6 Prof. W. C. Williamson. [Jan. 6, 



T into the denominator of the second term; they evidently differ 

 from our first equation p = 6T — a, in which a is independent of 

 temperature. 



We shall soon be in a position to communicate the results of this 

 investigation, giving full data. 



[January 18th, 1887. — We have alluded to Clausius's formula, 

 RT * c 



p — — - ; his latest published formula is, however, 



v—a. T(v + p) 2 ' 



RT c 



v — — , where 9= aT n — b. As the second term is here 



v v— a 0O + /3) 2 



also a function of temperature, it is evident that his last equation is 

 also not in accordance with the simple relation p = 6T — a]. 



III. "Note on Lepidodendron Harcourtii and L. fuliginosum 

 (Will.)." By W. C. Williamson, LL.D, F.R.S., Professor 

 of Botany in the Owens College and in the Victoria 

 University. Received November 27, 1886. 



In March, 1832, the late Mr. Witham read to the Natural History 

 Society of Newcastle-upon-Tyne the first public notice of the classic 

 specimen of Lepidodendron known as Lepidodendron Harcourtii. Still 

 later (1833) he published further figures and descriptions of the same 

 specimen in his work on ' The Internal Structure of Fossil Vege- 

 tables.' Additional figures and descriptions of the same object 

 appeared in the second volume of Lindley and Hutton's ' Fossil 

 Flora,' and in Brongniart's ' Vegetaux Fossiles.' But notwith- 

 standing all these publications the exact plant to which they referred 

 has long been doubtful. I hoped to have found either the original 

 specimen] in the museum of the Yorkshire Philosophical Society or 

 the sections described by Lindley and Hutton in that of the New- 

 castle Society ; but, though carefully sought for, I long failed to 

 discover either one or the other. 



In 1871, I laid before the Royal Society my memoir, Part II, " On 

 the Organisation of the Fossil Plants of the Coal-measures," in which 

 I figured (Plate 25, fig. 12), a plant that seemed to me to be identical 

 with L. Harcourtii ; and in Part XI (1880) of the same series of 

 memoirs, I gave further representations of the same plant (Plate 51, 

 fig. 10; Plate 49, fig. 11). Since the publication of the latter 

 memoir I have obtained a fine series of specimens, which appeared to 

 me to approach still more closely to the various representations of 

 Lepidodendron Harcourtii, referred to above, and which inclined me to 

 think that I had hitherto included two species under a common name. 

 The two forms unmistakably belong to a common type, to which I 



