Mr. W. J. Lewis's Crystallographic Notes. 139 



Which form of Darnell's element Bosscha used, unfortunately 

 he does not state ; but we may assume as extremely probable 

 that he made use of the form ordinarily employed, to which, 

 according to my measurements, belongs the absolute electro- 

 motive force 



010 /™nim£milMgr*\ 

 \ sec. / 



This value is greater, in the ratio of 1*1016 to 1*0000, than 

 that deduced by Bosscha. Now, supposing that Bosscha has 

 carried out his measurements free from error (a supposition 

 which of course cannot be rigorously correct), then the abso- 

 lute value of the resistance taken by him as the basis of his 

 measurements, i. e. the absolute value found by W. Weber for 

 Jacobi's unit, would be 10*16 per cent, too little. 



This calculation of the error is based on two somewhat un- 

 certain assumptions, briefly indicated above. On this account 

 it is a great advantage that an error in W. Weber's determi- 

 nation of the absolute resistance of Jacobi's unit, in the same 

 direction and of the same order of magnitude, can be deduced 

 in quite another way. According to W. Siemens the ratio of 

 Jacobi's resistance-unit to Siemens's is =0*6618. From our 

 numerous and multifariously varied measurements the absolute 



value of the Siemens unit is 0*9550 x 10 10 ( ' ). Accord- 



\ sec. / 



ingly the absolute value of Jacobi's resistance-unit would be, 



from our measurements, 0*6320 x 10 10 ( ' ) ; while M. 



7 \ sec. / 



Wilh. Weber found only 0*598 x 10 10 ('^^ , )--that is, a 



value about 6 per cent, less than that found by us. 



Hence the absolute measurement by M. W. Weber of Jacobi's 

 resistance-unit has turned out certainly from 6 to 10 per cent, 

 too little. 



[To "be continued.] 



XIX. Crystallographic Notes. 

 By W. J. Lewis, M.A., Fellow of Oriel College, Oxford*. 



DR. HUGO MULLER had the goodness, some time ago, 

 to send me some crystals of the isomerous compounds 

 Quercite and Inosite, which he had obtained from new sources — 

 the former from the leaves of the dwarf-palm (Chamcerops 

 humilis), and the latter from cochineal. 



* Communicated by the Crystallological Society, having been read 

 October 26, 1877. 



