On the Electrical Behaviour of Solid Insulators. 181 



sky of Italy, the neutral point which I discovered under the sun. 

 He sought for it in vain on the 1st and 2nd of July 1862. We 

 are not aware that it has been seen by any other observer than 

 M. Babinet. 



The observations published by Dr. Rubenson were made with 

 a very fine polarimeter executed by M. Duboscq. It was com- 

 posed, like the one which I used, of a pile of glass plates, and a 

 Savart's polariscope, and was fitted up with graduated circles and 

 other accessories, so as to give at once and very accurately with- 

 out any calculation of the sun's motion, the position of the plane 

 of polarization, the distance between the sun and the point ob- 

 served, the altitude and the azimuth of this point, and, finally, 

 the relative value of the polarization. 



At Rome the observations were made upon a terrace on the 

 top of the house No. 101 of the Via Sistina on Monte Vincio, 

 one of the highest points of the city, and commanding an exten- 

 sive view in all directions. At Segni the polarimeter was placed 

 on the top of the mountain on the upper part of which the town 

 is built. 



The most recent observations on the polarization of the atmo- 

 sphere were made between 1862 and 1864 by M. Andres Poey, 

 under the tropical sky of the Havana. He adopts my theory of 

 the polarization by refraction, and admits the resemblance be- 

 tween the phenomena and those of biaxal doubly refracting crys- 

 tals, and to those of uniaxal crystals when the sun is in the 

 zenith. He seems also to have observed the neutral point below 

 the sun. The general results of his observations will be found in 

 a short paper of four pages in the Comptes Rendus, torn. lx. p. 781, 

 Avril 17, 1865. 



XXII. On the Electrical Behaviour of Solid Insulators, 

 By Dr. W. von Bezold*. 



IT is well known that bodies are divided, as to their behaviour 

 in relation to electricity, into two classes — conductors and 

 insulators. While the former have been the subject of frequent 

 and searching investigations, the latter have of late attracted 

 but little attention, notwithstanding that it was in them that 

 electrical phenomena were first observed, and that consequently 

 they were long called preeminently electric, and the conductors 

 non-electric bodies. They have been regarded as almost per- 

 fectly indifferent in their relations to electricity, and were studied 

 only so far as the practical requirements of the experimentalist 

 demanded. A single phenomenon seemed continually to indi- 

 cate that this indifference was nevertheless not so complete as 



* From Poggendorff' s Annalen, vol. cxxv. p. 132. 



