474 



siderablj' influenced nnder special circumstances bj very strong 

 fields '). 



By putting n <^ J the effect of reducing the dimensions is at once 

 evident ; this is interesting with a view to tlie design of the usual 

 oplitalmic magnets which may also be iuiproved by the above calcu- 

 lations and experiences ; the maximum distance in this case is not 

 more than 2,5 cm. A type is now being made of 8,5 cm. diameter 

 of core, serving the double purpose of an intestinal magnet of less 

 strength than the above and at the same time of a very powerful 

 ophtalmic electromagnet. 



Ore separators have long been applied in metallurgy to separate 

 un magnetic from ferromagnetic or only paramagnetic powdered ore 

 by the dry or the wet method. These apparatus are variously con- 

 structed ; the principal magnetic organ is however essential and 

 common to them all and is a more or less finely ribbed polar arma- 

 ture. The best cross-section for a definite mean size of the grains 

 may be determined by means of the theory above-nientioned for a 

 prism semi-angle between 30° and 32°8'. 



Mineralogy. — "On the Tin of the Island of Flares" By Prof 

 Dr. A. WiCHMANN. 



(Communicated in the meeting of June 27, 1914). • 



During the last decenniums very contradictory answers were given 

 to the question regarding the occurrence of tin-ore in the Isl and 

 of Flores. The fact that the solution of this question does not only 

 regard the interests of a mining-scientifical nature, but is likewise 

 \Qvy interesting from a mineralogical and geological point of view 

 may justify the attempt (»f elucidating this subject. 



In -'.p fii-sf place we have to bring into remembrance the fact, 

 : the Sunda Islands the older geological formations gradually 

 •)fvir if we move in that range of islands in an easterly 

 itMi. till — beginning from Bali — only neogenic and pleisto- 

 cenic sediments are found, and at the same time tertiary and post- 

 tertiary eruptive rocks with their tufas begin to play a predominant 

 part. The question rises then : Does Flores make an exception to 

 this rule and do we find in this island remains of ancient granite 

 .>tücks, accompanied by deposits of tin. or are all the reports regarding 

 the occurrence of this ore only of a legendary nature ? 



1) 0. LiEBKNECHT and A. P. Wills, Ann. d. Phys. 1 p. 183, 1900. W. J. de Haas 

 and P. Drapier, Ann. d. Phys. 42 p. 677, 1913. 



