Conductibility of Minerals for Voltaic Electricity. 13 



times, and with several specimens, the surfaces having been pre- 

 viously purified with care. 



Three hundred and nineteen species have been submitted to 

 direct examination by the author. The comparison of his results 

 with those of previous experimenters shows in general a satis- 

 factory coincidence. Where divergences exhibited themselves, 

 they are to be referred to the variety of structure resulting from 

 difference of locality, and without doubt also to the fact of his 

 having made use of voltaic instead of machine electricity. 



The influences resulting from differences of structure are 

 exhibited by the experiments of M. Riess on the conductibility of 

 the sulphuret of antimony, native and artificial. These obser- 

 vations have been confirmed by those of Professor Faraday, and 

 more recently by MM. Karsten and Munck of Rosenschold, who 

 have found among other peculiarities, that native crystals of 

 realgar procured from Nagy-Ag are good conductors, while 

 M. Hausmann ranks the substance among the semi-conductors, 

 and M. Pelletier among the insulators. Sulphuret of zinc, in 

 pieces or in powder, is a conductor or an insulator according as 

 it is prepared in the dry or the humid way. Does not this 

 Explain the divergence of the results of M. Hausmann from those 

 of Mr. Fox with reference to blende ? Black sulphuret of mer- 

 cury very pure conducts well, while cinnabar is a perfect insu- 

 lator. The other sulphurets exhibit the same peculiarities. 



The diversity of electric propagation, according as it comes 

 from the pile or the machine, is rendered manifest by a crowd 

 of experiments. MM. Hausmann and Henrici distinguish as 

 semi-conductors a great number of substances, such as carbonate 

 and sulphate of lime, which with current electricity are perfect 

 insulators. The author has repeated the greater number of their 

 experiments and found them correct. This remark will be suf- 

 ficient to explain certain divergences which exhibit themselves 

 between their results and those of Mr. Fox, as also the complete 

 coincidence (if we except the sulphurets of bismuth and silver) 

 of his results and those of the English physicist. 



There are two aspects of the subject, the study of which would 

 be very interesting, namely, the relative conductibilities of non- 

 insulating minerals of the same dimensions and subjected to the 

 same conditions; and the variation of this property in different 

 directions through the same crystal. Unhappily difiiculties 

 almost insurmountable lie in the way of such an examination. 

 The manner of contact with the surfaces at the entrance or the 

 exit of the current can augment or diminish the deviation of 

 the rheometer two or threefold. The author has made numerous 

 experiments with an insulated platinum forceps, between which 

 the crystal was squeezed by means of a vice. But the fraction 

 of a revolution of the latter, more or less, caused such alterations 



