437 



the shorlest possible action and by nioaiis of very dilute solutions 

 of the acid, a great number of cojrosion-tignres, which appeared to 

 be elevations, instead of impressions in hv far the most cases. Thev 

 generally (tig. \0a, b, c) did not have any well detlniable shape, 

 and were moreover quite irregularly distributed over the surface of the 

 crystalplates^), only on |001| we succeeded sometimes in getting 

 some extended rectangular forms, proving the presence of a binary 

 axis and of two perpendicularly intersecting symmetry-planes. The 

 corrosion-figures on |100j and |010|, and also on the prism jllOj 

 of the crystals from Jhity and Bodenmais, pi'oved clearly in every 

 case the absence of a horizontal phine of synnnelri/; they were 

 however furthermore so abnormally shaped, that they could hardlj^ 

 be used for tlie control of the above deduced symmetry of the 

 ci-ystals. This case proves once more, that the method of corrosion- 

 figures used, eventually can give unreliable results, either by the 

 production of abnormal etching-figures or by a shape of the corrosion- 

 figures, which cannot sufficiently exactly be defined. 



A second trial to determine the physical symmetry in this case, 

 was based upon the idea, that because the principal axis c was of 

 polar nature, it would be possible, that its ends would manifest 

 opposite electrical changes on mechanical deformation or on heating. 

 Although we are strongly convinced of the truth that a negative 

 result can hardly be considered to be a decisive argument in this 

 question, we have nevertheless spent a considerable time in en- 

 deavouring to prove the pohxrity of the c-axis by means of Kundt's 

 method of dust-figures. Although we were able to obtain on this 

 occasion e.g. the alternative red and yellow powdering of the vertical 

 edges of prismatic quartz-crystals in a \'ery satisfactory way, however 

 all our numerous tentatives with plates of cordierite, as well with 

 the pinacoidal as with the prismatic plates, remained without a 

 positive result. In every case, if present, this piezo-, or pyro-electrical 

 polarity of the c-axis appears to be only so feeble, that it seems 

 impossible to prove its existence in the described way with any 

 certainty. 



It is a quite remarkable fact, which strongly corroborates the 

 value of the new method that even where all crystallographic methods 

 to find the smaller physical symmelry-difierences of crystals used up 

 to tiiis date, are failing, the new method however appears to be quite 

 able to elucidate the finer feature of symmetry of such crystals in 

 so comi)lete and persuading a way. Therefore an indubitable place 



^) In these photographs, the crossed hairs in the field are parallel to the 

 directions of optical extinction of the plates. 



29 

 Proceedings Royal Acid. Amsterdam. Vol. XVII. 



