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V. llemarkable Property of Mica. By P. Riess*. 



A PIECE of clean mica becomes covered like other sub- 

 ■*■*■ stances with a vast number of small drops of water at 

 very small distances from each other, when it is breathed upon 

 or held over steaming water. The layer of bubbles impairs 

 the reflecting properties of the mica, and renders objects 

 placed beneath it imperfectly visible; if by means of a sharp 

 knife a very thin leaf be pealed off the mica, and thus a new 

 surface given to it, and it then be again exposed to the aque- 

 ous vapours, the fresh surface remains perfectly transparent 

 whilst the surrounding parts become dim as before. This 

 curious fact is not accounted for by supposing no aqueous 

 vapour to be condensed upon the fresh surface, for a distinct 

 play of colours may be observed upon that part, showing it 

 to be covered with a continuous thin layer of water. A drop 

 of water, which remains in a globule upon an old surface of 

 mica, immediately spreads out and moistens the whole extent 

 of a fresh surface. 



From its extreme purity, therefore, the fresh mica surface 

 exerts so strong an affinity for aqueous vapour as to condense 

 it in a continuous film, whereas, if it has been exposed for 

 some time to the air, it can only condense the vapour in se- 

 parate little globules. This may be distinctly seen under the 

 microscope. 



A fresh mica surface shows this property not only in an at- 

 mosphere nearly saturated with aqueous vapour, as is pro- 

 duced by an exhalation, but likewise in common air when 

 far removed from a saturated state. It is well known that 

 mica is one of the best non-conductors of electricity; a piece 

 was chosen which did not in the least affect a charged elec- 

 trometer during the space of a minute; on pealing off a thin 

 layer, the fresh surface discharged the electrometer in a few 

 seconds, the old surface still remaining a non-conductor. 

 That the power of conducting was due to a continuous layer 

 of water, and could be removed by its evaporation, was proved 

 by the following experiments. 



A fresh mica surface placed in a box, the bottom of which 

 was covered with chloride of calcium, could not, after a mi- 

 nute or two, be found to conduct electricity; it became however 

 a conductor on the removal of the drying material and the 

 renewal of the air in the box. Whilst heated, a similar leaf 

 of mica did not conduct, but regained that power during 

 cooling. 



* Translated from Poggendorff's Annalen, No. 3, 1846, and communi- 

 cated by Dr. E. Ronalds. 



