step of the analysis to which Mr. Sylvester al- 

 ludes, at p. 78, as being wanted to make the development 

 complete. 



XLII. On the Composition of certain Mi?ieral Substa?ices of 

 Organic Origin, By James F. W. Johnston, M,A., FM.SS. 

 L. Sf E., F.G,S., Professor of Chemistri/ and Mineralogy, 

 Durham,"^ 



1. Middletonite, 



npHE substance for which I propose the name of Middle- 

 -^ tonite, occurs about the middle of the Main coal or Haigh 

 Moor seam, at the Middleton Collieries near Leeds. It pre- 

 sents itself sometimes in little round masses, seldom larger 

 than a pea, but generally forms thin layers, rarely thicker than 

 the sixteenth of an inch, interposed between the layers of coal. 

 These layers vary in extent from two or three to probably 

 twelve inches, and lie over each other at irregular intervals 

 near the centre of the coal seam, which is here about five feet 

 in thickness. 



It is hard, brittle, easily scraped to powder by a knife, in 

 small fragments is transparent, by reflected light of a reddish 

 brown, by transmitted of a deep red colour, and gives a light- 

 brown powder. It has a specific gravity of about 1*6, a re- 

 sinous lustre, and is void of taste or smell. By exposure to 

 the air for a length of time it blackens, and is then distin- 

 guishable from the mass of coal only by a slight peculiarity 

 in the lustre, which it still retains. It is unaffected by heat 



* Communicated by the Author. 



