180 Mr. L. Fletcher's Cry stallographic Notes. 



of 23*379 millim. The data for the calculation are : — 



Weight of a litre of water-vapour at 100° C. '80357 grm. 



„ „ „ mercury 13*579 grms. 



Tension of vapour of water at 20° C. . . 18-495 millims. 



The height of a column of mercury equivalent to the column 

 of water-vapour of height 23*379 millims., is 



•80357 18*495 23*379 



13*579 760 100 



The first factor is the fraction of a litre of mercury which a 

 litre of water-vapour at 100° C. equals. The second factor 

 reduces the height of this to millimetres. The third gives the 

 result at 20° C, supposing the vapour to follow Boyle's Law. 

 The fourth is the part of a litre there was to be considered. 



The expression for the mean free path in these surface- 

 vapours is then 



T-.fi47 ' 80357 100 18 ' 495 23 ' 379 1 



2 - L -' b4 ' ' 13.579 ' iUU • 760 ' 100 ' 18*495' 



r = *647. 



This gives L = -0000024 millim. 



If the law, according to which the density of the vapours 

 varies with the depth, were known, the free path of a molecule 

 in a gas at the ordinary pressure could be found. 



Physical Laboratory, Harvard College, 

 Cambridge, U. S. A., Jan. 27, 1880. 



XXV. Crystallographic Notes. By L. Fletcher, M.A., Fel- 

 low of University College, Oxford, Assistant in the Mineralo- 

 gical Department, British Museum*. 



[Plate V.] 



I. Copper. 



THE following forms have already been described as occur- 

 ring on native copper: — 



(100) (110) (111) (210) (520) (310) 



(311) (412) (18 10 5). 



The forms (3 1 1), (4 1 2), (1 8 1 5) have not been ob- 

 served on any of the crystals in the collection of the British 

 Museum. The form (3 1 1) was described by Rosef as occur- 

 ring on some rude crystals from Nijni-Tagilsk : the crystals 



* Communicated by the Crystallological Society. 



•*• Reise nach dem Ural, Ton G. Bose, vol. i. p. 313 (1837). 



